Storm Forecast

Storm Forecast
Valid: Sun 15 Apr 2012 06:00 to Mon 16 Apr 2012 06:00 UTC
Issued: Sun 15 Apr 2012 01:31
Forecaster: TUSCHY

A level 1 was issued for parts of CNTRL Italy mainly for heavy rainfall.

A level 1 was issued for Sicily, S-Italy, the Ionian Sea and W-Greece mainly for large hail, strong to isolated severe wind gusts and an isolated tornado event.

A level 1 was issued for parts of Bulgaria and Romania mainly for an isolated large hail and tornado event.

SYNOPSIS

An extensive upper trough covers most parts of Europe. Numerous embedded smaller vortices affect mainly the Mediterranean and surrounding areas with showers and thunderstorms. A cold and dry air mass over N-Europe precludes deep convection.

DISCUSSION

... Sicily and S-Italy, the Ionian Sea and Greece ...

The upper streamline pattern features a favorable set-up for deep tropospheric lift over a prolonged period throughout the forecast, but the main concern will be limited recovery time for the BL air mass, given yesterday's frontal passage. However, unusal overlap of cool mid-level downstream of the upper trough in a diffluent high-level streamline field balances marginal BL moisture, which itself recovers betimes due to the elongated fetch atop 17-18 C SSTs. Hence a broad warm sector evolves within the highlighted area with 500-800 J/kg MLCAPE forecast.

The wind field is supportive, but strongest winds remain still confined to the base of the upper trough over N-Africa and the mid-level streak probably first ejects out of the base beyond the forecast period. Nevertheless, 15-20 m/s SW-erly 6 km bulk shear affect the region all day long with strengthening 0-1 and 0-3 km shear forecast after sunset, as a 25 m/s streak at 700 hPa approaches from the SW. Hence, shear favors well organized multicells during the day and probably a mixture of multicells/isolated supercells during the night hours.

Initiation is hard to pinpoint with limited forcing during the day. After sunset, a potent vort max draws near from Tunisia with an overall increase in thunderstorm coverage. Expect topography to play an important role in initiation during daytime hours. Large hail and strong to isolated severe wind gusts will be the main hazard. During the night, an isolated tornado risk becomes more likely given better LL speed shear. For now, no area with more concentrated severe can be detected and therefore a general level 1 was issued. Thunderstorms continue all night long.

... CNTRL Italy ...

Placed beneath a temporarily weakening cyclonic vortex, the set-up remains unchanged compared to yesterday. Shear is weak, so storms will be slow moving. Those, which evolve next to the vortex's center may see stronger LL winds, which may increase the risk for some training activity. Slow storm motion or training both indicate a risk for heavy rainfall with thunderstorms. Also, GFS has 100-150 J/kg LL CAPE in its forecast, which may be a bit exaggerated (compared to yesterday's sounding reports with similar forecast), but any convergence zone may be the focus for a few waterspouts. Hence, also the coastal areas were added to the level 1.

... N-Bulgaria and parts of Romania ...

HIRLAM and GFS both agree in another day with strong, pulsating thunderstorm activity, probably maximized over S-Romania and N-Bulgaria. 500-800 J/kg MLCAPE will be adequate for a few strong updrafts and large hail events. Also, the LL CAPE field remains supportive for a few funnel/isolated tornado reports with that activity. Thunderstorms are daytime driven and therefore decrease in coverage and intensity after sunset.

... Iberian Peninsula and the SW-Mediterranean ...

Very cold mid-levels (500 hPa temperature at or below -30C ) overspread the area. Given expected BL moisture dispersal (marginal onshore and better offshore), CAPE increases from 100-400 J/kg MLCAPE over Spain to 500-800 J/kg offshore. Shear remains weak, so only expect marginal hail and gusty winds.

An isolated large hail event over the Balearic Islands is possible due to more robust CAPE, but confidence in higher storm coverage precludes a level 1 for now. Heavy rain accompanies those storms with slow storm motion forecast. Also, we can't rule out an isolated waterspout event during the day, before cold front/moisture boundary moves in from the NW after 21Z.

... Rest of the general thunderstorm areas ...

Mainly daytime driven, pulsating thunderstorms are forecast. Storms over the Mediterranean may be capable of producing heavy rain and marginal hail due to the slow storm motion and a cool lower troposphere.

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