Storm Forecast
Valid: Sat 03 Jun 2017 06:00 to Sun 04 Jun 2017 06:00 UTC
Issued: Sat 03 Jun 2017 06:47
Forecaster: TASZAREK

A level 2 was issued for CNTRL Germany mainly for mainly for large hail, severe wind gusts and excessive precipitation.

A level 1 was issued for Germany, E France and Switzerland mainly for large hail, severe wind gusts and excessive precipitation.

A level 1 was issued for NE Spain and S France mainly for large hail, severe wind gusts and excessive precipitation.

A level 1 was issued for Balkans mainly for large hail, severe wind gusts and excessive precipitation.

SYNOPSIS

Two large troughs are placed over N Europe. One is slowly and eastwardly approaching British Isles while the second is located over NW Russia and moves westward. Jet streaks extends from Finland up to Ukraine and over British Isles, but mostly outside instability areas. A weakening ridge covers CNTRL and S Europe. A shallow mid-level cut-off is located over SW part of Balkan Peninsula. Widespread thunderstorm activity with severe weather potential is expected in the belt stretching from NE Spain trough E France up to Germany on the bordering area between ridge and trough over NW Europe. Shallow surface lows are expected to develop over Spain and N Germany and will be divided by a small ridge over SW France. Another cluster of weakly sheared thunderstorm activity with some potential of producing severe weather will be placed on the E flank of the cut-off over Balkan Peninsula.

DISCUSSION

...W Europe...

A broad area of rich boundary layer's moisture (mixing ratios ~ 10-11 g/kg) overspreads Germany, Benelux, France, N Italy, N Spain and W parts of Mediterranean. Along with diurnal heating and evapotranspiration, a solid ML CAPE up to 1000-1500 J/kg develops over Germany. Weaker instability (ML CAPE ~ 600-800 J/kg) is expected over France while a partially strongly capped ML CAPE up to 1000-1500 J/kg over Spain. Although CI over Spain is a bit uncertain, isolated cells which will get trough the cap may be capable of producing large hail (steep lapse rates 15 m/s DLS), microburst type severe wind gusts (enhanced delta theta-e values) and excessive precipitation (mainly in the NE Spain with enhanced PW values).

Better confidence for severe thunderstorm development is over Germany, mainly due to better lift (low developing in the afternoon hours lobes of PVA), and the coverage of unstable airmass. Due to weak CIN, a widespread convective development is expected around noon hours, mainly over S, CNTRL and W Germany. Thanks to around 15 m/s DLS, cells will be merging into multicell clusters, and a few local supercells (due to local wind field interactions with mountains) cannot be ruled out as well. Due to rather weak airflow (~ 8-10 m/s at 700 hPa) and high PW values (> 30mm) one of the main threat will cover excessive precipitation, especially within training multicell clusters. Large hail up to 3-4 cm is also possible if the supercell thunderstorms will be present. Although both mid-level airflow and delta theta-e values are not impressive, a few local severe wind gusts events are possible. Convective activity should drop in the late evening hours, except E Germany (passage of the shortwave trough) and NE Spain, where thunderstorms will be active until late nighttime hours. Superimposing stratiform and convective precipitation over W Alps (Switzerland, E France) may also cause excessive precipitation and a local flash flooding event.

...SE Europe...

A surface advection of a moist air from the Black Sea and moisture pooling due to evapotranspiration and strong diurnal heating develops rich moist boundary layer with mixing ratios ranging from 10 to 12 g/kg. Along with steep lapse rates (associated with an eastwardly moving cut-off placed over Albania) ML CAPE up to 2000-2500 J/kg is possible according to GFS, mainly over E Greece and Bulgaria. Rest of the 50% area covers ML CAPE up to 800-1000 J/kg. Majority of Balkans remain under very weak vertical wind shear (< 10 m/s DLS) which indicate isolated pulse thunderstorms locally clustering into multicells. Main risk covering these storms concerns excessive precipitation (rich in moisture vertical profile weak storm motion), and microburt type severe wind gusts (enhanced delta theta-e values). Given locally strong CAPE with steep lapse rates, a 2-4cm large hail event cannot be ruled out, mainly over Bulgaria, E Greece and partially W Turkey. Convection should weaken in the late evening hours, but it will be ongoing during nighttime over Aegean Sea and W Turkey.