This short article discusses the conditions leading up to the devastating flash floods in the Valencia region on 29 October 2024. We also briefly discuss some storm characteristics from the perspective of the satellite data. Please note that we concentrate on the meteorology, but neither climatology nor the social aspect of the disaster, making this writing far from exhaustive.
The synoptic scale was characterized by a deep cut-off low over northwestern Africa and southern Spain. At the upper troposphere, a left exit region of a jet streak was located over southeastern Spain and shifted north during the day. Closer to the ground, a strong easterly low-level flow, with wind speeds up to 25 m/s at 850 hPa overspread eastern to central Spain with persistent warm air advection across the area. The low-level jet was lifted over the local topography on the eastern coastlines, initiating the development of the storms.
ECMWF analysis of the 300 and 850 hPa geopotential height, temperature and wind at 12 UTC.
Southerly mid-tropospheric flow advected steep lapse rates from the Sahara over the Balearic Sea. The models simulated a notable overlap of steep lapse rates (exceeding 7 K/km between 850 and 500 hPa) and abundant lower tropospheric moisture (exceeding 12 g/kg in the lowest 500 m). This yielded a high CAPE environment, especially over the sea and coastlines. Figure below shows the persistence of these ingredients just east of Valencia as the quasistationary storm was happening. High CAPE combined with substantial vertical wind shear, a situation with a high conditional probability of severe weather should storms form.
ECMWF 12 UTC forecast of 15 and 18 UTC lapse rates (red contours), specific humidity (color shading) and wind vectors at 500 m combined with a 15 and 17 UTC flash area (red dots) based on the Lightning Imager of the MTG. Note the position of the Valencia storm at the tip of the tongue of very moist airmass.ECMWF 12 UTC forecast of 18 UTC MUCAPE (color shading), 0-6 km bulk wind shear (barbs) combined with a 17 UTC flash area (red dots) based on the Lightning Imager of the MTG. Red marker just east of Valencia shows the location of the plotted forecast profile.
The storm produced high rainfall intensities over several hours, and at least three tornadoes rated IF1, IF1.5, and IF2. According to AEMET, the hourly record rainfall in Spain was broken in Turis, with 184.6 mm in 1 hour and 42 mm in just 10 minutes. Overall the station received 771.8 mm in 14 hours. Weather station Chiva reported 491.2 mm in 8 hours, 343 mm in 4 hours, and 160 mm in 1 hour.
Because extreme rainfall produced a much larger impact than tornadoes, we will mostly concentrate on the ingredients-based forecasting of this phenomenon, following the work of Doswell et al. (1996). The authors separate the ingredients for rainfall intensity and its duration. The intensity is a function of the supply of low-level moisture, its vertical flux, and the “efficiency”. The efficiency is the ratio of the amount of rainfall that reaches the ground to the amount of rainfall formed in the storm. The evolution of the forecast temperature and humidity profiles and hodographs just east of Valencia in the inflow zone of the storm shows an improvement in the ingredients for high rainfall intensity between 12 and 15 UTC.
Skew-T log p diagram of the ECMWF 12 UTC analysis and 15 UTC forecast and hodograph over the sea just east of Valencia.
By 15 UTC, these ingredients were present over the area:
High absolute moisture content in the lower troposphere rapidly advected towards the coast with wind speeds up to 25 m/s in the bottom 500 m.
High potential for rapid vertical flux of moisture due to high CAPE
High precipitation efficiency due to low-cloud bases, and high relative humidity in the low to mid-troposphere, resulting in a low potential for evaporation of rainfall.
Considerable depth of the cloud available for the growth of raindrops through collision and coalescence, given by the large distance between the lifted condensation level and the 0° isotherm (so-called deep warm cloud depth)
The intensity and the width of the precipitation swath were likely increased by the presence of strong vertical wind shear, both in the bottom 1 km and above it. Observations and idealized simulations have shown that precipitation rates are increased through shear-induced enhancement in the low-level updrafts (Smith et al. 2001, Nielsen and Schumacher, 2018Nielsen and Schumacher, 2020). The width of the precipitation area is also increased by the stronger shear (and conversely, strong inflow) producing wider updrafts (Mulholland et al. 2024). This was especially important in this case, which featured low cloud bases, which typically result in narrower updrafts.
Precipitation fell over the area in several rounds. The first convective system affected the Valencia region already in the morning hours. A decrease in activity was noted during the late morning with drier air in the low to mid troposphere advected over the region. The main phase with a quasi-stationary storm started around 14:30 UTC and lasted till 18 UTC.
ECMWF 12 UTC forecast of 15 and 18 UTC Q-vector convergence (blue and red contours), and vorticity (color shading) at 850 hPa combined with a 15 and 17:40 UTC flash area (red dots) based on the Lightning Imager of the MTG.
There were several factors contributing to the quasi-stationary nature of the storm in the area:
Persistent maximum of synoptic-scale lift at 850 hPa caused by warm air advection
Persistent strong easterly onshore flow ascending the local topography
Tongue of higher low-level moisture and CAPE upwind of the initiating storms resulted in back building
High relative humidity in the low troposphere combined with low cloud bases. This resulted in weak or absent cold pools that would push the initiation of new cells to other locations.
The storm showed the typical pattern for quasistationary rainstorms in the Mediterranean, with the higher moisture and CAPE remaining over the sea, while the storms form over the coastline and are advected further inland. This results in opposing directions of cell advection by mean wind and new cell formation towards the area of higher CAPE. This process applied in this case and is schematised below.
MTG Cloud phase RGB at 15:40 UTC combined with a schematic showing the distribution of CAPE in the area (brown and red contours), the orientation of the mean wind vector, and the region where new updrafts are initiated (ellipse with black stripes).
The storm that ensued looked extremely intense from the satellite’s perspective. It displayed a persistent, cold, and wide overshooting top with a long above-anvil cirrus plume advected in the direction of upper tropospheric storm-relative winds. Both features are shown in the figure below. The animation of the “Sandwich” (combination of visible and infrared channels) shows the long-lived nature of the features as well as the rapid expansion of the anvil. The above-anvil cirrus plume becomes more prominent towards sunset as the low sun angle creates longer shadows.
MTG Sandwich imagery at 16:10 UTC with colour representing the cloud-top temperature based on the IR channel.
The nature of the storm top features reflected the environment in which the storm formed characterized by high CAPE and strong upper tropospheric storm-relative winds, which helped to create a hydraulic jump downwind of the overshooting top (Homeyer et al. 2017). The quasi-stationary nature of the main updraft of the storm resulted in strong storm-relative flow both in the upper troposphere and near the surface. The storm-relative inflow reached up to 22 m/s in the bottom 1 km, which greatly increased the updraft width and the vertical moisture flux (Peters et al. 2020). Interestingly, although the storm had a persistent, strong, and wide updraft in the upper troposphere, there was no large hail reported to the European Severe Weather Database.
ECMWF 12 UTC forecast of the 18 UTC hodograph east of Valencia over the sea outside of the convective activity. Numbers on the hodograph represent altitude in km. Blue and purple vectors indicate the storm-relative winds in the lowest 1 km and at 10 km assuming the updraft of the storm was not moving.
Strong moisture flux through the charging region of the storm resulted in very high lightning activity. Some of the products available from the new Lightning Imager are shown below, showing a high density of lightning in the core of the storm. Some of the flashes were also detected from the southern side of the storm due to the angle of the scanning of the satellite, forming a curious U-shape around the edge of the anvil.
MTG Lightning Imager data at 16:35 UTC showing different products available to the forecasters: event groups, event group density, and flash sizes. More information on the the Lightning Imager and disseminated products can be found here.
In summary, a high-end environment for severe convective storms existed over the area from the perspective of high CAPE and strong vertical wind shear. This resulted in well-organized storms with strong updrafts and a high precipitation intensity. The continuous strong onshore flow of a very moist airmass, in combination with a persistent maximum in the low-level synoptic- and mesoscale-scale lift over the area kept the updraft redevelopment locked in the same position for several hours. This resulted in the extremely high precipitation intensity to persist for several hours.
Last but not least, ESSL would like to extend its deepest sympathies and condolences to the people of Spain, who have suffered from this horrifying natural disaster.
As of 20 January 2024, 9627 large hail (2+ cm) reports were submitted to the European Severe Weather Database for the year 2023. Out of these, 1931 reports involved very large (5+ cm), and 92 reports involved giant (10+ cm) hail. All three metrics were the highest ever recorded in the database, making 2023 the third record-breaking hail season in a row. Besides the raw numbers of reports, the number of days with each hail size category was also record-breaking. There were 229 days with large, 96 days with very large, and 13 days with giant hail. The relative difference in the number of reports to the other years was larger than for the number of days with a certain hail size. One likely reason is the ESSL and cooperating observers/observer networks becoming more efficient at collecting reports. Another reason can be that 2023 involved more very long-tracked hailstorms which produced large hail at many locations. For example in 2022, there were 5 hailstorms with hail swaths exceeding 200 km. In 2023, there were 13 such hailstorms, with one particularly long-lived supercell producing a 686 km long hail swath affecting 5 countries.
Number of reports and days with large (2+ cm), very large (5+ cm), and giant (10+ cm) hail between 2006 and 2023 according to the ESWD.
The country with the highest number of large hail reports was France (1502), closely followed by Italy with 1468 reports. However, in terms of the most damaging hailstorms, Italy took the commanding lead with 596 reports of very large, and 67 reports of giant hail, compared to 280 and 10 over France. 30.9% of very large and 72.8% of giant hail reports were submitted for Italy. The third most affected country by large hail was Germany with 1270 reports, out of which 142 involved very large hail.
Spatial distribution of large hail reports across Europe and surrounding territory in 2023.
Records were broken not just concerning the amounts of large and very large hail, but also in terms of the maximum hail sizes. Europe’s largest (photographed) hail record was broken twice in a mere 5 days in Italy. On 19 July, 16 cm hailstone was reported (see ESSL news item), followed by 19 cm hail on 24 July (see ESSL news item). In both cases, hail sizes were estimated using reference objects placed near the hailstones. The heaviest officially weighted hailstone also fell on 24 July with a scale reading 484 g. The hailstone had a diameter of 13 cm and thus the largest hailstones observed on 19 or 24 July likely weighed considerably more. Giant hail fell also in other countries: Slovenia (the largest hail reaching 13.8 cm), Bulgaria (13 cm), Croatia (13 cm), France (11 cm), Spain (11 cm), Bosnia (11 cm), Germany (10 cm) and Russia (10 cm).
Large hail had pronounced societal and economic impacts. At least 328 people were injured by hail, the real number likely being much higher as the lighter injuries caused by hail are not well covered by media. Most of the injuries (242) were reported from Italy. Hail also caused tremendous economic damage. Munich-RE report stated that Italian hailstorms cost billions of $ and the Gallagher-RE report put the figure to 3 billion $ with the overall damage cost for the convective storms in Europe at approximately 12 billion $.
Below, we present an overview of the (subjectively chosen) major hail cases in Europe. Please note that severe hailstorms occurred also outside of the chosen dates and not all of them are mentioned here. If you feel we’ve missed a particularly important case, let us know at tomas.pucik@essl.org. Very importantly, ESSL would like to express gratefulness to the Voluntary Observer Persons and Voluntary Observer Networks, who have significantly contributed to the record number of hail reports collected during this year. Their list can be found here.
Cases
6 July: Damaging hailstorms affected Spain and Italy. An 11 cm large hail fell in Herrera de los Navarros, damaging cars, their windshields, and roofs. Hail measuring 10 cm in diameter fell in several locations along the hail swath. This was the first storm to produce giant hail in Europe in 2023. Several other hailstorms affected Euskadi province. Hail reached up to 7 cm, causing 11 injuries in the town of Vitoria-Gasteiz and widespread damage to vehicles. In northwestern Italy, hail reached 6 cm and damaged numerous cars.
11 July: Many countries were affected by large hail with reports received from France, Germany, Switzerland, Italy, Austria, and Slovenia. France was the most affected with 258 reports. The largest hail fell in Neulise, reaching 10 cm in diameter, damaging roofs and cars. Supercells over France eventually grew upscale, resulting in a convective windstorm that traveled across northern Switzerland and southern Germany. Several hailstorms also formed over the southern Alps. 9.5 cm large hail fell in Bolzano province, Italy, which is a rare occurrence within the interior of the mountains. In Austria, hail up to 7 cm in diameter fell in Kärnten province.
12 July: Severe hailstorms affected France, Germany, Switzerland, and Italy. In France, hailstorms impacted the Rhône-Alpes region with the biggest hail estimated at 8.5 cm across. In Switzerland, hail was up to 6 cm across and the largest hail, estimated to be 10 cm across, fell in northern Italy, in Angolo Terme. The late evening hailstorms grew upscale, resulting in a damaging convective windstorm across northeastern Italy, Slovenia, and southern Austria. The highest measured wind gust reached 48.9 m/s.
13 July: Perhaps one of the most long-lived supercell hailstorms ever observed in Europe occurred on that day. The storm affected Slovenia, Croatia, Hungary, Serbia, and Romania, making it a “5-state hailstorm”, tracking for 686 km and lasting 9 hours and 15 minutes. The storm formed around 12 UTC in Slovenia, where it produced the largest hail, measured at 13.8 cm across, in Gorenja Lepa Vas. In Hungary, the supercell produced hail up to 5 cm across, and in Croatia, up to 6.5 cm across. The storm continued through Serbia and eastern Romania, producing hail up to 7 cm in diameter, dissipating after 21 UTC. Hail was wind-driven in many places, causing significant damage to roofs, cars, windows, facades of houses, and agriculture. In Vrbas, Serbia, at least 50 people were injured by hail.
19 July: Severe hailstorms affected Germany, Switzerland, Italy, Slovenia, Croatia, Bosnia, Serbia, Slovakia, Poland and Romania. Altogether, 447 hail reports were received in the ESWD and 4 separate hailstorms produced hail ≥ 10 cm across with 27 separate reports of hail of that size. Two of the giant-hail-producing storms also involved large hail tracks over 200 km long. The event started with a supercell forming on a southwestern flank of the convective system over Slovenia, tracking over Croatia, and dissipating over Bosnia. In Slovenia, the hail reached 9 cm in diameter. Over Croatia, the largest hail, measured at 13 cm in diameter, fell in Ribnik, and hundreds of roofs were destroyed in the Karlovac area. Over southeastern Slovakia, a supercell produced wind-driven hail up to 6 cm large, which caused serious damage to roofs, windows, and facades. The most serious impact was over Italy, where three supercells produced hail up to 10, 14 and 16 cm across. The estimated 16 cm hail fell in Carmignano di Brenta and set a new record for the European hail size that involves photographed hailstone. Dozens of villages and towns suffered car, roof, and window damage. At least 111 people were injured by hail here. Besides devastating hail, the event featured a derecho with an almost 1000 km long damage path that crossed southern Austria, Slovenia, Croatia, and Serbia and dissipated over southwestern Romania and northwestern Bulgaria.
Hail damage to the roofs, solar panels, facades, and windows. Photographs taken by Alberto Gobbi.
21 July: Multiple severe hailstorms were observed over Italy, Slovenia, Croatia, Bosnia, and Hungary. The situation started with an overnight hailstorm in Lombardia, Italy, with the largest hail measuring 13 cm and weighing 365 grams. In the afternoon a hailstorm in Bosnia produced an 11 cm large hailstone that fell in Prnjavor. In southwestern Hungary, wind-driven hail up to 9 cm large caused extensive damage to agriculture, roofs, and windows of houses.
22 July: Another day with severe hailstorms over Italy, Bosnia, and Serbia. In Serbia, a long-lived supercell produced wind-driven hail up to 9 cm in diameter with severe impact especially in and around Kraljevo. Here, the hail injured at least 2 people with one article mentioning that the emergency room was overwhelmed by people injured by hail there. In Italy, two supercells produced separate hail swaths in Emilia-Romagna. The largest hail, 10 cm across, fell in Terre del Reno. Hail also injured 5 people. The supercell that produced the largest hail also produced an IF3 tornado. The hail swath of the supercell was 230 km long before it moved over the Adriatic Sea. The total swath was likely much longer as the storm kept its intensity (judging by the satellite imagery) moving towards the Croatian coastline.
24 July: The true “hail day of the year” with 855 reports of large hail submitted to the ESWD. Large hail was observed across France, Switzerland, Italy, Slovenia, Croatia, Austria, Czechia and Slovakia. The number of hail reports per day almost matched the current record from 24 June 2021, which stands at 858 reports. 33 reports involved hail over 10 cm in diameter. In total, hail injured 119 people, all of them in Italy. 3 hailstorms produced swaths of hail over 300 km long, the longest being 546 km with the storm lasting 6 hours and 40 minutes. The first hailstorms formed in the early morning over France with a maximum hail diameter of 7 cm. The severe weather ramped up in the evening hours. The first hailstorm formed over Lombardia and tracked through the southern part of Lago di Garda, Veneto region, and across the Adriatic Sea to Istria, Croatia. The largest hail reached 12 cm near the lake and wind-driven hail occurred in multiple spots of the hail track. Two more supercells formed over Trento and tracked over Fruli-Venezia Giulia and then to Slovenia. The first produced hail “only” up to 9 cm, but it was wind-driven along much of the path. Extreme damage occurred in Mortegliano and its surroundings, with roofs and car windshields destroyed, house facades damaged, and windows broken. The second supercell produced larger hail with an approximately 50 km long swath of giant, 10+ cm hail. The largest hail fell in Azzano Decino, which was estimated at 19 cm across using the reference objects placed near the hailstone. The hail caused considerable damage, including complete punctures of some of the car windshields. In Slovenia, the hailstorm caused hail up to 10 cm across. It should be noted that Azzano Decino was hit twice by giant hail in a mere 2-year period. 10 cm hail fell in the town in the early morning hours of 1 August 2021. In Slovakia, 8 cm hailstones were reported from the northeastern part of the country.
Examples of hail damage to vehicles. Photographs taken by “Unwetter-Freaks”.
25 July: Two intense supercells produced giant, 10+ cm, hail in the overnight to early morning hours over Italy. The largest hail, measuring 12.5 cm in diameter, fell in Castiglione delle Stiviere. The storm that produced the hail copied part of the hail swath from the previous day and also impacted the area to the south of Lago di Garda with giant hail. Severe hailstorm also affected western Ukraine with the largest hail up to 8 cm in diameter.
6 August: Giant hail, up to 13 cm in diameter, was reported from Dulovo, Bulgaria. The hail was driven by severe wind in the area and caused significant damage to cars, roofs, facades, windows, and agriculture. Interestingly, such large hail fell only at one location and other locations received hail only up to 6.5 cm. The wind-driven hail continued into Romania, where the storm transformed into a bow echo with wind gusts up to
7 August: Multiple supercells formed over Lithuania and moved across Latvia during the morning hours, with one storm affecting southwestern Estonia. In Lithuania, hail reached up to 9 cm, and widespread damage to the cars, windows, roofs, and greenhouses was reported, especially in the Panevėžys area. One person was injured by glass shattered by hail in Vepriai. In Latvia, the largest hail ever recorded till that time, 8 cm in diameter, fell in Annenieki. In Estonia, the hail reached 8 cm across in Torgu. The highest impact came from a fast-moving and long-lived supercell that covered 380 km distance in 4 hours and 15 minutes as it moved from Lithuania to Estonia. Along much of its path, large hail was driven by severe wind gusts. Over Latvia, particularly in Tukums, Jelgava, and Talsi districts, wind-driven hail of 3 – 6 cm in diameter caused extreme damage to agriculture, forests, cars, windows, and facades of houses. 18 410 ha of farmland was destroyed by the hail. Wind gusts reached up to 32.6 m/s in the path of the storm. A historical castle in Jaunpils had many windows broken by the wind-driven hail, and more damage was caused by the subsequent heavy rainfall. Large hail also injured 3 people and killed or injured a large number of storks, cranes, and other birds. Some of them remain in different care centers because of blindness caused by the large hail damage to the eyes or the necessary amputation of wings due to broken bones.
Examples of wind-driven hail damage across Latvia. Photographs taken byAnita Zieda-Misuleand Iveta Bremane.
22 August: Several supercells produced very large hail over eastern Czechia and southern Poland. The largest hail fell in Klokočov, 8 cm in diameter. Significant damage to roofs, windows, and cars was reported from the area.
25 August: Very large hail fell in France, Switzerland, Italy, and Austria. The largest hailstone, measured at 9.5 cm across, was found in Bärnbach, Austria. In France, the largest hail reached 7 cm. The worst impact was reported from Ticino, where wind-driven hail up to 7 cm in diameter caused extensive damage to cars, roofs, facades, and windows. An unknown number of people were injured by hail.
26 August: 316 large hail reports were submitted to the ESWD for this day. Large hail occurred in Spain, Italy, Germany, Austria, Slovakia, Poland and Greece. The event began with supercells developing over Poland and N Slovakia. The largest hail measured 9.5 cm in diameter in Poland and 8 cm in diameter in Slovakia with one hail-related injury. In the afternoon hours, two long-lived supercells formed over Bavaria. The northern one produced hail up to 6 and the southern one hail up to 10 cm in diameter. Hail was wind-driven in both storms and fell in large quantities. The largest impact from the storms was in the Bad Tölz-Wolfratshausen area, where 80% of the roofs suffered serious damage. Besides that, cars, windows, and house facades were heavily damaged. An impressive video of the hailfall can be found here. Hailstorms also had a tragic impact on animals: birds, hares, deer, and calves were reportedly killed by hail. In some areas, 90% of the wildlife was killed or injured by the hail. Several horses on the farm were injured, suffering bone fractures from the hailstone impacts. In Spain, 10 cm large hail fell in La Sènia, Catalonia, injuring one person and causing damage to roofs and cars.
Examples of hail damage caused by the hailstorms in southern Germany. Photographs taken by Claudia Koestler, Harry Wolfsbauer, Manfred NeubauerandHarry Wolfsbauer and posted on Süddeutsche Zeitung
30 August: Severe hailstorms occurred over Bulgaria, Romania, northeastern Poland, Lithuania, and Latvia. In Lithuania, hail up to 9 cm was found following the storm, and in Latvia, hail up to 8.5 cm in diameter was observed.
17 September: Damaging hailstorms affected Spain and France. Altogether, 5 storms produced hail over 5 cm in diameter. Two storms were particularly noteworthy. The first crossed the Valencia region in Spain. Hail reached 8 cm in diameter, fell in large quantities, and was wind-driven as well. Vineyards and tree orchards were completely devastated. Impressive photos or videos from the event can be found here. Even larger hail occurred in France, northwest of Toulouse, reaching 11 cm across. Roofs, greenhouses, and car windows were destroyed by the hail.
The year 2022 was another record-breaking year for hailstorms in Europe. In total, 8224 large hail reports (≥ 2 cm in diameter) were submitted to ESWD. That is 2791 more than in 2021, which was already a record-breaking year. Very large hail (diameter ≥ 5 cm) was reported 1334 times and 18 reports involved giant hail with a diameter ≥ 10 cm. There were 213 days with at least one large hail and 94 days with at least one very large hail report. The period of 20 May to 10 July was particularly active concerning very large hail. Out of 52 days, very large hail was observed on 42 of them.
Large hail reports in Europe in 2022 based on the European Severe Weather Database.
By far the largest number of reports was submitted for France (2461), followed by Italy (993) and Germany (583). The three days with the most hail reports were 4 June (411 reports), 20 June (385 reports), and 25 May (334 reports). Hail injured 215 people and killed 1. The two most societally impactful events were the Casamassima (Italy) hailstorm on 19 August with 100 injuries and the La Bisbal d’Empordà (Catalonia) hailstorm on 30 August with 67 injuries and one fatality. Hailstorms had a very large economic impact, especially in France, where the insured losses reached 4.8 billion € according to Swiss-Re. While some of the hailstorms produced large hail only for 15 minutes, some lasted for more than 3 hours. The longest-lasting hailstorm occurred on 22 May in France, producing large hail for 5 hours in a hail swath over 300 km long.
Major hail events of 2022 formed in a large range of CAPE and shear values. That said, the majority of the events occurred with CAPE exceeding 1000 J/kg and 0-6 km bulk shear exceeding 15 m/s, which is a parameter space where the large hail typically occurs. Some of the giant hail-producing events had a rather unremarkable environment. A good example of such a case is the hailstorm of 1 July over Czechia, which produced hail up to 11 cm in diameter with CAPE around 1000 J/kg and 0-6 km bulk shear well below 20 m/s. Giant hail was produced briefly following the merger of two storms. Hail around 3 cm in diameter was observed pre-merger and no large hail followed the brief period of giant hail production. This shows the importance of storm-scale processes, which can’t be captured by looking at the large-scale environment.
Major hailstorms of 2022 in CAPE and 0-6 km bulk shear parameter space based on the ECMWF data. Background colors represent the relative frequency of hail > 2 cm acquired from the additive logistic regression model ARCHaMo for large hail (Battaglioli et al. 2023, submitted).
The biggest hailstorm cases of 2022:
20 May: Several hailstorms impacted northern France and western Germany. The largest hail, 8 cm across, fell in Sedan, France. Most hail damage was produced by a hailstorm that passed the northern suburbs of Koblenz, Germany, damaging roofs, windows, and cars. The largest hail reached 6 cm there.
22 May: A supercell tracked for more than 300 km across west-central France, impacting the towns of Niort, Poitiers, Chauvigny, and Chateauroux. Supercell produced hail for 5 hours with several peaks of activity, when the hail diameter exceeded 8 cm in diameter. The largest measured hail was 12 cm in diameter, but even larger may have fallen with reports of weight up to 780 g. Chateauroux was hard hit with hailstones up to 9 cm in diameter. 4 people were injured and the hail badly damaged 250 houses and 1000 cars.
One of the giant hailstones that fell on 22 May in France. Source: https://www.francebleu.fr/infos/faits-divers-justice/en-images-a-frontenay-rohan-rohan-des-grelons-comme-des-oranges-1653307396
25 May: Several supercells produced long hail swaths over southeastern Austria, Slovenia, northeastern Croatia, and Hungary. The largest hail measured 8 cm in diameter. Severe damage to buildings was reported in Kapela Podravska, Croatia, and Kiskorpád, Hungary.
28 May: South-moving hailstorm produced damage east of Milan. The largest hail reached 7 cm in diameter. A hail up to 8.5 cm in diameter was reported in the Smolan region, Bulgaria, damaging cars, roofs, and agriculture.
29 May: A woman was injured by very large hail in Italy. Hail up to 7.5 cm in diameter caused significant damage to roofs, cars, and greenhouses in Orizari, Bulgaria.
2 June: Severe hailstorms impacted southeastern Austria, Slovenia, and northern Croatia. Damage to crops, houses, and cars was reported. The maximum hail size reached “only” 5 cm, but hail fell in large quantities and was accompanied by severe winds in many locations with drifts up to 20 cm deep.
3 June: Hailstorms affected southern France. The largest hail measured 7.5 cm in diameter and the longest hailstreak was 150 km long.
4 June: Widespread hail damage was reported in central France, particularly in Vichy and surrounding areas. Hail reached up to 10 cm in diameter, severely damaging cars, roofs, and agriculture. 1 person was injured by hail in Vichy.
5 June: Hailstorms impacted eastern France, Switzerland, northwestern Italy, Slovenia, Austria, and Serbia. Hail up to 9.5 cm was reported in Frankolov, Slovenia, and an 11 cm hailstone fell in Kufstein, Austria.
19 June: A long-lived, right-moving supercell produced an almost 350 km long hail swath across France, affecting the outskirts of Orléans. The supercell produced hail for 4 hours. Roofs and cars were damaged along the path of the storm and the largest hail, measuring 8.5 cm, fell in Chambord and Saint Cyr en Val.
20 June: A long-tracked supercell crossed northern Bordeaux and destroyed more than 5000 hectares of agriculture with wind-driven hail in Dordogne. The largest hail diameter reached 7 cm. Serious damage to roofs and cars was reported in some parts of the hail swath. North of Pyrennees, a hailstorm produced giant hail with a maximum estimated hail size of 13 cm in Vic en Bigorre with widespread damage to roofs and cars. Long-lived supercells with hail also affected southern Germany and southeastern Czechia, but hail did not reach 5 cm.
21 June: Third day of severe hailstorms in France in a row. Wind-driven hail up to 9 cm in diameter impacts regions of Auvergne and Bourgogne, destroying agriculture, roofs, windows, and cars. House facades and sides of cars suffered extensive damage as the hail was blown horizontally in the strong wind. A video showing the combination of severe winds and very large hail can be found here. Another long-lived supercell produced hail up to 8 cm, severely damaging roofs and cars in the region of Morvan. Very large hail also fell in Italy, Slovenia and Serbia. In Serbia, hail up to 7 cm in diameter caused severe damage to agriculture and roofs in the Moravički region.
Damage to the vehicles by the wind-driven hail in Digoin, France. Photo taken by Christophe Asselin (Chroniques Chaotiques).Damage to the vehicles by the wind-driven hail in Digoin, France. Photo taken by Christophe Asselin (Chroniques Chaotiques).
23 June: Hail and windstorm impacted Podgorica, Montenegro with damage to agriculture and cars. Largest hailstones reached 5.5 cm. Hail up to 8 cm in diameter was observed in Sfélinos, northern Greece.
26 June: Severe hailstorms affected eastern France with hail up to 7 cm in diameter. One of the storms also impacted Strassbourg and its outskirts and many cars were damaged.
27 June: Very large hail up to 8 cm in diameter was reported from southern Germany as two supercells tracked along the northern edge of the Alps. 7 cm hail was reported in western Czechia.
28 June: 1 person was injured by very large hail in Kastoriá, Greece.
29 June: Widespread large to very large hail was reported in eastern Czechia and southern Poland.
30 June: Very large hail fell in France, Italy, northern Czechia, Poland, and Bosnia-Herzegovina. The largest hail, 8.5 cm across, was reported in Blamont, France.
1 July: Giant hail up to 11 cm in diameter fell in Rovensko pod Troskami, Czechia. Very large hail also fell in eastern Czechia and western Slovakia from two long-lived supercells. Splitting supercells produced hail up to 6 cm in diameter in the Veneto region, Italy.
5 July: Severe hailstorm impacted southwestern Serbia. Hail up to 7 cm in diameter damaged crops, roofs and cars.
4 July: Very large hail up to 6 cm in diameter injured 2 in Castel Maggiore.
7 July: Several supercells formed in Veneto, Lombardia, and Emilia-Romagna regions in Italy, each producing very large hail. Widespread damage to cars, windows, and roofs was reported. The largest hail fell in Ostiglia and was estimated to be 9 cm in diameter.
Very large hail that fell in Revere-Ostiglia area on 7 July. Photo taken by Luca Vezzosi.
20 July: Severe hailstorms occurred in Switzerland and eastern France. The most severe storm affected the region of Franche-Comté and the commune of Doubs, where hundreds of roofs, windows, and vehicles were badly damaged. The largest hailstone fell in Le Russey, estimated at 9 cm across.
27 July: Very large hail was reported from the Abruzzo province in Italy. The largest hail, 9 cm in diameter, fell in Teramo and Ascoli Piceno. Damage to cars, roofs, and windows occurred.
28 July: Left-moving supercell produced very large hail up to 8 cm in diameter near Lleida, Spain, damaging roofs, cars, windows, and greenhouses.
13 August: Southward moving supercell produced a long hail swath across Sardegna, a rather rare occurrence on the island. The largest hail was estimated to be 9 cm in diameter and fell in Alà dei Sardi.
17 August: Widespread large to very large hail was reported from southern France. The largest hail fell in Bonnétage, estimated at 8 cm across. In Catalonia, a woman was slightly injured by a 5 cm hailstone.
18 August: The event known especially for the powerful derecho producing wind gusts exceeding 60 m/s over Corsica also featured a number of damaging hailstorms. In the early morning hours, wind-driven hail injured 22 people in Sestri Levante and Lavagna in Liguria, Italy. Hail damaged cars, windows, and facades of houses. Around noon, a hailstorm struck Menorca with hail up to 7 cm in diameter. In the evening hours, another series of hailstorms impacted Italy, especially the regions of Marche and Tuscany. The largest hail, 11 cm in diameter, fell in Macerata Feltria damaging cars, and roofs and injuring 1 person.
19 August: Giant hail, reaching 10 cm in diameter, was reported from Casamassima, damaging cars, and windows. At least 100 people were lightly injured by hail. Most of the injuries were inflicted by broken glass. The number of injuries ranks as the third highest recorded in the ESWD for large hail events.
Very large hail that fell on 19 August in Italy. Photo taken by Michele Connena.
30 August: In the late afternoon, a supercell storm formed over the eastern Pyrenees. The storm moved southeastward and entered the district of Girona in Catalonia, producing a swath of very large hail (≥ 5 cm) between Esponnellá and Tamariu. Multiple reports of hailstones larger than 10 cm in diameter were collected with the largest stones estimated to be 12 cm. Impressive videos of the hailfall can be found here or here. Besides serious damage to roofs and cars, 67 injuries and even one fatality (a 2-month-old baby) resulted in the town of La Bisbal d’Empordà. 28 people had to be taken to the hospital, including one serious head injury. This was the first direct hail fatality in Europe since 1997. Furthermore, the number of injuries ranks as the fourth highest recorded in the ESWD for large hail events.
Giant hailstone that fell in Catalonia on 30 August 2022. Source: https://mobile.twitter.com/Supercelulas
8 September: Southeastward moving supercell over Lazio, Italy produced a swath of very large hail. The largest hail, 9 cm across, fell in Boville Ernica.
27 September: Serious hail damage to roofs, cars, windows, and greenhouses was reported from Serbia, especially in Pomoravski okrug region. Hail reached up to 6 cm in diameter.
23 October: Intense storm that resulted in a long-tracked tornado in Haute-Normandie, northwestern France also produced very large hail up to 7.5 cm in diameter.
Mini ECSS is around the corner and you can find the latest information about it here! The conference is held online on 27 and 28September. On both days the program will start with student presentations at 9 AM CEST (7 UTC) and there will be two invited talks each day starting at 2 PM CEST (12 UTC). In case you forgot, there is still a chance today (i.e. 26 September) to register to attend the conference!
All registered participants will be sent a link to the Bluejeans that we will use for the conference. The program can be found below or downloaded.
In the late afternoon of 30 August 2022, an extraordinary supercell storm formed over the eastern Pyrenees. The storm quickly started to move to the right of the mean wind as it entered the district of Girona in Catalonia, producing a swath of very large hail (≥ 5 cm) between Esponnellá at 16:50 UTC and Tamariu at 17:34 UTC, after which it moved over the sea.
Sandwich satellite image of the hailstorm over Northeast Catalonia on 30 August 2022 at 17:30 UTC, and surface station measurements of the wind, temperature (red), and dew point (green).
Multiple reports of hailstones larger than 10 cm in diameter were collected with the largest stones estimated to be 12 cm. Based on some videos, the hail fall was relatively dense for stones of that size. The impacts of the storm were high: Besides serious damage to roofs and cars, 67 injuries and even one fatality resulted in the town of La Bisbal d’Empordà. 28 people had to be taken to the hospital, including one serious head injury. Based on our study on hail impacts across Europe, this was the first direct hail fatality in Europe since 1997. Furthermore, the number of injuries ranks as the third highest recorded in the ESWD for large hail events.
Given the societal and economic impact of this hailstorm and the fact that the giant hail has already been reported 18 times in Europe this year, we look at the predictability of this particular event from the perspective of the large-scale, pre-convective environment addressing these two questions:
How likely was the convective initiation?
How likely were the initiated storms to produce giant hail?
Limiting ourselves to the large-scale environment we do not address two important sources of data: high-resolution convection-allowing models and nowcasting data, such as products based on radar
How likely was the convective initiation?
With an abundance of low-level moisture along the coastline and steep mid-tropospheric lapse rates advected from the interior of Iberia, high convective available potential energy (CAPE) was present to support the development of severe thunderstorms.
With this potential being present, the most important question was if a trigger strong enough to set this energy free would be available in this environment. The synoptic-scale lift was forecast only over extreme northeastern Spain and in the upper troposphere near 300 hPa, but not at lower levels. With the absence of fronts or other large-scale air-mass boundaries, the mesoscale lift had to come from an upslope flow of maritime air against the high terrain.
Locations of where large-scale upward (red contours) and downward (blue contours) vertical motion is to be expected based on Q-vectors at the 300 hPa pressure level.
Convective initiation across Spain was complicated by a substantial amount of convective inhibition (CIN), negative energy to be overcome before a storm can form, especially over the southern part of Iberia. Near the eastern coastline, CIN rapidly increased from the mountains towards the coastline, restricting the ability of the storms to tap into the moisture- and CAPE-rich air mass. The largest area of relatively low CIN (< 50 J/kg) existed over far northeast Iberia, where the supercell formed.
Combining the lowest CIN and the presence of at least some synoptic-scale lift over far northeast Spain, with hindsight it is possible to pinpoint this area as one with the highest probability of storm formation. Severe hailstorms are often isolated cells, rather than storms which are embedded in a larger convective system. On the 30 of August, the absence of widespread mesoscale lift and the presence of some CIN in the environment probably helped to limit the number of storms that formed to the one storm that produced the giant hail.
While it is easy to retrospectively explain the isolated nature of the storms on this day, beforehand it was not possible to state with certainty what will be the exact track of the storms or whether there will be three or no storms at all.
Forecast of the ECMWF IFS model of CAPE (colors) and areas of convective inhibition (whitish shading) at 15:00 UTC.
How likely were the storms to produce giant hail?
The supercell moved into the environment that has been found to be very conducive to severe weather, featuring high CAPE and strong vertical wind shear. Considering the model-simulated Skew-T and the surface observations from the area (temperature of 29, dewpoint of 23°C, and 5 m/s SSE wind), very large hail production was supported by:
High values of CAPE (MLCAPE ≈ 4000 J/kg) with large amounts of CAPE found in the temperature zone < -10°C
Vertical wind shear supportive of supercells. Very large hail occurs almost exclusively with this type of convection. Furthermore, strong shear resulted in a strong inflow into the storm. Based on the simulated hodograph, observed surface wind and observed storm motion, the surface inflow into the storm was almost 20 m/s. Strong inflow supports wide updrafts and wide updrafts lead to long hail embryo residence times in the favorable growth zone.
Unidirectional vertical wind shear (i.e. straight hodograph). Straight hodographs have been found to be more conducive to large hail growth than curved hodographs.
Both low-level shear and storm-relative helicity were quite weak in this case. Unlike for tornadoes, high values of these parameters are not necessary for very large or even giant hail.
ICON-EU forecast of CAPE, 0-6 km bulk shear at 15 UTC with the model Skew-T and hodograph for the location represented by the red cross. The red star within the hodograph marks the observed surface wind and the blue star the observed storm motion. The black arrow is the surface storm-relative wind (i.e. inflow at that level).ECMWF and GFS forecast soundings for 15 UTC. Note the underestimation of surface dewpoint.
Supercell also profited from being the only storm around with no disruption to its inflow and updraft. Its deviant motion to the right was also more pronounced than anticipated by the Bunker’s ID method, suggesting the presence of a strong mesocyclone.
Was it possible to make a confident forecast of the storm producing hail reaching 10 cm? Such a forecast would be useful, as 10+ cm hailstones have a higher probability of causing both damage and injuries compared to 5+ cm hailstones. However, hail diameter doesn’t linearly increase with increasing CAPE and shear. For example, increasing CAPE may even limit the large hail production beyond some point. There are likely other factors that influence the trajectory of hail embryos through the updraft and their residence time in a zone of abundant super-cooled water droplets. Some of these are covered in a lecture by Matthew Kumjian. Testing these factors against a large sample of very large or giant hail cases will perhaps bring us even closer to confident forecasts of such devastating hailstorms.
In conclusion, the combination of very favorable large-scale conditions for hail with isolated convective initiation resulted in a perfect scenario for a damaging hailstorm. Such knowledge provided a good chance to correctly nowcast the event once the storm entered a supercell stage.
On 18 August, severe storms occurred in a swath from Menorca through Corsica, northern Italy, Slovenia, Austria, and southern Czechia. The event featured giant hail up to 11 cm in diameter and extremely severe wind gusts up to 62.2 m/s. In total, 12 people died and 106 people were injured by wind and hail. All fatalities, and most of the injuries, were caused by a long-lived convective system that produced a swath of severe to extremely severe wind gusts. The windstorm event can easily be classified as a derecho, a particularly long-lived and severe convective windstorm as the official criteria for a derecho were clearly met: its damage path exceeded 1000 km in length and wind gusts > 32 m/s were measured in Corsica, Italy, and Austria along the path of the storm.
Temporal evolution of ESWD reports for the event between 00 and 24 UTC.
This article describes in more detail the evolution of the event and its relation to the environment.
Temporal evolution of the derecho The first severe wind gust report was observed at 06:15 UTC in western Corsica. The last severe wind gust report was reported at 17:45 UTC from Czechia. Severe weather reports indicate that the derecho lasted at least 11 hours and 30 minutes. However, it is almost certain that the severe wind gusts occurred already sooner than 06:15 UTC. The severe wind gusts probably occurred as soon as 05:30 UTC, when the bow echo began to form. This means that the severe wind production in the storm lasted likely for more than 12 hours.
17 August
21:00 – 00:00 UTC: Storms initiated over the Balearic Sea. One of the storms became a right-moving supercell and intensified.
18 August
00:00 – 03:00 UTC: The right-moving severe storm grew in size and impacted Menorca. Heavy rainfall and severe wind gusts were reported. Further storms formed to the north.
Airmass RGB showing a severe storm over Menorca and a pronounced trough over western Spain and the Balearic Sea. Courtesy: EUMETSAT.
03:00 – 05:00 UTC: The storms continued to grow upscale and became a squall line. In the same period, a supercell ahead of the main storm system impacted Lavagna and Sestri Levante in Liguria, Italy with wind-driven hail up to 5 cm in diameter. 22 people were injured in this event. Cars, roofs, and facades of houses were badly damaged by hail.
Wind-driven hail damage to the house facade in Sestri Levante, Italy. Photo by Gherardo Ghotti.
05:00 – 06:00 UTC: The squall line transitioned into a bow shape just ahead of reaching Corsica with an increase in lightning activity and rapid acceleration towards the island’s western coastline.
06:00 – 07:00 UTC: The bow echo passed Corsica with extremely severe wind gusts up to 62.2 m/s in Marignana, a station located on a hill. Two other stations in Northwest Corsica measured wind gusts above 50 m/s. 4 people died and 10 were injured in this time frame. Severe winds capsized boats or blew the boats against the rocks and beaches. Damage to the roofs and power lines was reported. The updrafts of the storm, as well as lightning activity, weakened as the bow crossed the island.
07:00 – 08:00 UTC: The bow echo passed the northwestern part of Corsica and continued over the sea. 1 person died in this part of Corsica and wind gusts reached up to 34.4 m/s. Updrafts increased in intensity again as the bow entered the warm sea, as indicated by an uptick in lightning activity.
08:00 – 09:00 UTC: The bow echo impacted the western Italian coastline of southern Liguria and Tuscany with extremely severe wind gusts up to 38 m/s, causing widespread wind damage to trees and roofs. Some roofs were completely destroyed or blown away. 2 people died and 45 were injured. Lightning activity weakened and also the wind damage became more isolated as the storm passed the Apennines.
Roof damage in Ortonovo, Liguria, Italy. Photo byCittà Della Spezia.
09:00 – 10:00 UTC: Lightning activity of the storm increased and new updrafts developed over the lowlands behind the Apennines.
10:00 – 11:00 UTC: The storm system moved over the Veneto and Emilia-Romagna regions of Italy. Severe wind gusts reached up to 38 m/s and 9 people were injured.
11:00 – 12:00 UTC: The lightning activity decreased again as the storms reached the border between Italy and Slovenia. 2 injuries were reported in this period.
12:00 – 13:00 UTC: The system produced severe wind gusts over Slovenia and moved over southern Austria, where the storms intensified again. At the same time, a severe storm over Menorca produced very large hail up to 7 cm in diameter.
13:00 – 14:00 UTC: The system produced wind gusts up to 38.6 m/s in south-central Austria, mainly in Styria and Carinthia. 2 people died and 13 were injured in Mettersdorf. High voltage power lines were brought down by the winds. Storms were now traveling north.
14:00 – 15:00 UTC: The convective system continued producing severe winds over Austria with a maximum gust of 34.4 m/s as it traveled north. 3 further fatalities occurred in Styria as trees fell on hikers trying to protect themselves from the storm.
15:00 – 17:00 UTC: The system started decaying shortly after 16 UTC over northern Austria. Still, it managed to produce severe wind gusts over south-central Czechia.
Further storm activity in the evening yielded very large to giant hail in Toscany and Marche regions, Italy. The largest hail fell in Macerata Feltria, measuring 11 cm across and injuring 1 person. Giant hail damaged cars, roofs, and solar panels.
Giant hail with size estimated at 11 cm across. Photo by Irene Gaggia.
The environment of the storms and its relation to their evolution.
The first storms over the Balearic Sea developed in the forward flank of the trough, in the exit region of a cyclonically curved jet stream. These storms were already forming in an environment of 0-6 km bulk shear > 20 m/s and plentiful CAPE. The transformation into the bow echo occurred in an environment featuring MLCAPE > 3000 J/kg, which is extraordinarily high, and 0-3 km bulk shear > 25 m/s, conditions very favorable for intense convective windstorms.
The very high CAPE values developed due to the presence of abundant low-level moisture and steep mid-tropospheric lapse rates that had been advected from Sahara. Such pronounced overlap of CAPE and shear was present west of Corsica and between Corsica and the coastline of Tuscany. Further north and northeast, CAPE was lower, but bulk shear remained favorable for well-organised squall lines (> 15 m/s in 0-3 km layer) as far as central Austria.
ECMWF forecast of MLCAPE and 0-6 km bulk shear combined with a forecast Skew-T and hodograph for a gridpoint west of Corsica.
Besides the extremely severe wind gusts, the environment was very conducive for (very) large hail. ESSL’s experimental model-based forecasts AR-CHaMo predicted a large hail probability (within 40 km of a point) of over 60% in an area between Corsica, central and northeastern Italy in the 24-hour period between Thursday and Friday 06:00 UTC.
24-hour probability of large hail and lightning based on AR-CHaMo and the combination of 3 models.
While the presence of high CAPE and very strong shear helped the intense bow echo to develop, the crucial point was the upscale growth from the isolated storm to the squall line between Menorca and Corsica. We speculate that the development of the cold pool within the storm helped with the upscale growth. While the maritime boundary layer remained very moist, the presence of drier air and steep lapse rates above 900 hPa could have created strong downdrafts. Another factor may have been the mesoscale lift ahead of the progressing cold front that removed the CIN from the environment west of Corsica. The storm then tracked along the warm front towards Italy, where the further lift was provided by a warm air advection regime and a convergence zone. This lift also helped to remove the CIN from the environment. South of the warm front, CIN values were again very high, preventing convective initiation at this stage. Note that along the coast of Tuscany, vertical wind profiles would support intense tornadic supercells, but the organization of convection into a large-scale bow-echo lowered the threat of tornadoes compared to the severe wind gusts.
Sandwich imagery of the storm about to impact Italy at 07:45 UTC, combined with surface observations at 07:00 UTC. Positions of the three ICON-EU derived Skew-Ts are shown using the letters A, B and C. Note the temperature gradient along the warm front and the increase in CIN south of the front.
Looking at the surface observations, a cold pool was present during all stages of the derecho. The temperature of the cold pool of the storm remained constant throughout its evolution, between 20 and 21 degrees. The temperature difference between the cold pool and the surrounding environment increased as the system moved from the maritime areas further inland. Over western Corsica, the temperature decreased by 6 – 7 degrees when the gust front passed. Near the coastline of Tuscany, the temperature dropped only by 4 degrees. The temperature difference increased towards Austria and Slovenia with a drop of 9 to 13 degrees. This case shows that a strong cold pool is not necessary for a powerful convective windstorm.
The derecho went through several cycles of strengthening and weakening. By far the most impressive phase of strengthening occurred just west of Corsica as the squall line developed into a bow echo. The squall line was originally traveling towards the east. As the bow began to develop, the storm accelerated forward and its direction of movement turned more towards the north. The central part apex of the bow echo accelerated to an incredible speed of 40 – 50 m/s, compared to a mean wind of 17 m/s, which suggests the presence of a very strong rear inflow jet. The storm moved in the direction of the 0-3 km shear vector, i.e. in the direction of the strongest lift along the cold pool.
Bow-echo evolution over Corsica as shown by radar between 06:00 and 06:30 UTC. The location of the most severe wind gust is shown by the star. Source: kachelmannwetter.com, meteologix.com
During the course of its path, the storm weakened twice as it passed the high terrain of Corsica and the Apennines. Downslope winds and perhaps lower CAPE disrupted the generation of new updrafts, which can be seen in the satellite imagery and lightning detection data.
A sequence of sandwich satellite images showing a lack of overshooting top development as the system crossed Corsica.
The updrafts also weakened as the system moved through northeastern Italy. Over Slovenia, numerous severe wind gusts were observed well away from the heavy precipitation cores, suggesting a persistence of the outflow. Further intensification occurred as the outflow encountered the main Alpine ridge, over which new strong cells erupted. This was followed by an intensification of severe wind gusts, reaching over 32 m/s at several stations. At this time, the environment was still characterized by 0-3 km bulk shear exceeding 20 m/s.
Sandwich imagery from 13:30 UTC shows intensifying updrafts over the Alps. Yellow squares represent severe wind gust reports based on the ESWD.
The system finally died after moving over the area of weaker 0-3 km vertical wind shear, higher LFC, and marginal CAPE over northern Austria and southern Czechia.
24 June 2022 is the first anniversary of the devastating tornado that impacted southeastern Czechia in 2021. Together with the Czech Hydrometeorological Institute and partners from other weather services and universities we have worked over the past months to process data from ground and aerial surveys that we carried out in the days after the tornado.
A 30-page report has now been published that summarizes the findings and contains information about the tornado, a detailed look at individual segments of the tornado path, and a discussion of challenges associated with the survey.
Key facts about the tornado and its damage path
Tornado formation: ~17:14 UTC, 1 km east of Břeclav Tornado decay: 17:53 UTC, 1 km south of Ratíškovice Maximum intensity: IF4 Path length: 27.1 km Maximum continuous path of IF2- or stronger winds: 15.3 km Maximum path width: ~ 2800 m, east of Břeclav* Maximum width of IF2- or stronger winds: 590 m, in Hrušky Minimum path width: ~ 250 m, in Hodonín The area impacted by the tornado: 21.9 km² The area impacted by IF2- or stronger winds: 6.1 km²
*It can not be ruled out that part of the beginning of the damage swath was caused by straight-line winds in a rear flank downdraft surge.
This short blog post provides an overview of severe weather associated with the recent windstorms and discusses why the windstorm of 16 – 17 February ended up as a prolific tornado producer.
Multiple severe windstorms affected Europe in the period of 16 – 21 February. 2814 reports of severe wind gusts were submitted to the ESWD in this period, the majority of them in the belt from the British Isles through northwestern France, BENELUX, Germany into Czechia, northern Austria, and Poland. The evolution of severe weather reports for the windstorm of 16 and 17 February is shown below
Temporal evolution of severe weather reports between 16 February 12 UTC and 17 February 12 UTC. Yellow squares represent severe wind gusts and red triangles tornadoes.
Each of the windstorms also involved a strongly-forced convective line that formed along the cold fronts ahead of the fast-moving short-wave troughs. In many areas, the passage of the convective line was accompanied by an increase in wind gusts severity. The environment featured marginal CAPE with a low Equilibrium level (suggesting low-topped storms) and strong vertical wind shear in the lower troposphere with 20 – 30 m/s of 0-1 km bulk shear. Very strong flow just above the ground explains the severity of wind gusts observed along the track of the storms.
500 hPa geopotential height, temperature, and wind (upper panel) and MLCAPE and 0-1 km bulk shear (lower panel) for 17 February, 18 February, and 21 February windstorms.
Due to the strong low-level shear, enhanced tornado threat accompanied the passage of all the windstorms. Between the night and morning hours of 17 February, a tornado outbreak occurred in a swath from northeastern Germany to south-central Poland, Malopolskie region. By 28 February, 22 tornadoes causing 2 fatalities and 5 injuries were reported in the ESWD, while further site surveys are ongoing by Skywarn Polska, and it is likely that the number of tornadoes will grow. 12 out of 22 tornadoes were strong and rated as IF2.
While the 17 February convective windstorm ended as a prolific tornado producer, the other two situations did not result in a single tornado despite a very strongly sheared environment. Overall, three windstorms occurred in this period: the 16 – 17 February windstorm (Dudley), the 18 February windstorm (Eunice), and the 20 – 21 February windstorm (Franklin). The 16 – 17 February and the 20 – 21 February windstorms featured well-organized convective systems with persistent bowing segments and inflow notches. The 18 February convective line was broken with no accompanying bowing segments.
The first difference among the three situations was in the orientation of the convective line, the direction of the mean wind (green arrow), and the direction of the movement of convective elements, such as bow-echoes or inflow-notches (red arrow). In all three situations, the convective elements within the line moved to the right of the mean wind. The largest angle between the mean wind and the orientation of the convective line was noted for the 17 February windstorm over Poland, suggesting the strongest lift on the leading edge of the convective line (perpendicular being the most “favorable” configuration). The individual convective elements also deviated most from the mean wind in this case.
Relationship of convective system orientation and motion of individual elements (such as bow-echoes) to the wind profile characteristics for different cases. Upper panel: Black lines denotes the orientation of the convective system, green arrows represent the mean wind, and red arrows the direction of motion of individual convective elements. Lower panel: Hodographs with 0-1 km SRH highlighted for observed storm motion of convective elements.
Hodographs based on the ICON-DE allow us to reconstruct the storm-relative helicity available to the convective elements in different situations. Considering the observed motion of the convective elements (marked in the hodograph by blue stars), the wind profile on 17 February over Poland had by far the most SRH. The inflow into the storm would be very strong (> 20 m/s) and contain almost purely streamwise vorticity (meaning aligned with the flow, making it “helical”). Strong inflow combined with large amounts of streamwise vorticity helped to create a favorable environment for the development of strong rotation in parts of the advancing convective system.
The reason for a higher amount of helicity was a stronger southerly component of the flow compared to the other situations. This is evident both from the hodographs and the surface station measurements. As the convective line crossed Central Europe between 16 and 17 February, 10 m wind over northern Germany had a smaller southerly component than over central Poland.
EUCom radar composite combined with surface station data (temperature and dew point shown in red and green color respectively) and ESWD reports (red triangles correspond to tornadoes and yellow squares to severe wind gusts).
The amount of helicity in the near-surface inflow seems to be the main difference between the tornado-producing convective windstorm over Poland on 17 February and the other windstorms. 17 February also had the most abundant low-level moisture content (and the highest CAPE values) as well as the line broken into many different convective elements (bow-echoes and inflow notches). Compared to the Kyrill tornado outbreak on 18 January 2007, cold pools were weaker on 17 February 2022.
2021 was a very big year for severe hailstorms across Europe. As of 31 October, we have received 5195 reports of large (≥ 2 cm) hail, 871 reports of very large hail (≥ 5 cm), and 29 reports of giant hail (≥ 10 cm) in the European Severe Weather Database. These numbers are far greater than in the previous years. The number of very large hail reports collected in 2021 is more than twice more than the second most active year, which was 2019. By 31 July, the sum of large hail reports exceeded the sum of reports in the past years collected over the whole 12 months.
2021 does not stand out as much when looking at the number of days with large and very large hail. The inflation in the number of hail reports was caused by very large amounts of reports submitted for some individual days, such as 24 June or 8 July. Any trends in hail reports can not be used to infer the trend in the frequency of severe hailstorms in Europe due to a large number of non-meteorological factors, such as the increases in reporting rates across different countries.
A number of large and very large hail reports and days between 2006 and 2021 in the period of 1 January – 31 October.
The 2021 severe hail season was particularly active in the pre-Alpine region, both on the northern (Switzerland, Austria, Germany) and southern flank (Italy), as well as across eastern Poland and southeastern Czechia. The largest hail was reported on 24 June in Poland with an estimated hail size of at least 13.5 cm. The largest number of injuries, 12, was reported from the 26 September hailstorm that hit Tuscany, Italy. 84.7% of the submitted hail reports included estimate or measurement of hail size.
Spatial and size distribution of 2021 hail reports across Europe till October 2021. Size ratio expresses the percentage of reports that include hail size.
Below you can find a selection and description of some of the most severe hail days and hailstorms in 2021. This list is non-exhaustive, so a severe hailstorm that affected your region may not be included. Red font indicates a particularly severe event. In case you feel that we have missed a particularly severe event in this list or it is not included in the ESWD, please let us know and/or submit your reports at eswd.eu!
ESSL would like to express gratitude to all voluntary observers and networks for submitting severe weather reports for their respective countries.
22 May 2021: Numerous severe hailstorms produced very large hail up to 7 cm across in Stavropolskyi Kray in Russia.
12 June 2021: Very large hail up to 9 cm across fell in the town Tábua, Portugal.
14 June 2021: 40 sheep were killed by large hail near Ankara, Turkey.
16 June 2021: Very large hail up to 6 cm across damaged numerous roofs and cars in Krasnodarskyi kraj, Russia.
19 June 2021: Severe hailstorms affected the Central, Midi-Pyrénéés, Bourgogne, Picardie and Franche-Comtéregion regions in France. The largest hail fell in Vercel-Villedieu-le-Camp with a diameter up to 10 cm, causing significant damage to cars and roofs. This was the first giant hail report in Europe in 2021. Very large hail up to 7 cm also fell in Luxembourg
20 June 2021: Very large hail was reported in the Rhone-Alpes region, France, and Luzern canton in Switzerland.
21 June 2021: Widespread area spanning a belt from southern France through Switzerland into northwestern Austria was affected by severe hailstorms. Southern suburban areas of Munich experienced hail up to 5 cm across. Severe hail damage was reported from the border area of Austria and Bavaria with the largest hail size of 6.5 cm.
22 June 2021: 7 cm hail was reported in southern Russia. Two long-tracked hailstorms affected southern Germany and northwestern Austria. Severe damage to cars from 6 cm large hail was reported from the town of Gmunden, Austria.
23 June 2021: Severe hailstorm with hail up to 7 cm across caused damage on the border of Austria and Czechia.
24 June 2021: 809 severe hail reports were submitted to the ESWD for this day, 615 of them from Poland. This day has broken the record for the maximum daily number of hail reports submitted to the ESWD, as well as the maximum daily number of hail reports submitted for a single country. Several hail-related injuries were reported to the ESWD from this day along with widespread hail damage. In some villages and towns, most roofs were damaged or destroyed by hail.
Very large hail was reported from Austria, Czechia, Slovakia and Poland. Giant hail was reported from Poland and Austria. In Poland, giant hail was observed in multiple locations with the largest hailstone, measuring at least 13.5 cm across, reported from Tomaszów Mazowiecki, breaking the national record. The largest hail fell from a left-moving supercell. In Austria, giant hail was reported in two locations with the largest hailstone falling in Hollabrunn and an estimated diameter of 12 cm. Supercell that spawned the violent tornado between Breclav and Hodonin in Czechia also produced hail up to 9 cm on the border of Czechia and Austria.
A photo of the largest hailstone of the 2021 hail season in Europe that fell in Tomaszów Mazowiecki, Poland. The lower-end estimate using a ruler in the hand of the person that took the photo was placed at 13.5 cm. Photo by Magdalena Cecotka.
25 June 2021: Severe hailstorms affected southeastern Austria, Croatia, Hungary, northern Slovakia and southern Poland. The largest 8.5 cm hailstone was reported from Czarny Potok in Poland.
28 June 2021: Very large hail was reported from Switzerland and southern Germany. 11 people were injured by hail in Switzerland and a giant, 10 cm, hail was observed in Wolhusen with significant damage to roofs, windows, and cars.
29 June 2021: Multiple severe hailstorms were reported in the Alpine area and parts of central Europe. A Hailstone of 8 cm in diameter was photographed in Tarcento, Italy.
30 June 2021:Very large hail up to 7 cm fell in northern Slovakia. A long track hailstorm across southern Poland produced giant hail up to 11.5 cm that weighed up to 200 g.
1 July 2021: Very large hail up to 7 cm across reported in Bulgaria.
8 July 2021: Very large hail was reported in Italy, Switzerland and Czechia. 169 large hail reports were submitted to the ESWD from Italy. The largest hail, 11 cm in diameter, fell in Rozanno, on the southwestern outskirts of Milan.
Very large to giant hailstones in Rozanno, Italy on 8 July 2021. Image source: Tornado in Italia (author unknown)
9 July 2021: Very large to giant hail was reported from Croatia (up to 9 cm across), Hungary (up to 9 cm across) and Poland (up to 11 cm across). The largest hail across Poland was again produced by a left-moving supercell.
13 July 2021: Very large hail was reported from northern Italy (up to 7 cm in diameter) and southwestern Czechia (up to 6 cm in diameter).
14 July 2021: Very large hail up to 8 cm in diameter was reported from eastern Austria in the early morning hours.
15 July 2021: Very large hail was reported in Serbia and Romania (up to 7 cm in diameter).
25 July 2021: Very large hail, up to 8 cm in diameter, observed in Lombardia.
26 July 2021: Several long-tracked hailstorms formed over southern Germany and northern Italy. The largest hailstone reached 8 cm in diameter. One of the hailstorms crossed the highway near Parma, severely damaging hundreds of cars that were undrivable afterward. Minor injuries were also reported. Very large hail was also reported in Austria and Poland.
Examples of damage inflicted by wind-driven hail on 26 July 2021. Photos were taken by Rosella Rotelli and inmeteo.net.
30 July 2021: Very large hail up to 7 cm across damaged roofs in Rochefort-Samson, France.
31 July 2021: Numerous severe hailstorms occurred in eastern Spain and northern Italy. Giant hail up to 10 cm across fell in Peñíscola, the Valenciana region of Spain. In Italy, the largest hail fell in Piemonte with a diameter up to 8 cm across multiple locations.
1 August 2021: An early morning hailstorm produced giant hail up to 11 cm in diameter in Azzano Decino, Friuli-Venezia Guilia region of Italy. Serious damage to roofs and cars was reported. In the afternoon, very large hail was observed in Romania.
15 August 2021: Severe hailstorms formed over eastern France, southern Germany and eastern Slovakia. Maximum hail size reached 6 cm.
23 August 2021: Very large hail up to 8 cm in diameter was reported from southern Russia.
18 September 2021: Damaging hailstorm affected Tivat, Montenegro in the morning hours. Large hail caused significant damage to windows, cars, and crops
26 September 2021: 12 people were injured by very large hail in Vaglia, Tuscany. Significant damage was done to cars, roofs, windows, and crops.
10th edition of the ESSL Testbed is upon us and will be held in four weeks: 14–18 June, 21–25 June, 5–9 July and 12–16 July. The second and third weeks are for invited experts, or participants who have already been to the Testbed before or other courses. Like last year, the Testbed will be held online. 44 participants have registered, 12 people for the regular and 10 for the expert weeks.
Compared to 2020, there are a few new aspects. First, we have improved our WeatherData Displayer, which has a new, darker layout, and now allows a smoother switching between different regions. Moreover, a transparency option for data layers has been added. There is also more NWP data in the displayer compared to last year, including ensemble output from ICON-D2 (DWD) and C-LAEF (ZAMG). The third convection allowing model is Harmonie (KNMI). A large part of the data processing is now done in the European Weather Cloud, with thanks to ECMWF.
Besides daily forecasts of convective storms, we are going to evaluate new tools developed for forecasting and nowcasting of convective storms. This year we will concentrate on:
Extreme Forecast Index and Shift of Tails for CAPE and CAPE-shear from ECMWF
C-LAEF ensemble prediction system from ZAMG
ICON-D2 ensemble prediction system from DWD
Modified Lightning Potential Index from DWD
KONRAD3D cell-tracking nowcasting tool from DWD
STEPS-DWD radar nowcasting tool from DWD
NowcastSAT satellite nowcasting tool from DWD
We are looking forward to all the participants and interesting discussions on these new products and the hopefully interesting weather situations!
We welcome external persons to join the daily Testbed weather briefings from Tuesday through Friday during Testbed weeks starting at 11:00 CEST (0900 UTC) until approximately 12:30 CEST (1030 UTC). To join, visit this link: https://bluejeans.com/720241930
