Storm Forecast
Valid: Sun 17 Jul 2011 06:00 to Mon 18 Jul 2011 06:00 UTC
Issued: Sun 17 Jul 2011 05:43
Forecaster: SCHLENCZEK
A level 2 was issued for N Italy for widespread large / isolated very large hail, severe wind gusts, excessive precipitation, and tornadoes.
A level 1 surrounding the level 2 was issued for large hail, severe wind gusts, excessive precipitation and tornadoes.
A level 2 was issued for the Balkan Mountains and S Romania for widespread large / isolated very large hail and severe wind gusts.
A level 2 was issued for N-central Ukraine and W Russia for widespread large / isolated very large hail and severe wind gusts.
A level 1 was issued for most parts of eastern Europe for large hail and severe wind gusts.
SYNOPSIS
A strong low pressure system over the British Isles will move very slowly towards Scandinavia. At the southern tip of this system, a 45 m/s upper jet streak stretches from S France towards N Italy. The cold front of this low stretches from N Spain via SE France and extreme N Italy towards E Germany and especially in N Italy, some instability is predicted in place with strong to very strong vertical shear.
Further east, moderate to high instability is found in a tongue of very warm and unstable air which stretches from Bulgaria towards W Russia. Even though vertical shear is rather weak, isolated severe hail is possible.
DISCUSSION
...N Italy, Slovenia...
In the vicinity of the upper jet's left exit region, cyclogenesis is predicted over N Italy. Although the resulting surface low is rather weak, easterly winds at the eastern side of this low lead to strongly enhanced directional and speed shear. Given 0-3 km shear around 15 m/s and 0-6 km shear around 20 - 25 m/s in place with some hundred J/kg nearly uncapped CAPE, well-organised convective systems may develop. SRH3 in order of 200 - 400 mē/sē is favourable for supercells which may produce large hail, severe wind gusts and also an isolated tornado. The QPF signals in GFS indicate that excessive convective rainfall is also expected over a broad region. A level 2 was issued for the area where the potential for widespread severe / isolated extremely severe events is highest. The surrounding area is likely to get isolated severe weather. Peak activity should be in the afternoon / evening hours when the left exit is directly over the region of interest.
Further upstream towards the Balearic Islands, the airmass is strongly capped and the number of severe events is probably below the level 1 threshold. Nevertheless, an isolated hail / wind gust event is not ruled out.
...Balkan Mountains, Romania, W / C Ukraine, W Russia...
Within an area of moderate instability, some hot-spots with CAPE around 2 kJ/kg / ICAPE in order of 3 MJ/mē are found. Current thinking is that some storms which will develop in this environment are likely to produce large hail or even very large hail and / or severe wind gusts despite 10 m/s or less of deep layer shear. The regions with the highest probability of hail / wind gust events are included in a level 2. Some of the 00 UTC soundings from W Russia already show CAPE in excess of 2 kJ/kg with a capping inversion at the top of the boundary layer. To the west, instability is a bit reduced but deep layer shear reaches 15 m/s in some places.
...France and British Isles...
Showers and thunderstorms are likely to occur in the vicinity of the upper cold core. Rather weak vertical shear and low instability should preclude severe weather, but as often seen in such cases, an isolated funnel / brief tornado cannot be ruled out.
