Storm Forecast
Valid: Sat 22 Sep 2018 06:00 to Sun 23 Sep 2018 06:00 UTC
Issued: Fri 21 Sep 2018 17:37
Forecaster: &BEGIN=2018092206
A level 1 is issued for the S Baltic Sea and adjacent coastal areas mainly for severe convective wind gusts and to a lesser extent for tornadoes.
A level 1 is issued for the NE Adriatic Sea and adjacent coastal areas mainly for flash floods and to a lesser extent for tornadoes and severe convective wind gusts.
A level 1 is issued for the E-most Black Sea for non-supercellular tornadoes.
SYNOPSIS
A large and at all levels pronounced cyclone is placed over N Scandinavia. Embedded in very strong mid-level flow around its S flank, a short-wave trough crosses Finland, the Baltic States, Belarus and Poland. Cold air advection enhances its amplitude but largely cancels its lift,rendering it little weather-active outside the influence area of the warm Baltic Sea. Several hundred kilometers ahead of the trough, a long cold front enters NW Russia, Belarus, the Ukraine, the Balkans and N Italy. South of ca. 45N, warm and mostly calm late summer conditions remain in place.
The frontal zone stays unseasonably intense from the N Atlantic into W Europe. A new frontal wave will cross the Celtic Sea Saturday night but with no convective developments yet. (Preview for Sunday: it will intensify into the next storm cyclone and cross central Europe on a more southerly track than its antecedessors.)
DISCUSSION
... NW Russia ...
Warm and moist air with a few hundred J/kg of CAPE is advected far northward in the broad warm sector of the Scandinavian storm cyclone. Vertical wind shear rises towards the NW, where 15 (10) m/s 0-3 (0-1) km bulk shear overspread the unstable air with strengthening mid-level flow.
ECMWF, GFS and ICON agree on isolated to scattered initiation from the late afternoon onwards. Convection will possibly organize into a band at the surface cold front, which will be little active until then due to its shallow "ana front" character. These storms may go on into the night. However, weak lapse rates, the limited CAPE magnitude and the increasingly flow-parallel orientation of the cold front keep the severe weather risk low.
... S Baltic Sea and surrounding coastal areas ...
Cold air advection on top of the warm S Baltic Sea (sea surface temperatures around 18C) produces steep low-level lapse rates and a few hundred J/kg of CAPE. Scattered thundery showers will form and travel eastward throughout the forecast period. Despite weak vertical wind shear and poor storm organization, the background flow is strong enough to support scattered wind gusts in excess of 25 m/s, maximized with the showers. Convection encounters frictionally enhanced low-level shear (10-15 m/s) as soon as it moves onshore, hence a brief spin-up of a tornado is not ruled out.
... N Adriatric region ...
As the tail of the cold front crosses the Alps, scattered storms are likely in a confined region around the Gulfs of Trieste and Rijeka in the 06 to 15 UTC time frame, where strong convergence occurs at the nose of Scirocco winds in an environment supportive for organized storms (CAPE > 1000 J/kg, deep-layer shear 20-25 m/s). Flash floods are the main risk onshore, while a few tornadoes may form offshore. The onset of strong to severe, dry and cool Bora winds will terminate the thunderstorm situation after a few hours.
... Tunisia, Malta, S Italy and surrounding sea waters ...
Scattered convection will still go on in an environment of low to moderate CAPE and almost no vertical wind shear around a slowly filling warm-core cyclone. As the wind fields further relaxes, a few waterspouts become possible at convergence zones. One or two flash floods are not ruled out in the afternoon, when convective activity over land is maximized.
...E-most Black Sea ...
A similar environment of moderate CAPE and almost zero shear is in place. A few waterspouts are possible late in the forecast period, when (not necessarily electrified) convection is predicted to form along a land breeze front.
