Storm Forecast
Valid: Fri 05 Feb 2010 06:00 to Sat 06 Feb 2010 06:00 UTC
Issued: Thu 04 Feb 2010 21:28
Forecaster: TUSCHY

A level 1 was issued for W/NW-France mainly for severe wind gusts and an isolated tornado.

A level 1 was issued for parts of Corsica and central Italy mainly for an isolated tornado and large hail risk.

A level 1 was issued for the east-central coast of the Adriatic Sea mainly for excessive rainfall and an isolated tornado.

SYNOPSIS

A southeastward building upper low affects most parts of west/central Europe with falling geopotential heights and large-scale low-end CAPE build-up. This causes numerous areas with showers/thunderstorms. Otherwise, cold and stable conditions prevail over N/E-Europe.

DISCUSSION

... France and SW/S-central UK until roughly 18 UTC ...

This extensive lightning area consists of numerous, different rounds of convection. A split-up was done for this region in the following paragraphs.

The pattern is in a transition stage during the daytime hours, as an active depression approaches from the Atlantic. During the day, gradual height falls continue over France at mid-/upper levels with constant cooling. Beneath roughly 3km, the streamline pattern becomes more diffluent with further weakening of the westerly wind field. A sharp cyclonic turn is present at high levels.

A vorticity lobe and attendant front over central France continues to push eastwards although further weakening in this environment is due. GFS has a good handling of the surface dewpoints and with modified, maritime air mass surging eastwards, some increase in the BL moisture is forecast with dewpoints somewhere around 5°C.

Diabatic heating and marginal BL moisture beneath very cold mid-levels / steep lapse rates result in a broad area with low-end SBCAPE. No real support in terms of forcing can be seen, as NVA strengthens behind the decaying vorticity lobe over east France. Strong high-level cyclonic turning is present but we do not expect this to be supportive for more widespread development. We will mention the chance for showers/isolated thunderstorms mainly over north-central France and SW-UK - probably peaking around noon - with some sleet and marginal hail.

Thunderstorms are more likely over SW/extreme S-France, where bette LL moisture resides with a gradual decrease during the afternoon hours. Marginal hail and strong wind gusts will be the main hazard with this activity.

... NW/W-France during the night hours ...

Of more interest will be the developing NW-SE aligned tropopause fold over NW-France with impressive baroclinicity. A 50-60m/s mid-level jet evolves over the far N-Bay of Biscay and to the north and this jet pushes to the SE until 06 UTC. No real surface front is seen in current model data but the most likely scenario will be an occlusion in form of SE-ward pushing wrap-around moisture, which affects the area at 21 UTC onwards. This may yield a broad swath of moderate BL moisture beneath very cold mid-levels and favorable dynamics in the left exit of the mid-level streak. Another plus for deep convection is the intense forcing, which crosses NW/W-France during that time. We would like to see a more compact/intense low level streak to accompany the occlusion for an organized squall line event, although the strength of lift may counterbalance that negative point. The focus for isolated, deep convection right now is a concentrated swath along the back side of the SE-ward sliding occlusion, as a well structured dark stripe builds in from the NW. Conditional instability may be released in that area of moist low-/mid levels along the front, impressive lift and a strong hydrolapse. 850hPa winds up to 25m/s only need weak convection/downward mixing for severe wind gusts at the surface, so a broad level-1 was issued. An isolated tornado event is also possible - although more conditional right now -due to very low-end LL CAPE release and a detoriating postfrontal air mass for organized LL mesocyclones.

... Central Mediterranean ...

Offcuts from the potential subtropical storm, which affected the Canary Islands and parts of the Iberian Peninsula during the past days is now placed over the western Mediterranean, just to the south of France. Latent heat release was the impulsive force in the past, but this feature now acquired cold core characteristica and hence it needs a baroclinic environment. This environment is present over S-France/N-Italy and therefore another round of intensification is forecast, as the depression moves over N-central Italy towards the Adriatic Sea. Due to its history, a plume of modified subtropical air accompanies the depression and is present in the broad warm sector. Best CAPE will be found offshore with a rapid decrease over land and probably maximized along the westward facing coasts, where some enhanced LL convergence and moisture pooling is forecast. LL shear is enhanced, with localized CAPE/shear overlap, maximized NE of Corsica and along the east-central coast of the Adriatic Sea, but the overall risk is conditional at best. The same for the large hail threat. The main risk for a level 1 is the enhanced rainfall risk, especially along the east coast of the Adriatic Sea due to orographic support.