Valid: Mon 05 Mar 2012 06:00 to Tue 06 Mar 2012 06:00 UTC
Issued: Sun 04 Mar 2012 21:06
The disturbed weather pattern over Europe continues. Numerous disturbances will play a role in determening thunderstorm probabilities. One upper wave atop the CNTRL Mediterranean progresses slowly to the SE while opening up. At the same time, a stronger upper low over the English Channel (06Z) shifts southeastwards, entering the far W-Mediterranean after sunset. This set-up assists in a NW-SE elongated surface low pressure channel with numerous embedded surface vortices. The strongest one evolves over the Ligurian Sea/Corsica and pushes a cold front towards the east. This front also serves as a boundary between better prefrontal BL moisture, replaced by significantly drier air in the postfrontal cold sector.
Over E-Europe the BL moisture remains too slim for deeper convection.
... Mediterranean ...
The first surge of enhanced DMC is bound to an upper wave, which weakens betimes while progressing southeastwards atop the Ionian Sea, Greece and Aegean Sea. BL air mass features enough moisture (e.g. surface dewpoints up to 10°C) for MLCAPE in the order of roughly 500 to 800 J/kg as 500 hPa temperatures temporarily drop below -25 °C. Temperatures below 850 hPa do not decrease significantly, so LL lapse rates remain modest. Nevertheless, GFS has a broad area of 50-100 J/kg LL CAPE, which may assist in an isolated waterspout event along any more persistent boundary on the meso-gamma scale mainly along the coasts. The fact that locally up to 800 J/kg MLCAPE may assist in more intense updrafts may also increase the waterspout risk due to vigorous upward motion atop those boundaries. DLS remains weak, so otherwise no organized convection is anticipated. The first round of DMC gradually diminishes in general after sunset and from west to east.
A second round is anticipated due to daytime driven convection over parts of CNTRL Italy, mainly concentrated along the mountainous areas. Again, BL moisture remains adequate for MLCAPE in the order of 300-500 J/kg beneath cold mid-levels. The forecast hodographs reval very short lengths due to near zero DL shear. Hence, pulsating and short lived thunderstorms with sleet and gusty winds are forecast. The activity decays after sunset.
Finally, the strengthening depression over the Ligurian Sea becomes the focus for heavy rainfall mainly over NW-Italy. This feature drifts slowly SE-wards but assists in a persistent onshore flow of moist air towards NW-Italy. Local models locally indicate QPF amounts near level 1 criterion, but very limited DMC precludes a level 1.
... France ...
A combination of well mixed maritime air, which infiltrates from the NW and the passage of a pronounced cold-core low (500 hPa temperature temporarily drop below -32 °C) result in a broad sector of modest destabilization (100-300 J/kg SBCAPE). Forecast soundings show elongated but thin CAPE profiles, which raises concerns about lightning production. Nevertheless, isolated thunderstorms can't be ruled out. Despite stronger DLS, limited depth of updrafts precludes better organized storms. Hence only short-lived thunderstorms with sleet and strong wind gusts are forecast. Thunderstorm probabilities rapidly decay during the evening hours as the mixed BL becomes replaced by the more stable nocturnal layer.