Valid: Sun 30 Aug 2015 12:00 to Mon 31 Aug 2015 06:00 UTC
Issued: Sun 30 Aug 2015 11:39
A waving frontal boundary is located over western France, the south of England across the North Sea into Denmark. The air mass situated south and east of this boundary is warm and moist with dew points of 14 to 20°C. Highest values are located over northern France, North Rhine-Westphalia and in the BeNeLux where nocturnal convective activity occurred. A strong south-westerly flow is overhead located in the mid to upper levels, providing moderate to strong shear, 10 m/s LLS and 20-30 m/s DLS. Strongest shear is situated over the English Channel along the frontal boundary. Near-surface winds over the continent are S to SE... over northern France... and E to NE over the northern BeNeLux and western Germany. This leads to 0-1 km and 0-3 km relative helicity values of 100-150 m²/s² and 200-250 m²/s²... respectively.
Satellite imagery of 11 UTC is showing mostly clear conditions over the aforementioned area, with strong surface heating and temperatures expected to climb to 28 to 33 degrees. Soundings of Trappes... 00Z... and Idar-Oberstein... 06Z... suggest the convective temperature for SB convection is 33 or 34 degrees. The associated SBCAPE is forecast around 2000 J/kg. Scattered to isolated initiation is expected, initially, mostly over western and northern France. Given the strong shear environment, we expect these storms to have reasonable potential for becoming supercells... with a risk of large hail (2-5 cm) and damaging winds. However, given high LCLs, tornado threat is likely limited.
During the evening and night hours, storms are expected to become more widespread as outflow boundaries trigger new storms and a couple of shortwave troughs pass over the area, providing some dynamic lifting. Likely storms will cluster into one or more MCSs, which will be or become elevated. The associated risk is mostly excessive rainfall... PW around 40-45 mm... and straight line winds. However, some storms may still be surface based and isolated, which will continue to have a threat of large hail and perhaps a tornado. Convective activity is forecast to shift into western and northern Germany in the late night and early morning hours.