Storm Forecast
Valid: Fri 27 May 2011 06:00 to Sat 28 May 2011 06:00 UTC
Issued: Thu 26 May 2011 17:27
Forecaster: TUSCHY

A level 1 was issued for parts of Italy mainly for a few tornado events, large hail and severe wind gusts.

A level 1 was issued for Slovenia, parts of Hungary, Slovakia and Poland mainly for a few tornado events, large hail and severe wind gusts.

A level 1 was issued for parts of Poland and Spain mainly for large hail.

SYNOPSIS

Weakening upper trough over central Europe moves estwards, crossing the alpine region during the night. An upper low over the Iberian Peninsula and another one over SE Euope result in scattered showers/thunderstorms over a broad area.

DISCUSSION

... Parts of Italy ...

During the afternoon hours, a weakening upper trough moves in from the west, assisting in LL cyclogenesis over N-Italy (already starting during the morning hours over far NW Italy). This configuration results in ongoing moisture advection over N-Italy during the forecast with increasing PBL moisture present during peak time heating with LL mixing ratios up to 10g/kg being well likely. Capping is weak, so initiation will be early during the forecast period, probably before noon. In addition, a sharp vorticity advection couplet crosses the highlighted area from the west with abundant mid/upper-level forcing. Widespread initiation is expected along and south of the Alps with thunderstorms spreading towards the east.

Shear will be supportive for multicells and a few supercells with DLS around 20 m/s and SRH-3 of 150-200 m^2/s^2 next to roughly 500-1000 J/kg MLCAPE (SBCAPE locally exceeding 1500 J/kg). This may help long-lived mid-level mesocyclones with a risk of large hail and strong to severe downdraft wind gusts. An inhibiting factor could be rapid clustering with a more messy storm mode and numerous storm interactions (favorable placement within right entrance of a strong upper jet results in strong upper divergence ). Hence, main severe risk exists during the initiation stage of any thunderstorm.

Betimes, aforementioned diffuse LL depression intensifies over N-CNTRL Italy, increasing LL shear/helicity within the BL. Hence, storms along the southern part of the gradually eastward moving cluster may see a more favorable environment for LL rotation with isolated tornadoes....tornado risk probably maximized between afternoon and evening hours. Therefore, the level 1 was expanded well to the south. Finally, excessive rainfall amounts are well possible within the level 1 area (especially just south of the Alps). During the night, thunderstorms move off towards the east.

... Slovenia, far SE-Austria, W-Hungary, parts of Slovakia and S-Poland ...

Rich BL moisture over the Balkan States (dewpoints in the middle tens) advects north/northeastwards during the forecast, also affecting the mentioned regions. In addition, a slowly eastward moving cold front approaches E-Austria and Slovenia during the late afternoon/early evening hours, again locally increasing BL moisture (pooling affect). Hence, MLCAPE around 1000 J/kg will be a likely scenario, as cold front moves in. Shear at all levels will be supportive for rapid thunderstorm organization and an attendant risk of large hail, severe wind gusts and a few tornadoes. Near parallel alignement of deep layer shear vector and frontal boundary may support rapid thunderstorm clustering, so discrete supercells are not likely despite far SE-Austria/W-Hungary and SW-Slovakia, where winds back a bit more. Thunderstorms keep going all night long with a gradual decrease in intensity and coverage.

... Spain and Portugal...

As an upper trough draws near during the forecast, widespread thunderstorm development is forecast. Until the afternoon hours, pulsating thunderstorms/a few multicells occur with isolated large hail being the main risk. Those thunderstorms keep going well into the night.

During the evening hours, winds at mid-levels increase from the south, as WAA sets in. This helps mid-level lapse rates to steepen rapidly over far S-Spain with modest elevated CAPE present. Combination of increasing shear and available CAPE increase the risk of large hail events during the evening/early night hours in the level 1 area. However, thunderstorm coverage decreases due to slowly decreasing CAPE, increasing capping and overall weak forcing.

... SE Norway and parts of Sweden ...

Placed beneath cold upper low, daytime driven thunderstorms are forecast. Shear remains weak, but strong LL CAPE signals are present within an environment with LCLs below 500 m and adequate BL moisture (regarding depth). A few funnels/tornado reports are well possible around peak time heating. However, no level will be introduce as mesoscale effects will play a major role (e.g. placement of convergence zones or best LL CAPE). Thunderstorms decay after sunset.