Storm Forecast
Valid: Fri 31 Aug 2012 06:00 to Sat 01 Sep 2012 06:00 UTC
Issued: Thu 30 Aug 2012 22:15
Forecaster: TUSCHY
A level 2 was issued for NE Algeria and N-Tunisia mainly for excessive rainfall amounts.
A level 2 was issued for parts of Italy, Slovenia and extreme SW Croatia mainly for large hail, severe wind gusts, heavy rain and an isolated tornado risk ( the latter risk maximized along the N-Adriatic Sea and over Slovenia).
A level 2 was issued along the S-ward facing slopes of the NE Italian Alps and far S-Austria mainly for excessive rainfall amounts.
A level 1 surrounds all level 2 areas and runs from SE-Spain to N-Tunisia and to N-Italy mainly for large hail, strong to severe wind gusts, heavy rain and an isolated tornado event.
SYNOPSIS
A large cut-off low gradually moves to the south and enters the far NW Mediterranean during the night. At the surface, a diffuse lee cyclogenesis is forecast to evolve over NW Italy. Ridging over SE/E-Europe keeps the weather stable.
An extensive cold front rapidly crosses the W-Mediterranean during the forecast with a marked slow-down along the Alps.
DISCUSSION
... NE Italy and W-Slovenia ...
A strong cut-off drips down from the westerlies with a progressive soutbound movement. Induced pressure fall in the lower troposphere creates lowering heights at low/mid-levels over NW-Italy. This pattern assists in a slow down of a SE-ward moving cold front over NE-Italy, as a southerly LLJ strengthens atop the N-Adriatic Sea during the day.
The area of most concern will be NE Italy, N-Adriatic Sea and W-Slovenia, where the SE ward moving frontal boundary, strong background forcing and strengthening LLJ merge. An extensive cluster of storms is forecast with a constant motion to the east. With 20-30 m/s DLS and augmented SRH-3, kinematics would favor well organized multicells/supercells, but strength of forcing and weakly capped air mass probably result in a large mess of clustered showers/thunderstorms with heavy rain. Nevertheless, either tail-end storms or prefrontal, more discrete storms would very well be organized with large hail, severe wind gusts and an isolated tornado risk. One area with enhanced tornado probabilities may arise along the N-coast of the Adriatic Sea, where LL CAPE is maximized.
We also added parts of N-C Italy into a level 2, as 1 kJ/kg MLCAPE and 25 m/s DLS overlap. Supercells may produce all kind of severe before the cold front exits the area to the SE during the evening hours.
Excessive rain may produce flash flooding especially in areas over far S-Austria (history of regionally record breaking summer rainfall) and W-Slovenia (burned areas due to summertime forest fires).
... Rest of Slovenia, extreme S-Austria and parts of Croatia ...
Despite large-scale subsidence, eastward moving surface cold front assists in scattered CI during the daytime hours. There exist two scenarios for CI:
I) Incoming cold front from the west with already active DMC intensifies along its way to the east. Main concern would be the tail-end activity, which would likely reveal supercellular structures with large hail, severe wind gusts and an isolated tornado risk. With 20 m/s DLS, eastward racing MCS would also pose a concentrated risk of severe wind gusts further to the north and also an heavy rain risk over extreme S-Austria.
II) Ahead of the surface cold front, discrete storms manage to evolve in a favorable environment for supercell development due to weak background forcing and prefrontal diabatic heating. Those cells would be able to produce large hail, severe wind gusts and an isolated tornado event. Actual cold front would still be able to produce a swath of strong to severe wind gusts as second and final severe event moves in from the west. Again, excessive rain over S-Austria would be able to produce flash flooding.
... Mediterranean ...
Southbound moving cold front reaches the coast of N-Algeria/Tunisia during the day and results in scattered to widespread CI. DLS upt o 20 m/s and moderate mid-level lapse rates assists in strong to severe thunderstorms with large hail, strong to severe wind gusts and heavy rain. A similar set-up is present over most parts of Italy. We highlighted the complete cold front in a level 1, as CI won't be widespread and either diabatic heating or orography will steer CI. A level 2 was added over NE-Algeria and N-Tunisia mainly due to an excessive rainfall risk with clustering storms.
The level 1 was expanded towards SE-Spain, where a maritime BL air mass is placed ahead of the cold front, supportive for isolated to scattered CI. Same risks are forecast compared to aforementioned areas.
... W-Germany, Benelux and E-France ...
Placed beneath the cold-core low, daytime driven thunderstorms with isolated large hail, strong wind gusts and heavy rain are expected. I won't exclude an isolated tornado event due to strongly veered profiles in the BL next to low LCLs. Also augmented LL CAPE over Benelux/far NW-Germany may support an isolated tornado event. Thunderstorms weaken after sunset.
