Storm Forecast
Valid: Thu 18 Aug 2022 06:00 to Fri 19 Aug 2022 06:00 UTC
Issued: Wed 17 Aug 2022 20:12
Forecaster: PUCIK
A level 3 was issued across Corsica and central Italy mainly for large to very large hail, damaging wind gusts, excessive rainfall, and tornadoes.
A level 2 was issued in a belt from the Balearic Sea towards NE Italy and the E Adriatic Sea for similar threats, but with less likelihood.
A level 1 was issued in a belt from N Alps through Germany and W Czechia into S Sweden mainly for excessive rainfall.
A level 1 was issued across S Finland and N Russia mainly for large hail and severe wind gusts.
A level 1 was issued across SE Ukraine and S Russia mainly for large hail and severe wind gusts.
SYNOPSIS
A deep trough is forecast to move from NE Iberia towards Italy. Between the trough and a ridge stretching from S Italy into the Balkans, a strong mid to upper tropospheric flow is simulated. Abundant low-level moisture combined with steep mid-tropospheric lapse rates advected from Sahara yields very high CAPE values over the central Mediterranean. Towards the E, a shallow mid-tropospheric low remains over S Russia, Ukraine, and the Black Sea. Closer to the surface, an ill-defined frontal boundary will be situated over Germany, S E France, N Italy, and the western Mediterranean.
DISCUSSION
... Balearic Sea through Corsica, Sardegna, Italy into the Adriatic Sea ...
An outbreak of (extremely) severe thunderstorms is expected over the area!
An extreme overlap of CAPE and shear is forecast across the area, especially over the seas and the adjacent coastlines. MLCAPE > 3000 J/kg will combine with 0-6 km shear between 25 and 35 m/s and 0-3 km shear between 20 and 30 m/s. Storms forming in this environment will have a high probability to become extremely severe. They will be initiated by the warm air advection in the early morning hours, followed by further initiation along the advancing cold front and ahead of the axis of the trough. The highest coverage is forecast in the late afternoon to early night hours. All models agree on the CAPE-shear overlap and storms being present somewhere within the Lvl 2. Large discrepancies in the presence and timing of storms can be found in convection-allowing models.
Further uncertainties in the forecast are:
1. The southward extent of the morning convection, which will very likely be plentiful near the coastlines of the NW Italy. This may impact where the track of the most intense severe weather will be in the afternoon and evening.
2. Convective mode. The whole situation will unfold with a mix of supercells and large convective systems. The exact convective mode over a certain location will determine the main severe weather type. Some of the high-resolution model runs suggest intense bow-echoes capable of extremely severe wind gusts. While this is not a typical evolution over the area, it is plausible given that the lift for storms moves fairly quickly east and that the 0-3 km shear is very favorable for convective windstorms. If supercells form, a threat of 5+ cm hail will be very high. Supercells may become tornadic given strong LLS along the coastlines.
3. How effective will the forcing combined with low-level lapse rates remove the stable layer with large CIN between the moist maritime boundary layer and steep lapse rates aloft? The best agreement in CIN removal among the models is currently for the area W of the Apennines.
Lvl 3 is issued for the area, where the highest confidence in extremely severe weather exists, but any storm forming within Lvl 2 will have a high chance of producing severe weather!
... N Alps to Denmark ...
Less shear and less CAPE are forecast further north. Still, some severe storms are possible, especially concerning isolated large hail and excessive rainfall. Interestingly, a large model discrepancy exists where exactly the storms form. Thus, a very wide lvl 1 is kept over the region.
... Finland ...
Further severe storms are expected over the area. Slightly less shear may decrease the severe wind gust risk a bit compared to the previous day. Storms will be initiated along a warm front and travel parallel to it, perhaps yielding some linear segments. Given decent CAPE, large hail may also occur.
... SE Ukraine / S Russia ...
Numerous storms are forecast over the region. Compared to the region to the W, more shear will result in better storm organisation and also a higher threat of large hail and/or severe wind gusts.
