Severe storm expected over France, UK, Ireland
Valid: Mon 24 Jun 2019 06:00 to Tue 25 Jun 2019 06:00 UTC
Issued: Mon 24 Jun 2019 03:02
Forecaster: VAN DER VELDE
A level 1 and 2 were issued across western France and southern UK mainly for severe wind gust, large hail and tornado chances, and in Ireland for spout-type tornadoes.
A level 1 was issued across southeastern Europe mainly for large hail and extreme rainfall.
The strong southerly flow east of an Atlantic low has advected a layer with steep low to mid level lapse rates out of the Sahara and Spain northward into central France, where high low level moisture converges in a thermal trough. Meanwhile, southeastern Europe continues to be under influence of an upper cold pool, with relatively weak flow but significant CAPE.
A level 1 over central France illustrates mainly the uncertainty of storm initiation, where MUCAPE over 1000 J/kg meets 500 m2/s2 0-3 km SREH and 20-25 m/s DLS. Td2m forecast to be 19-21C. Across western France and southeastern UK various models produce precipitation likely from elevated convection along the cold front, with weaker CAPE. The lifting action of shortwave troughs also reaches central France, and relatively weak CIN is calculated on the convergence line. However, models hesitate producing any storm in the otherwise severe weather supportive environment, except for an evening/night MCS developing most likely over Normandy, moving NNE-ward.
Initially convection will start as clusters of cells including rotating ones (supercells) even if elevated base, which can produce very large hail and severe gusts. If rooted in the boundary layer of the surface convergence line, then with 0-1 km shear exceeding 10 m/s and the strong SREH also tornadoes are possible. However, such collocation is not what COSMO model suggests, which keeps nocturnal development west of the line. But if indeed clustering into an MCS over the late evening, a squall line may form with the potential of organized severe wind gusts.
An upper low moving towards Greece will trigger numerous thunderstorms in the weakly capped environment with locally 1000-2000 J/kg MLCAPE. Weak shear and slow storm motion can create excessive precipitation sums in little time, while also large hail can be produced by strong updrafts, mainly across Bulgaria and E Romania.
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