Storm Forecast
Valid: Wed 24 Apr 2019 06:00 to Thu 25 Apr 2019 06:00 UTC
Issued: Tue 23 Apr 2019 22:33
Forecaster: PUCIK

A level 2 was issued across northern Italy mainly for tornadoes, large hail and excessive rainfall.

A level 1 was issued across northern Italy mainly for tornadoes, large hail and excessive rainfall.

A level 1 was issued across eastern France and western Germany mainly for large hail and severe wind gusts.

A level 1 was issued across Portugal and western Spain mainly for tornadoes and severe wind gusts.

SYNOPSIS

A deep cyclonic vortex is forecast over the Atlantic with its short-wave trough quickly translating from northern Iberia at Wednesday 06 UTC towards northern Germany by Thursday 06 UTC. Its geopotential gradient at the flanks and thus also wind field will weaken with time. Closer to the surface, an extensive southerly flow has developed in between of the low over the Atlantic and a high over eastern and northern Europe. Warm air advection regime will be underway across much of central Europe during the forecast period and a warm front will stretch from Scotland through Northern Sea into northern Denmark at Wednesday 12 UTC. A cold front will progress from France into Germany by the evening hours, in conjunction with the approach of the short-wave trough. Frontal system over the Mediterranean, located near Sardegna and Corsica, is forecast to weaken.

A large area of isolated to widespread thunderstorm is forecast across much of western and southwestern Europe. Some of the areas deserve closer inspection due to the enhanced chances of severe weather.

... Portugal, western Spain ...

Tongue of moist airmass advected from SW inland will yield marginal CAPE values, below 500 J/kg, mostly confined to the lower troposphere. Scattered thunderstorms are forecast to initiate over the area, forming small line segments and low-topped supercells in 15 to 20 m/s of 0-3 km bulk shear. With 850 hPa flow locally around 20 m/s, marginally severe wind gusts will be possible with line segments. As 0-1 km bulk shear reaches up to 15 m/s and combines with low LCLs, and CAPE confined to the lower troposphere, tornado threat will be elevated as well. The negative factor remains in the form of mostly straight hodograph with rather low SRH values.

... eastern France into western Germany ...

The cold front will act as an initiating mechanism over the area and models suggest that much of the convection will remain on the cool side of the boundary, elevated above the stable boundary layer. Several hundreds J/kg of CAPE, along with up to 20 m/s of 0-6 km bulk shear may yield several marginally large hail producing storms. Initiation is more questionable ahead of the advancing boundary, where forecast soundings reveal more CAPE with deep and dry boundary layer, supportive of severe wind gusts in high-based convection rooted in the boundary layer. Thus, development ahead of the cold front should be monitored for this type of threat.

... northern Italy ...

Forecast hodographs of several models reveal kinematic setup highly supportive of supercells. 0-6 km bulk shear ranging from 20 to 25 m/s will combine with high SRH, locally up to 300 m2/s2 in the 0-3 km layer and up to 150 m2/s2 in the 0-1 km layer. CAPE values will not reach very high values, but should be enough to sustain deep, moist convection with dewpoint values reaching between 12 and 15 deg C in the area. The scenario with several supercells developing over the area is supported also by some of the convection allowing models. In such case, all kinds of severe weather would be possible, but tornadoes would stand out given the degree of shear in the lower troposphere and low LCLs.

There are two uncertainties in the forecast that may reduce the overall risk. The first is the possibility of considerable cloudiness hindering the daytime heating and the second is the effect of the storms developing to the southwest of the area, over the Appenines, along the Liguria coastline. Despite the uncertainties, will keep a Level 2 for an area with the highest likelihood of one or two supercells capable of large hail and tornadoes.