Infrared satellite imagery over three supercells across Northern Italy, 24.07.2023 19:15 UTC
Source: Eumetsatview
Ahead of a trough over the North Sea, severe thunderstorms developed over Northern Italy. Consecutive supercells forming along the foothills of the Southern Alps on July 24, 2023, brought torrential downpours and extremely large hail to Northern Italy. A hail stone with a diameter of 19 cm was retrieved in the Veneto Region, being confirmed by the European Severe Storm Laboratory to be the largest ever retrieved hail stone in Europe.
While large parts of the Mediterranean were observing one of the most severe heat waves in the recorded history, with a ridge expanding from Northern Africa across the Mediterranean Sea, leading to the advection extremely warm air masses over Southern Europe. 850 hPa temperatures reached up to 30 °C, with the highest temperatures being observed in Sardinia and Sicilia, peaking at 48.2 °C both in Jerzu and Lotzorai on Sardinia.
500 hPa Geopotential and sea surface pressure (top), 850 hPa temperature (middle), and mixed-layer CAPE and KO index over Europe 23.07.2023 12 UTC - 25.07.2023 00 UTC, source:
wetter3
All the while, a strong trough formed over the North Sea, advecting unseasonal cold air masses into Central Europe, with 850 °C temperatures over Northern Germany dropping to 5°C. At the tip of the trough, a secondary trough developed increasing the advecting of very warm air masses in Northern Italy. Subsequently, a strong temperature gradient build up across Central Europe, with 850 hPa temperatures over the Po Valley reaching more than 25 °C.
The nearing trough provided mid-level lifting, along with a strong vertical wind shear. Further, a significant vertical wind direction shear was observed over the Po Valley, providing favorable conditions for supercell development. Atmospheric soundings ahead of the supercell formation showed abundant CAPE values with observed mixed-layer CAPE values reaching more than 1500 J/kg.
Radar Imagery across the Alps 24.07.2023 10 UTC - 25.07.2023 00 UTC, source: wetteronline
Severe convective activity was already observed early on July 24, 2023, with a supercell forming in the Southern Alps in the vicinity of the French-Italian border. During the afternoon hours, consecutive supercells formed in the Southern Alps. All the supercells were right-moving supercells drifting southeastwards into the Po Valley. Every supercell brought extreme downpours, violent convective downdrafts, and large hail. The supercell forming after 19 UTC to south of Bozen was particularly hail-active as it moved into the Po-Valley to the north of Venice. In the small town of Azzano Decimo in the Veneto Region, extreme hail was observed at around 21 UTC, with the largest retrieved hail stone reaching a diameter of 19 cm, which makes this the largest hail stone in Europe in the recorded history, confirmed by the European Severe Storm Laboratory (ESSL). This hail stone is only slightly smaller than the largest ever hail stone ever retrieved in Vivian, South Dakota on July 23, 2010, with a diameter of 20.3 cm.
Extensive giant hail damage in Tiezzo and Azzano Decimo (PD), NE Italy, on the morning of July 25th, 2023. Few hours after record-breaking 19 cm (7.5") hailstones. This is the new European record - the largest hail ever observed! Photos by @MarkoKorosecNetpic.twitter.com/4AMchQkOWl
— severe-weather.EU (@severeweatherEU) July 26, 2023
Earlier in the day, severe hail was also observed at Lake Garda, damaging many cars and houses. Overall at many sites in Northern Italy, hail sizes of more than 5 cm were reported, leading to widespread damages to cars, homes, and agriculture. This severe weather outbreak was the peak of a prolonged stretch of severe weather affecting not only Northern Italy, but also neighboring countries of the Balkans since mid-July. Recent years showed a sharp increase of severe hail activity over Northern Italy, stronger than anywhere else in Europe.
