Storm Forecast
Valid: Tue 28 Feb 2017 06:00 to Wed 01 Mar 2017 06:00 UTC
Issued: Mon 27 Feb 2017 22:56
Forecaster: DAFIS
A level 2 was issued for France and Switzerland mainly for severe convective wind gusts and tornadoes.
A level 1 was issued for France, Switzerland and SW Germany mainly for severe convective wind gusts and tornadoes.
A level 1 was issued for NW Italy mainly for excessive precipitation and tornadoes.
A level 1 was issued for Croatia mainly for excessive precipitation, severe wind gusts and tornadoes.
SYNOPSIS
An extended upper-level trough enters central Europe during Tuesday, affecting mostly France, Benelux, Switzerland and NW Italy. At the surface, several low-level vortices exist around North Sea as a reponse to the upper-level configuration. Another low pressure system is expected to form in NW Italy as a cold front shifts from France eastwards, over the Alps. Somw pulsating storms are also expected in N Denmark, NE Germany-Finland and in E Adriatc Sea.
DISCUSSION
.... France, Switzerland and SW Germany ....
A mid/upper-level wave shifts east and affects France around noon and Switzerland - Germany thereafter. The combination of an upper divergence and the left exit region of a 40 m/s mid-level jet results in a pronounced low-tropospherci wave ahead of its mid-level equivalent. The convergence zone that forms is pushed eastwards, towards Switzerland and Italy after 18z. Even though this plume lacks significant moisture (mostly advected from NW Mediterranean), strong DPVA is expected to override the WAA and we expect a growing convection along the eastbound moving convergence zone, probably in the form of a squall line. CAPE is forecast to reach 400-500 J/kg around noon as steep lapse rates form.
Some diurnal heating is expected as much of France will be under a broken cloud cover during the morning and it is uncertain how much diabatic heating will verify, but those regions which will experience diabatic heating will be prone to surface-based activity and tornado risk is increased. LLS will also exceed 10 m/s locally. Nevertheless, the main threat is expected to be the widespread severe convective wind gusts within these line segments especially in the region denoted with a level 2. Wind speed at 850 hPa may exceed 25 m/s and this may translate in similar intensity at the surface. DLS is forecast in the range of 15-30 m/s and the lack of directional shear limits the production of hail but any deeper updraft could produce an isolated hail event, though less than 2 cm in diameter.
The convective line may survive to reach Switzerland and Germany and at the NW part of the Alps some thundersnow events may verify, during blizzard conditions.
.... Italy ....
NWP models agree in an unobstructed SW flow of moist air masses resulting in high theta-e 850 hPa values and PW around 20 mm. High-resolution models forecast steep lapse rates around noon and the production of some hundreds of CAPE. Strong directional shear and low LCLs increase tornado threat onshore of Italy. Convective activity should diminish after 18z as the NW flow over the Alps will result in katabatic winds.
.... Croatia ....
As the vorticity lobe shifts over the Adriatic Sea during the night of Tuesday, strong kinematics overlap with substantial CAPE to produce multicells. Latest model runs suggest that effective PW will be in the range of 20-25 mm and LL flow from the SW in the range of 15-20 m/s. Excessive precipitation will be the main threat along the coasts of Croatia but also tornadic waterspouts may form into this strongly sheared environment.
