Storm Forecast
Valid: Sun 19 Aug 2012 06:00 to Mon 20 Aug 2012 06:00 UTC
Issued: Sun 19 Aug 2012 05:03
Forecaster: VAN DER VELDE

A level 1 was issued for Estonia, E Latvia, W Russia, N Belarus mainly for severe convective wind gusts, as well as tornadoes.

A level 1 was issued for S United Kingdom mainly for large hail and tornado chance.

A level 1 was issued for NW France, Belgium, Netherlands, N Germany mainly for severe convective wind gusts and large hail (mostly during the night).

SYNOPSIS

Between a high pressure area over central Europe and an Atlantic low pressure system, a southerly flow advected a dramatic tongue of mid-level Saharan Air Layer far northward into western Europe. This leads to 2m temperatures in the 35-40 °C range while providing deep nearly dry-adiabatic lapse rates in the same area. At the western fringe of this area, over western France into Belgium and Netherlands, low-level water vapor accumulates under the influence of convergence in thermal troughs, leading to rather high CAPE values (1000-2000 J/kg). The other area of interest is a zone of less strong instability near a more active depression affecting southern Finland, Russia, Estonia and Latvia.

DISCUSSION

...NW France, Belgium, Netherlands, N Germany...

The main dynamics seem associated with warm air advection into a Biscay low coming to the English channel in the evening. Another thermal trough moves from the Netherlands in the early afternoon to N Germany/S Denmark but is not expected to do anyhing convective-wise.
The assessment of instability needs to take into account the differences in low-level mixing ratio between models. Since the airmass is of Saharan/Spanish origin, it is very dry and hot at 1 km altitude and the question is whether mixing ratio can get large enough for a parcel to become positively buoyant. Animation of the 0-1000m mixing ratio reveals an unrealistically strong afternoon increase in present GFS model runs (at least 2 g/kg between 09Z and 15Z). The 09Z mixing ratio looks normal can be extrapolated. So, 12-14 g/kg should be the corrected GFS indication. This agrees with the maximum areas in Hirlam and also with the 00Z Bordeaux, Brest and Santander soundings (13 g/kg in the BL). The problem is that not only this moisture is advected to the north, but also the hot layer above which serves as a cap unless 40°C is reached, or the cap eroded by layer lifting. However, lifting processes are forecast to be weak.
So, as observations as well as models indicate, LFC-LCL differences or CIN remain too high to allow surface-based convective storms, although an isolated elevated convective storm is not ruled out. While the afternoon and early evening may remain rather quiescent, there is now quite a good number of models supporting a late evening and night (21-06Z) thunder episode mainly over the Netherlands. (GFS, ECMWF, diverse WRF, Hirlam 18Z produces at 03-06Z).
The question is whether the 20 m/s DLS will be effective at night, Probably not, and then some 5-10 m/s must be subtracted. It will remain sufficient for organized multicells with severe gusts as the main threat, with large hail as secondary threat. There isn't particularly strong background dynamics going on so the main drive would be strong local cold pools/downdrafts caused by low mid-level theta-e and high LCLs (2000-3000m) - although GFS delta-theta-e (20-28°) is inflated due to the unrealistic low level moisture problem. Still 16-20° is probable. Tornadoes do not seem likely if convection remains elevated, and onion-shaped skew-t profiles (prog soundings) are too dry. Even though Hirlam (more than GFS) produces spots with more than 10 m/s 0-1 km shear.

...southern UK....

This area is less capped since it is not overrun by the hottest part of the Spanish (Saharan) plume. LCL heights should only be 1000m and MLCAPE is also a lot weaker than on the mainland, however, here it will not be as capped. Also in favor are midlevel vorticity advection and positioning right under the jet, which creates deep layer shear of 30 m/s. 0-1 km shear can reach values of 10 m/s and SREH3 some 200-250 m²/s². Such values support supercells. Isolated large hail, severe wind gusts and tornadoes are possible. Initiation is most likely between 15-18Z.

...Estonia, E Latvia, W Russia, N Belarus...

Models agree about a patch of CAPE placed in a 20 m/s deep layer shear, 250 m²/s² SREH and strong 10-15 m/s 0-1 km shear, making this region favorable for development of supercells or bow segments mainly with severe wind gusts, marginally large hail and isolated tornado threat. Storm motion is fast and 35 kts mean 1-3 km wind speed overlaps partially an area of high delta-theta-e, supporting the severe gust potential. The hail size is probably reduced by limited CAPE and relatively low cloud tops.