Storm Forecast
Valid: Fri 12 Oct 2012 06:00 to Sat 13 Oct 2012 06:00 UTC
Issued: Thu 11 Oct 2012 21:18
Forecaster: TUSCHY

A level 1 was issued for parts of the W/C Mediterranean mainly for heavy rain, large hail, strong to severe wind gusts and an isolated tornado event.

A level 1 was issued for far NW France mainly for strong to isolated severe wind gusts, isolated large hail and an isolated tornado event.

SYNOPSIS

A very complex mid/upper tropospheric streamline pattern affects Europe. Two cold-core vortices are centered over NW/NE Europe with weakening ridging in-between. Both vortices approach each other and try to merge although complex interaction first begins after the end of the forecast period. 500 hPa heights atop the Mediterranean decrease from NW to SE and result in increasingly favorable conditions for DMC.

At lower levels, a slowly consolidating warm front over the Balkan States and a cold front over the W-Mediterranean will be the main surface features. We do not want to exclude a sporadic lightning event over NW Switzerland/SW Germany/SE France with a weakening cold front but limited coverage precludes any lightning area.

DISCUSSION

... W/C Mediterranean ...

Tough forecast ahead with diffuse forcing mechanism in place. No pronounced vort max., embedded in the brisk W-erly flow and an ill-defined LL vortex over Italy keep any confidence level for CI low regarding location of highest thunderstorm coverage.

A diffuse LL depression over C Italy stretches an extensive warm sector over the Mediterranean which shrinks from the NW during the night as a cold front accelerates to the SE. This configuration assists in the following CAPE dispersal: South of a line Sardinia - S Adriatic Sea, MLCAPE gradually exceeds 1000 J/kg with local peaks aoa 2000 J/kg around and south of Sicily. Also offshore areas (especially those along mesoscale convergence zones and where warmer SSTs exist) will see higher LLCAPE values.

Shear will be variable, bound to traversing vort maxima but in general keeps its magntiude of 10-15 m/s with local peaks of 20 m/s. Both 3 and 6 km bulk shear reflect similar shear magnitudes with locally enhanced SRH3 values.

Highest severe concern probably exists over the S-Tyrrhenian Sea, Sicily, S-Italy, the S-Adriatic Sea and parts of the Ionian Sea, where impressive CAPE and modest shear support well organized multicells and isolated supercells. The main risk will be large to very large hail, strong to severe wind gusts and heavy rain. An isolated tornado event seems possible. However, models agree in limited CI given weak forcing, so only a broad level 1 area was added.

Higher thunderstorm coverage exists to the north, e.g. between Corsica and the C-Adriatic Sea, but weak shear and limited CAPE lowers the overall severe risk. Nevertheless, numerous clusters of showers and thunderstorms with heavy rain affect those areas and an isolated tornado/waterspout risk with weak background flow and augmented offshore CAPE exists. Only coastal areas were added in a level 1 mainly due to an isolated tornado risk and heavy rain with onshore moving storms. I'm a bit concerned about isolated training storms along the east coast of the C-Adriatic Sea with favorable high LL moisture content/modest influx from the S and temporarily enhanced forcing. Overall conditions only favor confined time frames for training storm activity and hence no upgrade was performed with that outlook.

Further west, at and south of the Balearic Islands, the southbound moving cold front will be the focus for enhanced severe thunderstorm activity. Models disagree somewhat in how progressive the cold front will be with an overall slow down seen in most models south of the Balearic Islands. Repeatedly developing showers and thunderstorms along that cold front probably affect the same area for a prolonged period of time with heavy rain being the main risk. 20 m/s DLS atop the cold front and some veering with the front itself may assist in a few well organized multicells with strong to isolated severe wind gusts and an isolated tornado event. The overall risk gradually moves south and affects N-Algeria and N-Tunisia during the late afternoon and night hours. Large hail, strong wind gusts and heavy rain will be the main hazard.

... S-UK, the English Channel and NW-France ...

A pronounced vort max affects the area during the overnight hours with a rapid cool-down at low to mid-levels. Hence, moderate SBCAPE evolves over the offshore areas with cold EL temperatures and 20 m/s 850 hPa winds forecast, so DMC is likely with temporarily better organized updrafts. Strong to isolated severe wind gusts, an isolated large hail and tornado event are all possible. A confined level area was added for the region with the best overlap of shear/CAPE/forcing.