This was a classical supercellular tornado. This type of tornado forms in a specific type of supercellular thunderstorm, which has the peculiarity of having a vortex of rising air inside — called a mesocyclone, and this is where tornadogenesis starts. Rainfall in the thunderstorm produces a downdraft, called rear-flank downdraft (RFD) in this case, which enters the mesocyclone from the back. The combined updraft (from the mesocyclone) and downdraft (from the RFD) create a tornado.
As tornadoes are much more associated to the U.S., this risk is often underplayed in Europe and simply described as a rare phenomenon. But, are tornadoes so rare in Europe?
Tornado Occurrences in Europe
While the Great Plains of the U.S., a vast region stretching some 3,200 kilometers (2,000 miles) north-south across the U.S. and 800 kilometers (500 miles) east to west, bordered by the Rocky Mountains, are most well-known for their devastating tornado outbreaks, Europe also sees a significant number of tornadoes every year. The European Severe Weather Database (ESWD) aims to collect observations and reports of severe weather events, such as hail, severe wind, snowfall, etc. into a unified single database. The database also reports tornadoes and waterspouts (a specific type of tornado which does not make landfall) across Europe. So, what can we learn from the ESWD?
In 2017, the ESWD reported 209 tornadoes or waterspouts in Europe*. This number is surprisingly high, as newspapers will only report a few of them. Some of these tornadoes are triggered by cold fronts within extra-tropical cyclones (ETC), while others are formed in supercells, like the Caserta tornado. In order to model tornadoes in the new upcoming RMS® Europe Severe Convective Storm (SCS) High Definition Models, RMS filtered out waterspouts, as they are not touching land — and ETC tornadoes, as their losses will be represented within the overall ETC loss. This provides a new set of observations with only SCS-related tornadoes.
Even though some of the ESWD reports go back to Ancient Rome, only recent observations can be used for a more complete analysis. Between 2010 and 2016, we calculated that on average 108 SCS-related tornadoes were observed each year within the RMS model domain, with a maximum of 170 SCS-related tornadoes in 2017, against only 78 in 2011. This large number of tornado reports greatly contrasts with the experience Europeans have with tornadoes and shows that tornado risk is clearly underestimated in Europe. This can be explained by the very localized and weak nature of European tornadoes compared to their North American counterparts.
We have seen that European tornadoes occur more frequently than we think. The Caserta tornado is one recent example, but will probably be soon forgotten by everyone, except for the people in Caserta itself. But have we had more severe tornadoes in the recent years? I would like to come back to several historical tornadoes (both in the recent and distant past), which might have been forgotten, but are worth mentioning.
August 8, 2015: EF4 tornado in Mira, Italy
The last major European tornado occurred in Italy on August 8, 2015, in Mira, close to Venice. As in the case of Caserta, this tornado formed within the mesocyclone of a supercellular storm. Large hailstones up to five centimeters in diameter were also observed. One person died and 72 were injured by this event. In addition, about 250 houses were damaged. Watch some footage and pictures of the tornado here.
June 24, 1967: EF5 tornado in Palluel, France
The last EF5 tornado reported in Europe* goes back to 1967, in Palluel (Pas-de-Calais), northern France. This tornado was part of a larger outbreak, which caused 15 deaths in total (six killed by this EF5 tornado).
November 23, 1981: Largest European tornado outbreak, U.K.
The largest tornado outbreak in Europe occurred in November 23, 1981, though this event was not related to an SCS event. On this day, a cold front moved across the U.K. and produced a significant number of tornadoes, and at the time, a campaign estimated that there were 104 tornadoes. However, in 2016, Apsley et al. showed that there were some duplicates in the observations and that a revised number of 90 reports was plausible.
September 10, 1896: EF2 tornado in Paris, France
In 1896, an EF2 tornado hit the very center of Paris, starting in the Jardin du Luxembourg and continuing north-eastwards for six kilometers, causing severe damage to buildings and killing five people. This tornado was well studied due to its impact on the capital city. Read more about this tornado here (in French).
October 17, 1091: EF4 tornado in London, U.K.
Paris was not the only capital city hit by a tornado. For this event, we need to go back to 1091. On October 17, 1091, a tornado, with a strength corresponding to EF4, hit London and destroyed 600 (mostly wooden) houses. It also damaged London Bridge and the church of St Mary-le-Bow, which should be well-known to everyone working in the City. The event is known to have caused two fatalities. Imagine the damage if such tornado would happen again in London nowadays.
Fujita and Enhanced Fujita Scales
The Fujita scale was introduced in the 1970s by Tetsuya Theodore Fujita, a Japanese-American researcher, as an intensity scale for tornadoes. Because of their extreme wind speeds and narrow footprints, it is challenging to measure the wind gust speeds of tornadoes. The scale is therefore based on damages and windspeeds, and are derived from an interpolation between the Beaufort scale and the Mach number scale. In the 2000s, the Enhanced Fujita scale replaced the Fujita Scale, to align wind speeds more closely to the observed damages caused by tornadoes.
Both scales rate tornadoes in six categories, from 0 to 5:
EF0: No or minor damage
EF1: Moderate damage (damage to roofs, windows, mobile homes)
EF2: Considerable damage (severe damage to roofs, home foundations, vehicles, treefall)
EF3: Severe damage (destruction of entire stories, severe damage to large buildings)
EF4: Devastating damage (destruction of houses, vehicles blown away)
EF5: Incredible damage (total loss)
Modeling Severe Weather Risks
The RMS® Europe Severe Convective Storm HD Models will provide a pan-European risk management tool, serving multiple use cases from underwriting to portfolio management and capital adequacy. The models cover the full spectrum of events, from localized tornadoes and hailstorms to large derechos, including a consistent stochastic event set for 17 countries and giving users insights on sub-peril correlation between hail, straight-line wind and tornado risk.
The development is based on latest scientific research and uses a large range of datasets, to best capture the multiple aspects of this key European peril. The models complement the suite of European climate peril models, providing users a holistic view of risk across the domain.
* European domain of the new RMS Europe Severe Convective Storm High Definition Models
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Michèle joined RMS in 2013, and is based at the Moody's RMS Zurich office as part of the Global Climate Product Management team, focusing on European climatic hazard models. As part of her role, Michèle is the senior product manager for the new Moody's RMS Europe Windstorm HD Models and the Moody's RMS Europe Severe Convective Storm HD Models.
She holds a master’s degree in Atmospheric and Climate Science from ETH Zurich.