Storm Forecast
Valid: Fri 07 Aug 2020 06:00 to Sat 08 Aug 2020 06:00 UTC
Issued: Thu 06 Aug 2020 22:50
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for south-central Italy, Montenegro, Albania, W Greece and surrounding sea waters mainly for excessive convective precipitation and non-supercellular tornadoes (waterspouts).

A level 1 is issued for E Greece, W Bulgaria, North Macedonia and S Serbia mainly for excessive convective precipitation and to a lesser degree for large hail.

A level 1 is issued for parts of Spain mainly for severe convective wind gusts.

SYNOPSIS

An extensive blocking anticyclone centered over the Baltic States influences most of Europe with warm and dry summer weather. A mid-level cut-off low creates (relatively) cooler and more unsettled conditions from Italy to the Balkans. Over W Europe, a SW-erly flow with warm air advection spreads from Iberia to the British Isles and the North Sea ahead of an Atlantic depression.

DISCUSSION

... Italy to S Balkans ...

Scattered thunderstorms are expected in vicinity of the cut-off low. The airmass features meagre lapse rates, but plentiful maritime moisture with 2m dewpoints often between 18 and 23C in coastal areas, yielding CAPE up to 1000 J/kg or locally more.
Most storms occur in the afternoon over south-central Italy, Albania and W Greece. Efficient rainfall production in the moist air and slow storm motion or anchoring over the hilly to mountainous terrain may promote some dangerous flash floods. Poor storm organization keeps the other severe weather risks low.
Weak capping beneath the rather cool mid-levels suggests that some storms can also propagate to coastal and offshore areas in the night to morning hours. In that case, a few waterspouts are possible, most notably along land breeze fronts.

At the forward flank of the cut-off low, slightly steeper lapse rates and moderately enhanced 0-6 km shear (10-15 m/s) can support some better (mainly multicellular) storm organization from Bulgaria to Serbia with a slightly enhanced hail risk in the afternoon. Warm air advection wrapping around the cut-off low may carry some of these storms towards the W Balkan states later in the evening and night. By then, convection will turn elevated and partly embedded, and the main risk shifts back to heavy rainfall.

... Spain ...

Scattered afternoon storms are foreseen over orogrphic features in a hot and dry, deeply mixed airmass in inland Spain. Some severe (possibly dry) downbursts are possible and covered with a level 1, though the storm coverage will likely be rather low.
In case the storms can ingest some sea breezes propagating inland, more CAPE and enhanced vertical wind shear could promote better storm organization and a hail risk, but the strong capping will likely inhibit this scenario.

... east-central Europe ...

Owing to discrepancies about the low-level moisture, the forecast models remarkably disagree on the presence and magnitude of CAPE in the synoptically tranquil situation over east-central Europe, most notably the Czech Republic, Poland and parts of the Ukraine: Whereas ECMWF shows only a few patches of some hundred J/kg, GFS suggests wide areas with a CAPE magnitude around 1000 /Kg (with generous precipitation signals) and ICON simulates a more narrow CAPE zone but with maxima in excess of 2000 J/kg over SW Poland (without precipitation signals). On Thursday, surface observations seemed to support the reluctant ECMWF scenario and storms were consequently absent. For that reason, only a low-probability lightning belt is issued for Friday along a convergence zone whose more localized lift may be just enough to compensate the background of large-scale subsidence. In case yet more storms form in the afternoon to evening, isolated heavy rain and marginally large hail are not ruled out.