Storm Forecast
Valid: Fri 30 Mar 2018 06:00 to Sat 31 Mar 2018 06:00 UTC
Issued: Fri 30 Mar 2018 00:32
Forecaster: PISTOTNIK

A level 1 is issued for SE France and N Italy for excessive convective precipitation, large hail and to a lesser extent severe convective wind gusts.

A level 1 is issued for parts of the Bay of Biscay and N Spain for severe convective wind gusts.

A level 1 is issued for the W Mediterranean Sea mainly for severe convective wind gusts and to a lesser extent for large hail.

A level 1 is issued for parts of Azerbaijan, Armenia, N Iran and E Turkey for large hail and severe convective wind gusts.

SYNOPSIS

An active zonal flow covers much of Europe, supported and sustained by a stronger-than-average temperature gradient between still winterly cold air over Scandinavia and Russia and subtropical air over S Europe. A series of short-wave troughs and accompanying surface cyclones travels eastward in this flow. The dominant ones move from the Bay of Biscay and Spain to the Alps and from the mid-Atlantic into Spain, respectively, from Fri 06 UTC to Sat 06 UTC. They become the focus for possible organized convection.

DISCUSSION

... S-central Europe ...

The flow backs to the southwest ahead of the pronounced mid-level trough and advects warm air over Italy and the greater Alpine region. Under synoptic lift and steepening lapse rates, daytime heating may result in a few hundred J/kg of CAPE in the strongly sheared and helical flow (0-3 km shear and storm-relative helicity on the order of 15-20 m/s and 200-500 m^2/s^2, respectively).
Convective initiation is expected where the lift oft he entering cold front is supported by upslope flow. A cluster of storms may work its way eastward across SE France (06 to 18 UTC) and N Italy (after 15 UTC). Prolonged upslope flow may result in localized flash floods in case of training convection. Tail-end storms over the plains may become multi- or supercellular with an additional risk of 2-3 cm sized hail (or large amounts of smaller hail) and severe wind gusts.
Strong isentropic lift in the warm air advection regime may carry a few storms into Austria, Slovenia, Croatia and Hungary overnight. They will be elevated and should not pose any severe weather risk.

... W and SW Europe, W Mediterranean ...

Isolated to scattered, low-topped convection is expected over wide areas in the maritime polar air behind the cold front. Low CAPE and weak to moderate vertical wind shear suggest single- or at best multicellular convective mode. Cold air advection will further handicap convective initiation. The severe weather risk is mostly on the low side.
The exception is the belt overspread by the second short-wave trough, which will enter N Iberia after 15 UTC. The low-level wind field reacts and backs ahead of it, resulting in enhanced 0-3 km shear (~15 m/s) and storm-relative helicity (~200 m1^2/s^2). Multicells and bowing lines could bring isolated to scattered severe wind gusts in case the lift ahead of the sharp trough axis can still initiate and/or sustain storms beyond the daytime heating period.
In the night, the short-wave trough will also overspread the western Mediterranean Sea, where around 500 J/kg of CAPE may build. It is difficult to assess whether its lift will break the pronounced cap, but storms that form have an enhanced potential to organize and bring severe wind gusts as well as marginally large hail. This convection may move onshore in Corsica and Sardegna late in the forecast period.