Storm Forecast
Valid: Wed 30 Dec 2009 06:00 to Thu 31 Dec 2009 06:00 UTC
Issued: Wed 30 Dec 2009 06:28
Forecaster: PUCIK

A level 1 was issued for Western Iberia mainly for severe wind gusts, tornadoes and excessive precipitation.

A level 1 was issued for Southern to Central France mainly for severe wind gusts and tornadoes.

SYNOPSIS

Two major disturbances will be present in the forecast area - the first one in the form of a negatively tilted trough covering most of Western Europe and the second one in form of a deep low over Western Russia. Strong southwesterly to westerly flow at mid and upper levels will be present in the region of polar front zone with the highest velocities around the base and on the forward flank of the aforementioned trough. Slight ridging is forecast over the Central Mediterranean as warm air spreads northward.

At the surface, whole Europe is under the influence of the complex system of surface lows with several centers, one over western Russia and others over the Atlantic ocean - south of Ireland and a developing low affecting Iberia by Wednesday evening. The main frontal zone will stretch from Central Europe, where a warm front will translate slowly northwards ( with front lying from Southern Poland to Southern England by Wednesday morning. Cold front will affect Iberia during the passage of the surface low.

DISCUSSION

... Western Iberia...

With strong prevailing westerly or west-southwesterly flow, a moist subtropical airmass will be advected inland. Marginal values of CAPE were sampled by 12 UTC Tuesday Lisboa sounding and GFS or ECMWF predictions are simulating CAPE values in order of few hundred J/kg. The degree of instability will naturally decrease further inland and the major question in this scenario remains how far will the unstable airmass progress. Brisk southwesterly flow with over 20 m/s at 850 hPa and up to 50 m/s will create a strong, unidirectional wind shear conditions, conducive to well organised convection. Low level shear values will locally exceed 15 m/s and with the presence of the boundary-layer instability and rather low LCLs, isolated tornado event is not ruled out. Flow locally up to 25 m/s at 850 and 700 hPa level suggests that even weaker convection might be accompanied by rather strong and isolated severe wind gusts. At the coastal areas, convection might be ongoing for a very long period, with several waves of showers or thunderstorms possible, so that locally excessive precipitation event might be recorded. It is difficult to pick up the strongest threat time frame, but per models, best instability/shear overlap will exist between 12 and 18 UTC while the highest precipitation threat might be around 00 and 03 UTC as the strongest synoptic scale forcing is expected along with the arrival of the surface low.

... Southern to Central France ...

In the extent of the warm and moist airmass, south of the warm front a mid-level impulse is expected to move over the region. Several model runs suggest the development of a separate center of the low over France within the surface trough that extends to the east. Somewhat backed low level flow is expected on the forward flank of the trough. 06 UTC observations show dewpoints around 10°C and southeasterly winds over Southwestern France. With the arrival of the impulse a band of enhanced, but marginal CAPE is simulated by models, with values up to 500 J/kg, decreasing as the impulse progresses inland. Enhanced low level flow is predicted in the proximity to this impulse, with 850 hPa flow up to 25 m/s. In the strong wind shear conditions and enhanced SREH values, well organised convection is possible with one of the probable modes being a convective line moving rapidly northeastwards along with the favorable forcing with probability of embedded supercells. Isolated severe wind gusts or tornadoes are possible as high low level wind shear is forecast with values locally over 15 m/s. Main uncertainity is the formation of the surface low, its track and the extent of the band of unstable airmass. Nevertheless, Level 1 is warranted for this region for a timeframe 06 - 12 UTC mostly.

...Croatia to Southern Hungary and Central Romania...

Elevated instability is forecast in the vicinity of the surface trough that develops in the exit region of the mid-level jet. Favorable wind shear conditions are predicted over this region, so that a potential will exist for well-organised convection. Detrimental factor is, that the destabilisation is expected after the sunset and mostly around the warm front. GFS is suggestive that the convection will be mostly embedded in the large scale stratiform precipitation of the front. Therefore, any severe convective threat is very limited and no Level is introduced at the moment.