Large hail impacts across Europe

A new study, resulting from cooperation of researches from ESSL and Munich-RE, called “Large hail incidence and its economic and societal impacts across Europe” has just been published in the Monthly Weather Review.

This study addresses the large hail incidence in Europe and its economic and societal impacts from multiple perspectives. Two main datasets used in the study were the European Severe Weather Database and the NatCatDATABASE of Munich-RE. While authors acknowledge that ESWD still does not allow for a homogeneous climatology of large hail across whole Europe, one can still learn many things from the data, such as the peak month of large hail occurrence in Europe (Fig. 1), hail size distribution, or how many injuries were reported with large hail.

Figure 1. Month with most frequent large hail occurrence calculated over 2° latitude x 2°
longitude grid. Figure © Copyright 2019 AMS.

Damage description that can be entered in the ESWD was used to investigate how the different types of hail damage depend on the reported hail sizes. It was found out that damage to trees, crops or greenhouses is typically reported with hail sizes of 2 – 4 cm in diameter, but damage to roofs, vehicles and windows usually occurs with hail size larger than 4 cm (Fig 2.). Furthermore, damage to vehicle windows was never reported with hail smaller than 5 cm in diameter.

Figure 2. Relative frequency of impact type as a function of maximum hail diameter.
Figure © Copyright 2019 AMS.

Authors also looked at how hail-related monetary losses have evolved since 1990 over Germany based on the data from NatCatDATABASE of Munich-RE. It was found out that while time serie of annual hail losses is dominated by outliers with very high losses (e.g. the event of 27 to 28 July 2013, which claimed over 4 billions of $ in total losses), the number of annual hail loss days shows a statistically significant rising trend (Fig. 3). Besides increasing vulnerability, the frequency of severe storm environments capable of producing large hail has increased as well, as demonstrated by ARCHaMo models Rädler et al. (2018) applied to the ERA-Interim data.

Figure 3.Relation of mean annual probability of hail, calculated for a 0.75° ERA-Interim grid box and averaged over the area of Germany with each grid point having the same weight, to the hail-related annual losses (in millions of USD) for a. large and b. very large hail and to the number of hail loss days for c. large and d. very large hail across Germany between 1990 and 2018. Figure © Copyright 2019 AMS.

More information can be found in the article: Púčik, T., C. Castellano, P. Groenemeijer, T. Kühne, A.T. Rädler, B. Antonescu, and E. Faust, 2019: Large Hail Incidence and Its Economic and Societal Impacts across Europe.Mon. Wea. Rev.,147, 3901–3916, https://doi.org/10.1175/MWR-D-19-0204.1

Severe weather outbreak of 6 – 10 July 2019

In the recent days, parts of western and southern Europe have experienced an extraordinary outbreak of severe weather. Between 6 and 10 July 2019, 480 reports were submitted to the European Severe Weather Database (www.eswd.eu). The severe weather included flash floods, large hail, tornadoes, and damaging wind gusts.

During this period, the largest reported hailstones fell on 10 July in Pescara, Italy, and were estimated at 14 cm in diameter. This almost ties the previous record hailstone size in the database, 15 cm, observed on 20 June 2016 in Timișoara, Romania.

The storms had large societal impacts with 10 fatalities (6 in Greece) and 90 injuries (48 in Greece alone, but some media mention that up to 100 people have been injured), mostly from damaging wind gusts and hail. Most fatalities and injuries were reported on 10 July in northern Greece, following a convective windstorm that hit Chalkidiki. The highest number of severe weather reports originated from Italy, where more than 150 reports were collected during the 5 subsequent days of this severe weather outbreak.

Severe weather reports from the European Severe Weather Database between 6 and 10 July 2019.

What caused such a long-lasting outbreak of severe convective storms that impacted areas like Northeastern Italy several times in a row? The reason was a stalling frontal zone over the Alpine range, which initiated storms in a very moist air-mass with high convective available potential energy (CAPE) that frequently exceeded 2000 J/kg. Such high CAPE values supported strong updrafts in forming convective storms. At the same time, strong winds in the mid and upper troposphere (between 3 and 10 km above the surface) south of the front, created strong vertical wind shear. The wind shear helped the storms to organize into supercells (storms with rotating updrafts) or intense squall lines and bow-echoes, which are known for the strong winds they can produce. The figure below shows that, high CAPE (colours) and strong vertical wind shear (wind barbs) were collocated over southern Italy and Greece on 10 July. The outbreak only ended as the frontal zone finally moved southeast, followed by drier and cooler air in its wake.

00 UTC 10 July 2019 ICON forecast of 12 UTC CAPE (colors) and 0-6 km bulk vertical wind shear (barbs and contours). A forecast sounding and hodograph are shown, valid for the location of the red cross.

What happened on the individual days of the outbreak? On 6 July 2019 severe weather was reported from France and Italy. Over France, very large hail up to 8 cm in diameter and wind damage occurred. Over Italy, the regions of Friuli-Venezia Giulia, Veneto and Piemonte experienced very large hail. The area nea Vercelli was particularly hard hit with roofs completely destroyed by hail and cars heavily damaged with windows broken out. Severe wind gusts were reported from Switzerland.

7 July 2019 featured the most extensive area of severe weather that occurred in two belts: from southeastern Slovakia to Ukraine and from northeastern Italy towards Romania. Very large hail, up to 7 cm across, was reported from Croatia and a swath of severe wind gusts extended along the Adriatic Sea coastline and from Serbia to Romania. A 32 m/s wind gust was measured in western Romania in the evening hours and 30 m/s wind gust was measured in Zadar, Croatia overnight. Furthermore, a tornado was reported from Syurte, Ukraine, which occured with a long-lived supercell that also produced very large hail and downbursts over Slovakia.

7 July 2019 15 UTC Meteosat-10 visible and infrared – BT enhanced satellite imagery combination (sandwich) with ESWD reports from the previous 3 hours. Green triangles represent large hail (number inside indicates the diameter), yellow squares severe wind gusts and red triangles tornadoes. Data source: EUMETSAT, ESSL.

On 8 July 2019, severe thunderstorms again impacted northeastern Italy with very large hail. However, arguably the most severe weather happened in Spain, where severe flash floods killed a person in the town of Morriones. Besides flash flooding, very large hail up to 8 cm, a F1 tornado and severe wind gusts were reported from the northern part of the country.

The axis of severe weather shifted more to the south on the following day, 9 July 2019. In the afternoon, isolated storms produced very large hail up to 8 cm in diameter over Italy. Later on, storms clustered into a large convective system and produced swaths of damaging wind gusts, which extended over the Adriatic Sea towards Croatia, Montenegro and Albania.


9 July 2019 17:30 UTC Meteosat-10 visible and infrared – BT enhanced satellite imagery combination (sandwich) with ESWD reports from the previous 3 hours. Green triangles represent large hail (number inside indicates the diameter), yellow squares severe wind gusts and red triangles tornadoes. Two pronounced above anvil cirrus plumes are visible atop large convective system over the Adriatic Sea. Data source: EUMETSAT, ESSL.

The most societally impactful severe weather happened on the last day of the outbreak, 10 July 2019. Day started with an F1 tornado that injured 3 in Milano Marritima, Italy. By the early afternoon, the storms moved south and intensified while producing giant hail, up to 14 cm diameter, injuring 20 people in the Abruzzo region of Italy. In the afternoon, Puglia was impacted by damaging hail and wind gusts.

Subsequently, several of the storms originating over Italy crossed the Adriatic Sea and impacted first Albania and then Greece. Numerous injuries were reported to have resulted from damaging wind gusts and hail. In the evening, the Chalkidiki peninsula was particularly hard hit with 6 fatalities. Some media sources are mentioning over 100 injuries. On the same day, 2 people were also killed by lightning in northwestern Turkey.

This outbreak was quite rare, but it is difficult to say how extraordinary it was, because the ESWD only since 2006 collects observations of large hail and local severe wind gusts, phenomena that often escape detection by traditional observation networks. Analyses of severe weather conditions are showing that severe weather conditions with high CAPE have become more frequent (read more here), and are likely to increase during the century (read more here).

For more information contact: Dr. Tomáš Púčik (tomas.pucik@essl.org)

Giant hail in Europe

Yesterday, 11 June 2019, multiple severe storms occurred over parts of Central Europe. Several of these storms were prolific hail producers and two of them even produced hailstones reaching a diameter of 10 cm or more, which we call giant hail. Such large hail was reported from Gorzów Wielkopolski-Ustronie and Wojcieszyce in western Poland with the largest stone measuring 12 cm. This makes it the biggest oficially measured hailstone in this country according to our partner Skywarn Polska.

Giant hail also occurred in Stari Trg ob Kolpi, southern Slovenia, and Brod Moravice, northern Croatia, with maximum reported hail diameter of 11 cm. This hailstorm actually tracked very close to the path of another giant hail producing storm last year that struck Crnomelj, Slovenia, with hail up to 12 cm in diameter.

With these events ocurring, one might wonder how rare such large hail actually is in Europe. Before 11 June 2019, giant hail was reported 91 times in the European Severe Weather Database (www.eswd.eu) across many different regions in Europe (see figure below) and 42 times since the founding of ESSL on 1 January 2006. Giant hail comprises on only about 0.38% of all large hail reports (minimum diameter 2 cm) submitted to the database. Such hail can cause very serious damage, injuries and occasionally be fatal to humans and animals.

Very large and giant hail reports across Europe based on the European Severe Weather Database (www.eswd.eu).

Have we observed even larger hail sizes in the past over Europe? The answer is actually yes: The largest reported hail sizes in recent years are 15 cm on 20 June 2016 in Sânandrei in western Romania and 14.1 cm on 6 August 2013 in Undingen in southwestern Germany.

A climatology of thunderstorms across Europe

A new study on the climatology of thunderstorms, “A climatology of thunderstorms across Europe from a synthesis of multiple data sources”, has been published in Journal of Climate. The study was led by Mateusz Taszarek from Adam Mickiewicz University in Poznań and co-authored by Tomáš Púčik and Pieter Groenemeijer from ESSL, among others.

Different datasets were used to investigate the climatology of (severe) thunderstorms across Europe, namely the ZEUS and EUCLID lightning detection networks, SYNOP observations, soundings, ESWD reports and the ERA-Interim data. Weaknesses and strengths of each of the datasets were discussed, as well as similarities and differences in the context of annual number of (severe) thunderstorms days and their annual cycles across various parts of Europe.

For example, the mean annual number of thunderstorm days based on lightning observations over Romania was lower compared to the ERA-Interim dataset, but higher over Hungary, southwestern Slovakia, the Czech Republic and southern Germany. Compared to SYNOP observations, lightning detection networks show higher number of thunderstorm days over most of Europe.

Mean annual number of thunderstorm days based on different datasets.
Figure © Copyright 2019 AMS.

While there were numerous differences between results obtained the individual datasets, the annual cycle was reproduced similarly by all of them. Datasets show that the thunderstorm season peak shifts from south to north from May to August over the continental parts of Europe and then shifts to the Mediterranean area in the autumn. Inland areas of Spain experience the peak in thunderstorm activity in May to June and the eastern coastal areas experience the peak in September to October.

Day of the year with peak thunderstorm activity.
Figure © Copyright 2019 AMS.

You can read more here:
Taszarek, M., J. Allen, T. Púčik, P. Groenemeijer, B. Czernecki, L. Kolendowicz, K. Lagouvardos, V. Kotroni, and W. Schulz, 2019: A Climatology of Thunderstorms across Europe from a Synthesis of Multiple Data Sources. J. Climate,32, 1813–1837, https://doi.org/10.1175/JCLI-D-18-0372.1.

Early season tornado outbreak over Turkey

While much of Europe remains under stable conditions, severe weather outbreak occurred over Antalya province, southern Turkey, between 24th and 26th January 2019. Outbreak included numerous instances of heavy rainfall resulting in flash floods, tornadoes, severe wind gusts and large hail (Fig. 1).

Fig. 1 Reports of severe weather between 24th and 26th January over Turkey based on ESWD.

On 24th January, three tornadoes affected the province, one of them rated F2, killing 1 and injuring 6 people. 2 F1 tornadoes occurred as well, injuring 1 person. Tornadoes inflicted considerable damage to homes, roofs and greenhouses.

On 25th January, flash flooding has killed 2 people in the same area.

On 26th January, a strong tornado, rated F2, struck Antalya airport, injuring 11 persons at the site. 8 passengers were injured on a transfer bus that was overturned and dragged by severe winds. 3 airport employees were injured in another shuttle. The event has gained significant attention on the social media as many videos and photographs were taken of the tornado. Tornadic storm would later produce additional F1 tornado and also instances of very large hail, damaging greenhouses.

This tornado outbreak is interesting from two aspects. The first is its occurrence in the middle of winter and out of the convective season throughout much of Europe. However, recent research on tornado climatology (Groenemeijer and Kuhne, 2014; Kahraman and Markowski, 2014) shows that January is actually the month with peak tornado activity over this part of Turkey (Fig. 2). A relatively warm sea with strong flow aloft combined to create marginal CAPE, low cloud bases and pronounced vertical wind shear in the lower troposphere (Fig. 3)

Fig. 2 Month with peak tornado activity over Europe from Groenemeijer and Kuhne (2014).

The second interesting aspect is that it shows the potentially high societal impact that tornadoes may inflict when striking vulnerable infrastructure, in this case an airport. Had the tornado been stronger and/or larger, the impact could have been much worse, with hundreds to thousands of people in danger. While tornadoes are considered rare in Europe, this is actually the second time in less than two years that a tornado got in close proximity of an airport, after the Vienna Schwechat airport incident on 10th July 2017. Tornadoes are in general an underestimated threat in Europe (Antonescu et al, 2017) and this recent case demonstrates a strong need to include tornadoes in national weather warning systems.

Fig. 3 Forecast of CAPE and bulk 0-3 km vertical wind shear for 26th January 2019, 06 UTC, based on 00 UTC run of ICON.


Deadly flash floods in 2018

In our recent blog post about very large hail events of 2018, we mentioned that hail produces large damage. However, it is rarely deadly, in contrast to flash floods. Based on the data from  the European Severe Weather Database, by 12 December, flash floods have killed 152 people across Europe, parts of northern Africa and the Middle-East.

While most of the heavy rainfall events were reported in central Europe, the most deadly flash floods occurred in the Mediterranean area, including the 5 events with the highest number of fatalities, ranging from 12 to 21.

Heavy rain and deadly flash flood reports across Europe in 2018. 5 events with the highest number of fatalities are indicated.

Who was at most risk during the flash floods? Out of 35 events with more than 1 fatality we identified 16 that involved vehicles being swept away by the floods. Because not all reports include detailed description of the fatality circumstances, the ratio of events including fatalities in cars is likely even higher. The deadliest flash flood also involved a vehicle. In a tragic event on 25 October, a flash flood swept away a bus in Jordan, killing 21 persons onboard.

Besides vehicles, several events involved a group of hikers being swept away by flash floods in the narrow canyons. The first such event occurred in Israel, on 5 May and resulted in 10 fatalities. On  1 August, 5 hikers were killed on Corsica and 10 hikers drowned in the Pollino national park in Italy on 20 August.

This shows that data from the ESWD can be used not only to identify the areas with the highest severe weather incidence, but also to compare the impacts of severe weather phenomena or to find out which groups of people are at most risk in a given severe weather type.

Upgrade of severe weather database ESWD

Based on feedback from ESWD users collected at meetings in November 2017 and the ESWD User Forum in March 2018, the European Severe Weather Database has been updated to Version 4.4.

A number of changes have been made, particularly to the submission form. The most important changes are:

1. Kyrgyzstan, TajikistanTurkmenistan and Uzbekistan were added, so it is possible to enter reports for these countries. ESSL aims to learn more about storms in Central Asia!

2. The submission form has been simplified substantially.

3. Place and time accuracy have become required fields.

An updated version of the submission form. Time and place accuracy (in red boxes) are now required fields. Drop-down menu can be accessed by clicking at “More Details”

4. Funnel clouds can not be submitted into ESWD anymore.

5. The country can now be changed in the submission form.

6. Impacts of the event can now be indicated by ticking checkboxes. Each type of severe weather has a different set of impacts that can be selected. This step streamlines the reporting of impacts into the ESWD, and makes it easier to compile statistics of severe weather impacts of the storms across Europe.

An example of impact choices for large hail event type.

You can try out the new version by accessign the ESWD website. We welcome your feedback at eswd@essl.org!

Major hailstorms of 2018 across Europe

Severe hailstorms can cause extensive economic damage to both crops and properties. Report by Munich RE shows that damage of individual events can exceed billions of dollars as the risk has increased in the past decades. Hail damage to properties, such as cars, roofs and windows, becomes substantial when the diameter of hailstones approaches and exceeds 5 cm. Each year there are a number of such events when hail of this size occurs across Europe and this year, 2018, was no exception.

By looking at the ESWD dataset, we found 26 days, when hail exceeded 5 cm and caused significant damage to the properties. The spatial distributions of these major hail events and associated hail sizes can be found in in Fig. 1. The largest hail size was observed on 8 June 2018, when the town of Črnomelj, Slovenia was hit by giant hail up to 12 cm in diameter, destroying hundreds of roofs and cars. The hail swath on that day (40 km) was much shorter compared to long-lived hailstorms of 1 May in Poland (220 km) or the hailstorm of 30 June in southern Russia (200 km).

Fig. 1. Severe hail reports associated with 26 damaging hailstorm events over Europe in 2018. Size and colour of the symbol represent the observed hail size.

These damaging hailstorms occurred in an environment of moderate to high CAPE and bulk 0–6 km wind shear exceeding 15 m/s, conditions which favour strong updrafts and well-organized convection, including supercells. Our research shows that such conditions may become more frequent in the future. As we compare with the sounding database developed by Pucik et al (2015), most of these events occurred in typical conditions for damaging hailstorms (Fig. 2). Several of these events formed in rather weak vertical wind shear (i.e., bulk 0-6 km shear ranging from 10 to 15 m/s) but these cases were confined to the proximity of complex orography, where shear could be strongly enhanced locally.

Fig. 2. Environments of damaging hailstorms over Europe in 2018 (dots) compared to the relative frequency of hail > 5 cm (colour bar) as a function of CAPE and 0-6 km bulk shear based on Pucik et al (2015). Colour of dots represents the maximum observed hail diameter with each event.

Description of individual events

01.05.2018 (Poland). An isolated supercell cut a 220 km long hailswath across the regions of Mazowieckie and Podlaskie in northeastern Poland with hail up to 5 cm in diameter.

02.05.2018 (Slovenia). Very large hail was recorded in the districts of Lendava, Murska Sobota, Gornja Radgona and Ljutomer in northeastern Slovenia, with hailstones up to 6 cm in diameter. Hail damaged cars and roofs.

15.05.2018 (Bulgaria). Very large hail hit several villages and towns within a 120 km long path tracking through the districts of Vraza and Pleven in northwestern Bulgaria with hailstones up to 7 cm in diameter. Extensive damage to cars and trees was reported from the town of Pleven.

24.05.2018 (Spain). Hailstorm hit the town of Garciaz in Extremadura province, southwestern Spain, with hailstones up to 7 cm in diameter. Cars, roofs and fruit plantations were damaged by the storm.

26.05.2018 (France and Italy). Violent thunderstorms hit the regions of Aquitaine and Poitou-Charentes in southwestern France with a 100 km long hailswath originating near Bordeaux and devastating vineyards in the area. Hail up to 6 cm in diameter was reported. On the same day, very large hail, up to 7 cm in diameter was also reported from Piemonte province, northwestern Italy.  

27.05.2018 (Turkey). Severe hailstorms hit Samsun province, northern Turkey. Six people were injured in Muratbeyli village as 6 cm large hailstones smashed car rear windows and roof tiles.

04.06.2018 (Italy). A right-moving supercell brought violent hailstorm to the town of Noceto in Emilia-Romagna province, northern Italy. Very large hail up to 8 cm in diameter damaged cars.

08.06.2018 (Slovenia and Croatia). Two very severe hailstorms occurred in Slovenia and Croatia. The town of Črnomelj in southern Slovenia was particularly hit as supercell produced hail up to 12 cm in diameter. Hundreds of cars, roofs and solar panels were seriously damaged. Another supercell hit the districts of Karlovačka, Zagrebačka and Krapinsko-Zagorska in central and northern Croatia. Hail up to 10 cm in diameter hit Grabovec town, damaging cars.

11.6.2018 (Germany and Czech Republic). Violent, wind-driven hailstorm from supercell damaged roofs, facades and windows in  Furth im Wald town in Bayern state, southeastern Germany, and along the Czech-German border. Hail up to 6 cm in diameter was observed.

12.6.2018 (Ukraine). Supercells produced very large hail over Uzhhorod town and Irshava district in Zakarpatska province, southwestern Ukraine. Car rear windows and roof tiles were smashed by hailstones up to 6cm in diameter. On the same day, a violent hailstorm hit villages in the northern parts of Bihor County in western Romania. Hodoš village was worst hit by hailstones up to 6cm in diameter, destroying roof tiles.

13.6.2018 (Serbia). Supercell produced wind-driven hail with diameter up to 6 cm over central and east Serbia. Extensive damage to crops, windows, roofs, cars and facades of houses was observed.

26.06.2018 (Turkey). Violent hailstorm with hail up to 6 cm in diameter caused damage in Düzce province, northwestern Turkey.

28.06.2018 (Ukraine). A violent hailstorm hit areas in Mykolayivska province, southern Ukraine. Hailstones up to 6 cm in diameter caused extensive damage in Yuzhnoukrainsk town.

30.6.2018 (Russia). An isolated, long-lived supercell produced more than 200 km long hail swath with hail up to 8 cm in diameter in the districts of Krasnoarmeyskiy, Timashevsk, Bryukhovetskaya and Pavlovskaya in Krasnodar Region, southern Russia. Novokorsunskaya town was worst hit by very large hail which caused extensive damage to crops, cars, car windshields, roofs and windows. Additionally, very large hail up to 6 cm in diameter also caused extensive damage to cars and houses in Pavlo-Ochakovo area, Rostov province.

01.07.2018 (Russia). A violent hailstorm hit Stavropol region, southern Russia. Hail up to 7 cm in diameter hit the local areas of Bekeshevskaya town.

04.07.2018 (France). Several hailstorms produced very large hail up to 8 cm in diameter over France. 11 people were injured in the Poitou-Charentes region village of Saint-Sornin, where 7 cm hail produced extensive damage to roofs. Houses in villages south of La Rochefoucauld town were badly damaged and rendered uninhabitable by water damage following the very large hail. In Aquitaine region, southwestern France, very large hail up to 6 cm in diameter caused significant damage in some places east of Pau city.

14.07.2018 (Italy). A severe hailstorm hit Piemonte province, northwestern Italy. The town of Chivasso was hit by giant hail up to 10 cm in diameter. Cars and roofs were damaged during the hailstorm.

16.07.2018 (Italy). Several instances of very large hail, up to 8 cm, were reported from Marche province, eastern Italy. Hail caused extensive damage to cars in the town of Pesaro and surrounding villages.

21.07.2018 (Bosnia and Herzegovina). Very large hail up to 8 cm reported from Srpska territory, northern Bosnia and Herzegovina. 1 person was injured by 6 cm large hail in Prijakovci village, north of the city of Banja Luka. Roofs and cars were damaged by the storms in several villages.

24.07.2018 (Turkey). A strong hailstorm hit parts of Kastamonu province in northern Turkey causing extensive damage to cars and houses in Kuzyaka and Seyh villages by hailstones of at least 5 cm in diameter.

05.08.2018 (Czech Republic). Very large hail up to 6 cm was reported from villages in Zlín and Olomouc regions, eastern Czech Republic. Cars and roofs damaged by the hail.

07.08.2018 (France). Very large hail up to 6 cm (weighing 150 g) was reported in Basse-Normandie region, northern France. Roofs, cars and vegetation were damaged.

09.08.2018 (France). Very large hail up to 7 cm fell over Marseille city and Aubagne town, southern France.

28.08.2018 (Spain). Very large hail up to 7 cm in diameter was observed in Euskadi region, northern Spain.

02.09.2018 (Italy). Very large hail up to 8 cm was reported from Abruzzo province, eastern Italy with extensive damage to cars and roofs.

05.09.2018 (Spain). A severe hailstorm hit Albalate del Arzobispo town in Aragón province, northeastern Spain with giant hail up to 11 cm in diameter causing damage to roofs and cars.

13.09.2018 (Turkey). Severe hailstorm with hail up to 6 cm in diameter hit Kastamonu city in northern Turkey. Extensive damage was inflicted to cars and houses.

Flash floods in Austria and Slovenia

The convective season is beginning over Europe and the severe thunderstorms of yesterday, 16th April, are a good example of that. We have received a number of severe weather reports from Slovenia and southeastern Austria, where flash floods and large hail up to 3 cm in diameter occurred in the late afternoon (Fig. 1). The event that received the most attention was a flash flood in Graz, with rainfall totals of up 80 mm. A number of streets, as well as cellars, parking lots and even the shopping mall, were flooded.

Fig. 1 Severe weather reports collected for 14 April 2018 across Slovenia and southeastern Austria. Blue circles denote heavy rainfall reports and green triangles large hail reports.

Anticipating situations conducive to flash flooding may be quite tricky, as it often takes a quasi-stationary, long-lived thunderstorm to deliver several waves of heavy rainfall over an area. Yesterday was no exception. Thunderstorms formed in an environment featuring moist, southeasterly flow at the surface, CAPE values on the order of several hundred J/kg and an increasing southerly flow aloft (Fig. 2). A very moist profile in low to mid troposphere along with low cloud bases was evident, both from the forecast sounding over southeastern Austria and from the observed sounding in Zagreb, Croatia (Fig. 3). These conditions suggested a potential for very heavy rainfall with thunderstorms as little precipitation would evaporate when falling from the cloud.

Fig. 2 A 15 UTC forecast ICON-EU forecast of 10 m wind (arrows) and 2 m dew-point (colours), initialized at 00 UTC. Forecast sounding was taken at the location marked by red star.

Fig. 3 12 UTC sounding taken at Zagreb. Courtesy of University of Wyoming.

Even so, the increasingly southerly flow aloft meant that thunderstorms could hardly stay confined to one location and would move northward with time. Yet, that has not happened, as one can see from this radar animation shared by ZAMG, which reveals a number of instances of storms stagnating over one particular location. This is because storms were propagating (propagation represents the movement of the thunderstorm caused by the formation of new cells) along the convergence zone towards the moister and more unstable airmass in the south. With a mid- to upper-tropospheric flow almost parallel to the convergence zone and the propagation cancelling out the advection of individual cells to the north by mean wind, some of the thunderstorm clusters became quasi-stationary and produced excessive rainfall.

Fig. 4 16:00 UTC satellite (combination of visible and infrared channels) and radar (OPERA composite) imagery. 2m temperature (red numbers), 2 m dewpoint (green numbers) and 10 m wind (barbs) are plotted for individual stations. Thick black line denotes the approximate location of convergence zone. OPERA radar data © EUMETNET, satellite data © EUMETSAT.

This case shows how demanding it can be to correctly anticipate flash flood situations with thunderstorms when numerical models underestimate the rainfall sums, which is often the case with convective phenomena.

Convective windstorms in 2017 Episode 3: 11 August

The third most socially impactful convective windstorm case in 2017 occurred on 11 August, in a belt from the northern Czech Republic to northern Poland with 6 fatalities and many injuries.

Situation began with a quasi-linear convective system over Austria and the Czech Republic, which formed around 10 UTC, paralleling strong prevailing flow in low to mid troposphere. However, at this point, only marginally large hail and heavy rain occurred as the system remained elevated. System progressed northward towards the borders of the Czech Republic and Poland, encountering progressively warmer airmass near the surface. As soon as the system became surface based around 15 UTC, it began producing severe wind gusts (Fig. 1). Widespread forest blow downs were noticed already on the Czech side of the border. As the system moved towards north-northeast, it encountered increasingly favourable conditions for severe convection, strengthened and eventually transformed into a large bow-echo at 18 UTC. Just before this transition, an embedded supercell formed within the system, as evidenced by high reflectivities near the apex of the system (Fig. 2). Besides severe wind gusts, large hail up to 5.5 cm was observed at this point. Transition into the bow-echo was accompanied by a rapid increase in the severe wind damage reports. As the system moved offshore to the Baltic Sea after 22 UTC, it produced a 42 m/s wind gust at Milejewo near the coastline. A continuous swath of wind damage was noted from the northern Czech Republic all the way towards the Baltic Sea coastline in a 7 hour long rampage. A spectacular shelf cloud accompanied the passage of the bow-echo (Fig. 3)

Fig. 1 Chronological progression of severe wind reports in a convective windstorm of 11 August 2017 across northern Czech Republic and Poland.

Fig. 2 Composite image of maximum reflectivity in vertical column at the hourly time steps between 15 and 23 UTC. Courtesy of Mateusz Taszarek and IMGW.

Fig. 3 Shelf cloud observed on the leading edge of the bow-echo near the town of Krotoszyn at 18:15 UTC. Photo by Mateusz Taszarek

At the height of the storm, 180 000 customers were out of power and many roads were blocked by fallen trees. Damage to the forestry was widespread (Fig. 4) with 39 200 ha of forests completely destroyed and 40 500 ha partially damaged. The volume of fallen wood reached almost 10 millions of m³. Together with 20 000 damaged buildings and financial costs of the storm estimated in the range of 500 millions to 1 billion €, this was likely the most impactful convective storm to hit Poland in decades.

Fig. 4 Aftermath of the 11 August 2017 convective windstorm in Poland. Photo by Grzegorz Zawiślak.

This convective storm was also record breaking for the ESWD. Altogether, more than 1200 severe weather reports were collected, which constitutes most reports ever submitted per event in the history of the database.

Environment capable of such extreme convective windstorm featured a deep cyclone at 500 hPa centered over the Alpine range with a belt of 20 + m/s southerly flow stretching from Croatia towards western Poland (Fig. 5). A short-wave trough was translating from Austria northwards. At the same time, a wavy frontal boundary extended from northwestern Austria through the Czech Republic into eastern Germany and western Poland (Fig. 6). In this setup, a warm and humid airmass has advected over Poland, with 2 m dew points exceeding 20 °C. Combination of high values of CAPE and a strong lower tropospheric shear, exceeding 20 m/s in the 0-3 km layer, created very favourable conditions for development of a bow echo (Fig. 7). Lift provided by cold pool from the already ongoing convective system and a well defined convergence zone located across western Poland resulted in widespread initiation of new convective cells, which quickly merged into a large convective system.

Fig. 5 ECMWF forecast of 500 hPa geopotential height (black contours), temperature (colors) and wind (barbs) for 11 August 12 UTC, initialized at 00 UTC.

Fig. 6 ECMWF forecast of 850 hPa geopotential height (black contours), temperature (colors) and wind (barbs) for 11 August 12 UTC, initialized at 00 UTC.

Fig. 7 COSMO-DE forecast of CAPE (colors, J/kg) and 0-3 km bulk vertical wind shear (barbs and contours, m/s) for 11 August 15 UTC, initialized at 00 UTC. Forecast sounding and hodograph correspond to the location marked by a blue star.

ESSL would like to thank Skywarn Polska and Amateur Meteorological Society of the Czech Republic for many submitted reports documenting the windstorm case. Furthermore, thank you goes to Artur Surowiecki for information on the impacts of the storm, Mateusz Taszarek and IMGW for the radar data, and Grzegorz Zawiślak for the photo of windstorm damage.

Additional information regarding the windstorm impacts in the Czech Republic can be found here (in Czech) and regarding the forest damage in Poland here (in Polish).