Storm Forecast
Valid: Tue 02 Apr 2019 06:00 to Wed 03 Apr 2019 06:00 UTC
Issued: Mon 01 Apr 2019 22:23
Forecaster: DAFIS
A level 1 was issued for France, Benelux and Germany mainly for severe convective wind gusts.
A level 1 was issued for parts of the UK and Ireland mainly for severe convective wind gusts and large amounts of small hail/graupel.
Al level 1 was issued for Italy and NW Balkans mainly for severe convective wind gusts and large hail.
SYNOPSIS
The zonal flow over Central and Western Europe will be disturbed by a trough that brings unseasonably cold air masses which create instability over a large part of Europe. The enhanced southwesterly low-level flow south-east of the surface low, which has multiple centres between the North Sea, the UK an France, will bring warm and moist air masses from the south over France, Benelux and Germany where thermodynamics suggest that some organized storms may form. On the cold sector of the low-pressure system, in Ireland and West UK, very cold air masses and large-scale lift are expected. A short-wave trough SE of the main trough will cross Italy and NW Balkans where we also expect some rounds of thunderstorms, potentially and locally severe. Finally, the eastern parts of the Mediterranean and Middle-East will see several storms during Tuesday and Wednesday, as cold air masses in mid-troposphere and diurnal heating of the surface work synergetically to create thunderstorms.
DISCUSSION
.... France, Benelux and Germany ....
As the approaching trough from the NW brings large-scale lift due to a wide PVA, the southwesterly flow near the surface brings warm and moist air masses from the south. The first storm cells should appear around noon in France, especially where sunny conditions in the morning will provide the necessary lift for the first convective cells. The DLS only locally exceeds 15 m/s in the GFS solution, so only a few storm cells are forecast to propagate NE for more than an hour or two. More cells should appear later in Benelux and W Germany into a similar environment. Straight-line forecast hodographs limit the tornado threat, but the strong mid-level flow can easily be translated to the surface, so severe convective wind gusts cannot be ruled out.
.... UK and Ireland ....
Under the core of the cold trough (even -38oC at 500 hPa), Ireland and the UK will be under the influence of strong N-NW flow. Strong surface fluxes are expected over the ocean, providing moisture to the lowest levels which can feed the instability over land, where diabatic heating of the land will provide lift to unstable air masses. Locally large amounts of small hail/graupel (even if storm cells will be moving fast) are expected, but also strong to severe wind gusts can verify, especially in Ireland in the afternoon, where there is a mid-level jet streak.
.... Italy and NW Balkans (mostly Croatia) ....
A short-wave trough will cross the area on Tuesday providing some lift, but diurnal heating will be the main driver of convective phenomena over land. The steep mid-level lapse rates suggest the deep updrafts will be able to produce large hail, only up to 2 cm given the meagre CAPE in the NWP model outputs (400-500 J/kg) and the weak DLS (10 m/s). By examining the vertical profiles in numerical models, dry mid-level air masses over Italy may initially inhibit convection, but in the afternoon may enhance cold pools, so a threat for severe convective wind gusts was also issued.
