Severe thunderstorms expected over NW Europe and Spain
TomSlavkovsky API developers last edited by pavelneuman
Valid: Tue 23 Jul 2019 06:00 to Wed 24 Jul 2019 06:00 UTC
Issued: Mon 22 Jul 2019 21:21
- Levels 1 and 2 were issued for parts of France and the UK for large hail, severe wind gusts, and excessive precipitation.
- A level 1 was issued for parts of Ukraine, Russia, and Belarus mainly for excessive precipitation and large hail.
An intensifying ridge in W Europe brings exceptionally warm air masses up to the UK and the North Sea, and a jet stream builds on its forward flank, curving east towards South Scandinavia and Poland. At the same time, two troughs are forming on the west and east side of the ridge, leading to an Omega Blocking. Downstream from these two troughs, we expect DMC events mostly covering a large part of E Europe on Tuesday 23/07. Moreover, storms are also expected in Spain and NE Portugal, which may produce locally severe wind gusts.
France and UK
The advection of very warm air masses over France with up to 25oC at 850 hPa will create a strong cap extending north up to the UK, so we do not expect surface-based convection to use the fat CAPE profiles aloft, where very steep lapse rates create about 2.500 J/kg CAPE.
Nevertheless, late in the afternoon and early night of Tuesday a short-wave trough is forecast to reach the west coasts of France and the UK, with strong PVA. Current thinking is that the unstable air masses in extreme NW France and SW UK will penetrate the cap, given the strong synoptic lift, and will lead to severe weather events.
The exact time of convective initiation is questionable, but any storm that will form inside the level-1 and 2 areas will be able to produce severe wind gusts, large hail and locally excessive precipitation.
A jet-streak of 20-30 m/s that will cross SW UK increase the threat level for severe convective wind gusts and large hail. In France, NWP models (for e.g. GFS, AROME, ICON) show CAPE values exceeding 2.000 J/kg with strong low/mid-level helicity, suggesting that storms may quickly become supercells and very large hail is possible near the coasts of Normandy and Brittany, but according to IFS storms will stay offshore.
After midnight the storms will continue propagating N-NE over the UK, mostly elevated. In the early morning of Wednesday 24/07 the main threat will be excessive precipitation in central parts of the UK.
Ukraine, Russia, and Belarus
In the SE side of a low/mid-level vortex embedded in a wider long-wave trough over Ukraine, a slow-moving cold front will build several storms while propagating east to Russia.
The slow storm motion and high PWAT suggest that storms will be able to produce excessive rainfall. DLS in the range of 10-20 m/s overlapping with some hundreds of CAPE, may enhance the possibility of local large hail events. On the eastern side of the level-1 area, high-delta-theta-e values increase the probability of downbursts during slow-moving storms.
Unstable air masses over the Black Sea and decreasing low-level winds behind the cold front in the afternoon increase the possibility of waterspouts near the coasts.
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