Storm Forecast
Valid: Sat 16 Sep 2006 07:00 to Sun 17 Sep 2006 06:00 UTC
Issued: Sat 16 Sep 2006 06:39
Forecaster: DAHL

SYNOPSIS


High-over-low upper flow configuration existing over Europe ... with vigorous vort maxima present at periphery of the upper low over the central Mediterranean. Quite moist low-level air is present E of and beneath this feature. Until Sunday 06Z ... the upper low is progged to weaken some ... though shear should remain supportive of potentially severe evolution.

DISCUSSION

S Italy ... Adriatic Sea ... Ionian Sea ... W Balkan States

Focus for potentially severe TSTM development will persist along the cold front currently stretching from the north Balkans southwards into southern Tunesia. Shape of the thermodynamic profiles is somewhat uncertain as no 00Z ascents are available from the pre-frontal air mass ... but it does not seem likely that lapse rates have steepened substantially compared to Friday's 12Z ascents ... which in addition is not supported by model fields. CAPE should be on the order of 1000 J/kg ... though it might locally be somewhat higher where BL moisture is richer than what was revealed by Friday's 12Z launch from e.g. LIBR.

Current activity should continue across the Adriatic Sea and the W Balkans over the next couple of hours ... though shear will gradually weaken to 20 m/s (0-6 km) until late afternoon/early evening ... which should still promote scattered severe TSTMS.

Expect a few mesocyclones ... capable of large hail and severe wind gusts ... and maybe also a brief tornado though threat is somewhat uncertain given essentially no information about the LL thermodynamic setup of the pre-frontal air mass.

Another ... potentially severe MCS should form ahead of peripheral vort max over the Ionian Sea early Sunday morning ... which may graze southern Italy.

central and northern Italy ... Tyrrhenian Sea ... W/NW Mediterranean Sea

Interesting thermodynamic setup is present in the wake of the front ... where steep LL lapse rates coinside with very moist (10 to 16 g/kg mixing ratios) and essentially saturated boundary-layers across the W abd central Mediterranean. Shear should be rather weak ... providing favorable conditions for waterspouts. Storms developing close to the upper jet at the cyclonic shear side of the E periphery of the upper low ... may benefit from sufficiently strong DLS to become supercellular ... with an enhanced hail ... severe wind-gust and tornado threat.

Activity over France will develop in similar thermodynamic environments conductive to non-mesocyclonic tornadoes ... and a couple of events are expected.

Coverage of waterspouts in such situations is unknown ... but indications are that it is sufficiently high to warrant a categorical risk.