Storm Forecast
Valid: Sat 01 Apr 2017 06:00 to Sun 02 Apr 2017 06:00 UTC
Issued: Sat 01 Apr 2017 02:55
Forecaster: TASZAREK

A level 1 was issued for E Germany, part of W Czech Republic and part of NW Poland mainly for large hail and severe wind gusts, and in lesser extent for excessive precipitation.

A level 1 was issued for parts of W Mediterranean mainly for large hail, severe wind gusts and excessive precipitation.

A level 1 was issued for parts of SW France and NE Spain mainly for excessive precipitation.

SYNOPSIS

Large long-wave trough with the axis extending from British Isles up to W Mediterranean travels slowly eastward and creates favourable conditions for DMC on its eastern flank. Azores high builds up behind trough. Ridge with warm polar airmass (> 10C at 850 hPa) covers E and SE Europe and convection is unlikely in these areas. Exception is S Turkey, where weakening mid-level cut-off results in weakly sheared minor thunderstorm activity. Weakly sheared thunderstorm activity is also expected in the cold section of the trough in S British Isles and W France. Better storm organization and possible severe weather is possible over E Germany and W Mediterranean where small to moderate CAPE overlaps with moderate vertical wind shear. Jet stream extends from Iceland up to Spain and Tunisia, but except some parts of N Algieria it seems to be outside main CI and instability areas.

...E Germany, W Czech Republic, NW Poland...

Small wave with weak low-level low moves through Germany and NW Poland in the afternoon hours. Cool mid-levels aloft (-25C at 500 hPa) with diurnal heating develops steep lapse rates exceeding 7 K/km. Along with southern low-level warm air advection and building moisture due to evapotranspiration (MIXR ~ 10 g/kg), afternoon CAPE develops up to 800-1200 J/kg. DMC is expected to fire-up around 12-14 UTC and with moderate (and locally small) vertical wind shear (DLS ~ 10-15 m/s), isolated and multicell storm mode is expected. Although NWP models are suspected to overestimate low-level moisture and thus instability, these storms (if form) may be capable of producing large hail up to 2-3cm (given steep lapse rates and moist boundary layer) and local severe wind events (elevated delta Theta-E values). Given not very high storm motion and local trainings of multicell clusters, excessive precipitation cannot be ruled out. Elevated convection may maintain over NE Germany and NW Poland until late evening and night-time hours. Forecast for this area is somehow uncertain due to different model scenarios (differences in predicted instability) and uncertain CI due to lack of a well-pronounced QG-lift.


...W Mediterranean...

Better source of QG-lift is available within the trough over W Mediterranean where elevated mixed layers from Africa (> 7.5 K/km) overlap with 8-10 g/kg mixing ratios and builds up CAPE up to 1000-1500 J/kg. CI is expected along with diurnal heating in the afternoon hours and thanks to persistent lifting should remain until nigh-time over water surface. Deepening low cutting off from the main wave provides enhanced values of vertical wind shear (~ 15-20 m/s) and may support the development of supercells and multicell clusters. Given strong lapse rates, moderate instability and possible mesocyclones, large hail up to 3-5cm along with local severe wind events cannot be ruled out. Due to weak motion of the trough and its rather stationary position over Corsica and Sardinia in the second half of the day, maintaining stationary convection superimposing with large-scale precipitation may result in an excessive precipitation and local flash flooding event. Thunderstorm activity in the eastern part of this area should continue through the morning of the next day (Sunday).

According to some model scenarios excessive convective rain is possible over parts of SW France and NE Spain, mainly due to convection in these areas persiting all day long.