Severe weather outbreak in Slovakia and Poland on 4 – 5 October 2020

Severe weather outbreaks in autumn are quite typical over the Mediterranean but rarer over continental parts of Europe. On 4 and 5 October, an unusual synoptic-scale situation led to an outbreak of severe thunderstorms over eastern Slovakia and Poland.

During these two days, a total of 36 large hail reports (with 5 reports of hail exceeding 5 cm in diameter), 71 reports of severe wind gusts (with several instances of F1 damage), 4 reports of damaging lightning and an F1 tornado report were entered into the European Severe Weather Database (eswd.eu). The map below gives an overview of the impacts. While severe convective wind gusts are not uncommon with deep low pressure systems over this part of Europe, hail reaching 7 cm or more in diameter is exceptional for this time of year.

Severe weather reports on 4 and 5 October 2020. Data source: eswd.eu

The outbreak was caused by severe thunderstorms developing in a very warm and moist airmass that had been advected northward from the southern Mediterranean Sea (Fig 2). Mixing ratios of 10 – 12 g/kg allowed 500 – 1500 J/kg of MLCAPE to build over the area. Combined with strong vertical wind shear (exceeding 20 m/s in the 0-3 km layer), thunderstorms quickly turned severe as they organized into supercells and bow-echoes.

IR satellite imagery combined with an ECMWF forecast of specific humidity at 500 m AGL (g/kg, filled contours) and lapse rates between 850 and 500 hPa (K/km, red to purple contours). Forecast sounding and hodograph are representative of the location indicated by red cross.

More information concerning the F1 tornado over southern Slovakia can be found here. ESSL would like to thank Skywarn Polska and the Slovak Hydro-Meteorological Institute for providing severe weather reports!

Optimal use of satellite data in forecasting severe convection

In November, ESSL and EUMETSAT are introducing a new course called “Optimal use of satellite data in forecasting severe convection“. The course will concentrate on how to effectively use satellite data in nowcasting severe convection and will provide both the theoretical background on the basic dynamics of severe convective storms, as well as a satellite perspective on each discussed topic. In the afternoon, we will apply the gained knowledge in a forecasting / nowcasting exercise using the selected case studies. Click on the link above to find out more.

The course will last 4 days between 16 and 19 November and will be held online, so there is no need to travel anywhere. The early fee period has been extended till 30 September. So, if you are interested, please do not wait too long with registering here.

Severe storms impacting northern Italy and Vienna on 12 August 2019 with pronounced overshooting tops and above-anvil cirrus plumes apparent in the satellite data.

A Challenging Tornado Forecast in Slovakia

Tornado forecasting can be very challenging, especially in low CAPE – high shear environments and when lower tropospheric shear is only locally enhanced. Such was the case of an F1 tornado that hit the village of Lekárovce in eastern Slovakia on 3 October 2018. This tornado is the main subject of a paper “A Challenging Tornado Forecast in Slovakia” that has been recently published in the journal Atmosphere. The study was led by Miroslav Šinger from Comenius University and Slovak Hydrometeorological Institute in Bratislava and co-authored by Tomáš Púčik from ESSL. 

Roof damaged by the tornado and a toppled concrete cross. Author: Martin Žec.

Authors of the study attempted to reconstruct the environment leading up to the tornado based on the observational datasets and compared it with the model data that was available to forecasters in the morning hours before the tornado. One of the main aims of the paper was to show whether observational data or the higher resolution run of the local model would improve the ability to identify conditions favourable for tornadogenesis.

Tornado occurred in the early afternoon hours underneath strong west-northwesterly mid to upper tropospheric flow at the flank of a deep low-pressure system. Enhanced lower tropospheric moisture combined with mid-tropospheric lapse rates of 6.5 K/km allowed for a build-up of marginal CAPE reaching 200 – 300 J/kg.

Model forecast of 3 October 2018 12:00 UTC (a) Average lapse rate in 900–600 hPa layer (black isolines) (°C·km−1) and dew-point temperature at 2 m (color scale) (°C); (b) surface-based convective available potential energy (CAPE; color scale) (J·kg−1) and deep-layer shear (wind barbs: full barb, 5 m·s−1; half barb, 2.5 m·s−1; flag, 25 m·s−1). Country names shown in italics; tornado location highlighted by blue star.

But while the deep-layer shear was very strong, models predicted a decrease in 0-1 km shear between morning and early afternoon hours with surface wind veering from East to West. However, easterly wind direction persisted over eastern Slovakia for much longer, yielding strong lower-tropospheric shear as the storms developed in the afternoon. Study shows that using both observational datasets and the higher resolution version of the local area model would alert the forecaster to the presence of strong lower tropospheric shear over the area of interest.

Hodographs representing the environment before tornadogenesis based on observational datasets (red), model forecast available to the forecasters (black), and the experimental higher resolution run (blue). Crossed circles represent the storm-motion based on Bunker`s method, orange represents the real vector derived from the radar data. Values of low-level wind shear (LLS) and storm relative helicity (SRH) in 0–1 and 0–3 km layers for individual hodographs are shown in top-right corner.

More details can be found here: Šinger, M.; Púčik, T. A Challenging Tornado Forecast in Slovakia. Atmosphere 202011, 821.

UPDATE – ESSL events in 2020 online

ESSL has decided to carry out all its events in 2020 online. We have collected experience with organizing such events and have received mostly positive feedback. That being said, we aim to organize event on site in Wiener Neustadt in 2021 as soon as the virus situation allows it. We are looking forward to welcoming participants personally to our expanded facilities.

Tornado outbreak of 1967: How bad would it be today and could we forecast it?

The tornado outbreak of 2425 June 1967 remains the second deadliest tornado outbreak over Europe since 1950 after the tornado outbreak of 9 June 1984 over Russia. Over the course of two days, one F2 tornado, four F3 tornadoes, one F4 tornado, and one F5 tornado struck France, Belgium and Holland (Fig. 1), resulting in 15 fatalities and 234 injuries


Figure 1. Tornado tracks from 24 (red) and 25 (blue colour) June 1967.

In a study from 2018, a team led by Bogdan Antonescu looked at the details of the outbreak and at what would be the consequences if a similar tornado outbreak will occur 50 years later (i.e., 2017). This was done by transposing the seven tornado tracks from the June 1967 outbreak over the modern landscape. Due to urban growth, it is possible that tornadoes could cause even more impact than in 1967. Based on the statistics of fatality and injury rates associated with European extracted from the European Severe Weather Database, a similar tornado outbreak with the one that occurred in 1967, would result in 55–2580 injuries, and 17–172 fatalities. In the worst-case scenario, with tornado tracks moving over highly populated areas over the region,  up to 146 222 buildings could be  impacted with 2550–25 440 injuries and 170–1696 fatalities. This study clearly shows how impactful such a tornado outbreak could be to society.

A follow-up study that has just been published in Weather and Forecasting (link) hindcasts the tornado outbreak using an WRF-ARW simulation, with initial and boundary conditions provided by ERA-40 reanalysis and the highest-resolution domain with 800-m grid spacing. The model simulated an environment conducive for tornadic supercells with CAPE exceeding 2000 J/kg, 0–6 km bulk shear between 20–25 m/s and Storm Relative Helicity reaching 300 m²/s² in the 0–3-km layer. The model was also able to explicitly simulate a number of supercells over the region of interest (Fig. 2).  


Figure 2. Timestamps of radar reflectivity echo at 14, 15, 16 and 17 UTC on 24 June 1967.

One of the questions posed by the article is how would a forecast of such an event look like today? To do this, an ESTOFEX forecaster was presented with a set of forecast maps for both days (without knowing the dates) and asked to provide Day-1 outlooks. The forecaster issued Level 3 (the highest risk of severe weather) for both days, on  (Fig. 3). This paper demonstrates that, with our understanding of severe convective storms and state-of-the-art numerical modelling, a forecast of a tornado outbreak over Europe is possible. Given how much societal impact significant tornadoes can cause, addressing their threat should be part of any convective storm forecast.


Figure 3. ESTOFEX hindcast outlooks for a) 24 June 1967  and b) 25 June 1967.

Full versions of both studies can be freely accessed here:

Antonescu, B., J.G. Fairman Jr, and D. M. Schultz, 2018. What is the worst that could happen? Reexamining the 24–25 June 1967 tornado outbreak over Western Europe. Weather, Climate, and Society, 10, 323–340.

Antonescu, B., T. Púčik, and D. M.Schultz, 2019. Hindcasting the First Tornado Forecast in Europe: 25 June 1967. Weather and Forecasting, 35, 417–436.

Severe weather season 2019: summary

In the beginning of January, we published a series of posts on social media about severe weather in 2019. This blog puts together this information and graphics, and provides an overview of the most prominent severe weather events of the year. 

2019 was a comparatively active severe weather year. ESSL received 3275 reports of large hail as well as 12 027 severe wind gust reports, 792 reports of tornadoes and 3924 of heavy rain. This year has the highest number of large hail and heavy rain reports since the ESWD was established in 2006, and it was exceptional especially regarding the large hail. For example, more than 96 hail-related injuries were reported with 15 hail events, while hail injuries were reported only 96 times in the whole history of the database.  In addition, 14 events included giant hail, reaching or exceeding the size of 10 cm. Further fatalities and injuries were reported with severe wind gusts: 66 and 414, respectively. 

An overview of large hail and severe wind gust reports across Europe in 2019.

Tornadoes caused 3 fatalities and 97 injuries and were reported mostly near the coasts of the Mediterranean Sea. 531 out of 792 tornadoes occurred exclusively over water, and can thus be called waterspouts. There were 28 strong tornadoes during the year, of which 27 were rated F2 and 1 F3. Heavy rain caused 145 fatalities and 77 injuries. Most of the reports came from Central Europe and from Italy. 

An overview of tornado and heavy rain reports across Europe in 2019.

In total, severe weather killed 394 people and 1112 people were injured during 2019. Most of the fatalities were attributable to heavy rainfall, followed by avalanches and lightning. Most of the injuries were caused by severe wind gusts, followed by lightning and hail. While heavy rain comprised 37% of all fatalities, it contributed to only 7% of injuries. Tornadoes caused about 1% of fatalities and 8% of injuries. This shows how differently the impact of severe weather phenomena can be. 

Severe weather related fatalites and injures across Europe in 2019.

Below, you can find a list of 10 major severe convective storm events across Europe in 2019. Please note that this overview was selected subjectively by the ESSL staff and is not an exhaustive list of all major severe weather events that occurred in Europe in 2019. 

An overview of major convective storm events across Europe in 2019.

26 January: Tornado outbreak over Turkey

The year started with an active severe weather period over the southeastern Mediterranean. On 26 January 2019, a local outbreak of tornadoes occurred over southern Turkey. 15 tornadoes were reported in total, two of which were rated F2. One of the strong tornadoes struck Antalya airport around 8 UTC, overturning a bus with passengers that resulted in 11 injuries. Numerous videos of this event appeared online. Besides tornadoes, storms also produced hail up to 4 cm in diameter over the land, damaging crops and cars.

4 June: Convective windstorm and tornadoes over western Germany and the Benelux

On 4 June, first thunderstorms formed over northern France in the late afternoon and quickly moved northwest, forming a large convective system. A swath of severe wind gusts was reported over Benelux with the highest measured gust of 30 m/s. In addition, several tornadoes occurred including two strong, F2 tornadoes: one in the Netherlands and one in western Germany. 

10 to 11 June: Damaging hailstorms

10 June marked the beginning of an active period of severe weather over many parts of Europe. The most notable event on 10 June was a hailstorm that struck parts of Munich in the afternoon hours. Wind-driven of hail up to 6 cm in diameter, caused extensive damage to hundreds of cars and roofs. This event is set to be the most costly thunderstorm-related loss in 2019, with estimated losses of $ 800 million. 

One day later, severe weather activity continued, and while no major economic losses were reported like the day before, the storms produced even bigger hail. Giant hail was reported from two locations: From the border of Slovenia and Croatia (up to 11 cm in diameter) and across western Poland (up to 12 cm in diameter). For Poland, this marked the largest documented hailstone.  

1 July: Widespread severe weather over Europe

372 severe weather reports were collected in a belt from eastern France through Switzerland, northern Italy, southern Germany, the Czech Republic and Poland into Ukraine, making this day perhaps the most widespread severe weather outbreak of 2019 over Europe.  Severe storms produced numerous reports of large hail, up to 7 cm in diameter and severe wind gusts. Such large hail was reported both in France and the Czech Republic. The strongest wind gust speed was measured to be 35 m/s in eastern France. Five people were injured by gusts in France and six in Italy, where the severe wind also caused one fatality. 

10 July: Damaging wind gusts and giant hail over Italy and Greece

This was the last of a multi-day outbreak of severe storms over Italy and parts of the Balkans (more information on the whole outbreak can be found in a previously published blog post). On this day, severe thunderstorms had already formed over Italy during the morning with two tornado and several severe wind gusts reports. Shortly after 10 UTC, giant hail, up to 14 cm in diameter, was observed over the town of Pescara in the region Abruzzo, Italy. The hail that was combined with severe wind gusts injured 20 people. Further hail and severe wind gusts were reported across Italy later in the day. 

Injuries were reported also with storms that impacted Greece during the afternoon. However, the most severe event unfolded in the evening around 19 UTC, when a storm system that had travelled hundreds of kilometres from Italy, rapidly strengthened as it crossed the Aegean Sea and hit the Halkidiki peninsula. There, 6 people died and more than 120 were injured due to the severe wind gusts. Many of the injuries resulted from loose outdoor furniture being flung into groups of tourists seeking shelter. 

9 August: Severe wind gusts and tornadoes over western Europe

Severe storms occurred in a belt from southern France to the northern Netherlands. Severe weather began in the afternoon after 14:00 UTC with severe wind gusts and large hail over northern France. The most severe period occurred between 15 and 16:30 UTC with isolated supercells producing swaths of wind damage, including an F2 tornado that crossed from France into Luxembourg and caused 19 injuries in the town of Pétange, Luxembourg. Over eastern France, two weather stations clocked the wind gust measurements of 35.8 and 42.7 m/s. Later on, two F1 tornadoes were reported from the Netherlands between 19 and 20 UTC. One of them occurred directly in the centre of Amsterdam. 

22 August 2019: Deadly lightning strikes in Poland and Slovakia

Weak thunderstorms formed over central Slovakia during the morning hours and moved towards the High Tatras mountains on the border between Slovakia and Poland. Hundreds of hikers were caught unprepared by the storm, resulting in a total of 5 fatalities and 159 injuries. The worst lightning strike was reported from Mt. Giewont in southern Poland with 4 fatalities and 156 injuries. People were knocked down from the mountain or received burns as they held on the steel cable supporting them on the steep ascent to the mountain. Rescue efforts continued till the late night. 1 person died and 3 were injured on the Slovakian side of the Tatras. The Mt. Giewont lightning strike set the record for the highest number of directly-caused lightning injuries in the ESWD.

11 – 12 September 2019: Flash floods over Spain

Within an easterly onshore flow bringing moisture rapidly inland, a quasi-stationary convective line formed over València in the late-night hours of 11 September 2019. After hours of extremely heavy rainfall, flash flooding followed in the morning of 12 September. By 03 UTC on 12 September, 359 mm of rain fell at the Benniarés station with a maximum 6 hourly sum of 191 mm. Station Ontinyent reported 296 mm of rain by 05 UTC, with 242 mm falling since midnight. 5 people perished during the flash floods, which brought widespread disruption to the region and left several settlements completely flooded. Besides heavy rain, an F1 tornado was reported shortly after midnight on 12 September.

4 October 2019: Giant hail over Greece

While large hail is not unheard of during the autumn over the Mediterranean, flash floods are usually the dominating risk of severe storms during that time of the year. However, on 4 October, a supercell that tracked over the Attica region, Greece, produced an impressive swath of large hail. Giant hail up to 11 cm in diameter was reported from 3 different villages. Of course, cars, roofs and windows were badly damaged by this storm.  

1 December 2019: Flash floods over southern France 

December did not start well for some in southern France. After high amounts of rainfall in the past period, another round of heavy rainfall led to flash floods that severely impacted the Provence region. Weather stations reported 130 – 150 mm of rainfall falling during the day. While such amounts are not too high for the given region, previous rainfall exacerbated the situation. The flash flooding resulted in 5 fatalities and many people had to be evacuated from their homes. 

ECSS2019 Conference Photo

Taken in the poster area during today’s morning coffee break, the official ECSS2019 Conference Photo is now available for download here:

ECSS2019 Conference Photo (photographer: Thomas Schreiner, ESSL. CC license: BY-SA)

It also has been published on the ECSS webage.

Frequency rise of future European thunderstorm hazards

Researchers at the European Severe Storms Laboratory (ESSL), Munich Re, the Ludwig-Maximilians University Munich (LMU) and the German Aerospace Centre (DLR) could show that damaging convective weather events including lightning, hail and severe wind gusts are likely to become more common across Europe until the end of this century. An increase in convective instability as a result of rising humidity near the earth’s surface was identified as main reason for the increase in hazard frequency.

They applied a set of additive logistic regression models (AR-CHaMo) to an ensemble of 14 regional climate simulations to predict the frequency of severe thunderstorms and its associated hazards in Europe under projected anthropogenic climate conditions.

Simulated annual 6-hourly periods with (a), (b) and (c) hail ≥ 2cm (d), (e) and (f) wind (g), (h) and (i) with hail ≥ 5 cm. The first column (a, d and g) represent the historical period (1971-2000) while the second and third column show the percent change at the end of the century (2071-2100) for the (b, e and h) RCP4.5 and (c, f and i) RCP8.5 scenarios. Trends are called (very) robust where the change is larger than (twice) the initial standard deviation of the model ensemble. (Very) robust changes are indicated by (large) black dots in (b, c, e, f, h and i). Areas where models already diverge greatly for the historical period are displayed in gray in (a, d and g). From Rädler et al. (2019).

They projected a slight decrease in thunderstorm occurrence in southwestern and southeastern Europe, but an increase in the probability of severe weather. Large hail (≥ 2 cm) is projected to become 40%-80% more likely across central Europe in the RCP8.5 scenario by the end of the 21st century (Figure c ) while very large hail is projected to become more likely across most of Europe, with a doubling possible in parts of central and northeastern Europe (Figure i).  As a consequence of the rising hazard probabilities, risk models will need to be adapted and public warnings and precautionary measures should be issued as storms approach.

More information on this study can be found in this open access article: 
Anja T. Rädler, P.H. Groenemeijer, E. Faust, R. Sausen and T. Púčik, 2019: Frequency of severe thunderstorms across Europe expected to increase in the 21st century due to rising instability, npj Climate and Atmospheric Science, DOI:10.1038/s41612-019-0083-7

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.