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.

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.

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.

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, 2018 Nielsen 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.

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.

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.

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.

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.

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.

Our ESWD team extracted the 10 most deadly flash flood events from the European Severe Weather Database for Spain. Here is the overview:

* The number of fatalities of this medieval event comes with a large uncertainty.

** Number of fatalities as of 2 November 2024. The official numbers are not final yet and likely to change.

The fact that nearly all events occurred in the month of October is striking. The flood event of 29 October 2024 is the deadliest in Spain in more than 60 years and the deadliest so far this year in the whole of Europe and the Mediterranean, i.e. in all regions covered by the ESWD.

We plan to post an in-depth meteorological analysis of the event early next week.

Note: This article was originally posted on 31 October 2024 at 18 UTC and updated at 20 UTC. The “1962 Vallés floods” event was added with the update. The 1962 event ranks on the third position now. We are thankful for a hint to include this important event. Another update on 2 November 2024 at 20 UTC corrected fatality numbers for events on 25 September 1962, 14+15 October 1879, 18+19 October 1973, and 29 October 2024.

Online: 12 March 2024, 13:00 – 15:00 UTC
Participation free of cost. Pre-registration required.

We present MTG-related information relevant to forecasters:

• The status of the MTG commissioning (Stephan Bojinski, EUMETSAT)
• The Forecaster Testbed 2023 in retrospect and lessons learned from expert workshops (Alois Holzer, ESSL)
• Testimonial from forecasters who participated in the Forecaster Testbed 2023
• Information on how to register for the Forecaster Testbed 2025 (Natasa Strelec Mahovic, EUMETSAT)
• Interesting cases from 2023 revisited (Tomas Pucik, ESSL)

Please register here:
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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.

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. 

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. 

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.  

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. 

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. 

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.

A new study by ESSL, Adam Mickiewicz University, NSSL, and the Freie Universität Berlin, lead-authored by ESSL researcher Francesco Battaglioli, has been published in the AMS Journal of Applied Meteorology and Climatology

In the study, entitled “Modelled Multidecadal Trend of Lightning and (Very) Large Hail in Europe and North America (1950-2021)” they reconstructed the climatology of lightning and (very) large hail in Europe and North America since 1950 using statistical models (AR-CHaMo) trained with about 24 million lightning observations and more than 44000 hail reports.

They found that, across Europe, large (≥ 2 cm) and very large (≥ 5 cm) hail is most common across northern Italy, south-western France, and eastern Spain. Hail frequency has increased across large parts of Europe. This increase is caused by rising humidity in the lowest layers of the atmosphere.

In North America, hail is most common across the U.S. Great Plains. Unlike Europe, the modelled hail frequency has only increased in certain areas, while it decreased in others. Areas with increasing hail include the High Plains of Colorado and Central Canada.

Focusing on two hail-prone areas, northern Italy and central Oklahoma, temporal changes in large hail frequency can be shown using “Hail Stripes”, much like those used to illustrate temperature changes, first by Ed Hawkins. These stripes show that, across northern Italy, very large hail is now 3 times more likely than it was in the 1950s.

Since 1950, the season in which large hail occurs has lengthened across Northern Italy, especially because of an earlier start in Spring.     

The AR-CHaMo models also have important applications in hail forecasting and are used for experimental real-time forecasts available at the website stormforecast.eu. The so-called Convective Available Potential Energy, a parameter often used by forecasters, was shown to be of limited value. Instead, the part of this potential energy released high in the storm cloud at temperatures below -10 °C was found to be a much better predictor for large hail.

This work was funded by the German Ministry of Research and Education (BMBF) within the project CHECC, part of the ClimXtreme Research Programme. Stormforecast.eu has been developed in the Project PreCAST supported by the Austrian Science Fund (FWF) and the European Centre for Medium-Range Weather Forecasts (ECMWF).