Storm Forecast
Valid: Sat 27 May 2017 06:00 to Sun 28 May 2017 06:00 UTC
Issued: Sat 27 May 2017 06:51
Forecaster: TASZAREK

A level 1 was issued for N Spain, N France and Benelux mainly for the large hail, severe wind gusts and excessive precipitation.

A level 1 was issued for E British Isles mainly for large hail, severe wind gusts, excessive precipitation and in lesser extent for isolated tornado event.

SYNOPSIS

Spanish plume with a steep lapse rates and rich boundary layer moisture continues to develop over W parts of Europe. A typical for this feature ridge extends from NW Africa up to N Germany. A small shortwave providing lift moves trough British Isles, N France and Benelux in the afternoon hours. Although high shear values are displaced from the best instability areas, severe thunderstorms capable of producing large hail, severe wind gusts and heavy rain are possible. A broad thunderstorm activity within the long-wave trough is also possible on the other side of the ridge over Balkan Peninsula and parts of E Europe. However, due to small wind shear and weak instability severe weather potential over this area is strongly limited. Isolated storms with a heavy precipitation potential may develop over W and CNTRL Alps, but due to strong uncertainty regarding CI, no level threat is issued for this area.

DISCUSSION

... W Europe ...

The mixture of elevated mixed layer with lapse rates exceeding 7 K/km along with low-level moisture pooling (mixing ratios ~ 12-13 g/kg) and strong diurnal heating, develops a moderate to high instability (ML CAPE ~ 1500-3000 J/kg) in the belt from N Spain up to Benelux. DLS is not very high and amounts mostly 10 m/s (apart from N edge of France where 15 m/s may be available) which limits supercell potential and overall severity of convective cells. However, thanks to high thermodynamic instability large hail locally exceeding 3-4cm cannot be ruled out. Very large hail may occur if thunderstorms will develop in the better shear environment over N edge of France. Slow motion of the cells and high PW (~ 35mm) will also favour heavy rain events and a possible local flash flooding. Local severe wind gusts due to microburst phenomena are also involved. Limitation of this forecast is associated with the uncertain magnitude of the cap which may limit CI, especially over N France. The magnitude of lift seems to be also not very high, even besides WAA.

Severe thunderstorms will be also possible in the warm sector over E British Isles. Although these stay under weaker thermodynamic instability area, trough centre is closer and thus provide better lift and overall kinematics. Thunderstorm developing in this area may be capable of clustering into multicells capable of producing severe wind gusts and heavy rain. Few large hail incidents exceeding 2-3cm also cannot be ruled out. A marginal tornado potential also exists over tritple-point in E-CNTRL British Isles where low-level shear and helicity will be enhanced.

Majority of surface-based thunderstorms capable of producing severe weather will develop in the afternoon hours over N Spain, N France and E British Isles. In the evening hours, thunderstorm should move to Benelux and become elevated. A band of storms (squall line) moving through North Sea is also possible.

... SE Europe ...

Strong diurnal heating, enhanced low-level moisture due to evapotranspiration and lack of cap will favour "internal-airmass" convection over Balkan Peninsula, E Ukraine, SW Russia and Turkey. ML CAPE is predicted to be mostly small, locally moderate (up to 1000 J/kg). Along with weak environmental wind shear severe weather potential within these storms is strongly limited. However, few local events of heavy rain and severe wind gusts cannot be ruled out, but the overall threat seems to lie below level 5% severe probability.