Storm Forecast
Valid: Sat 07 May 2011 06:00 to Sun 08 May 2011 06:00 UTC
Issued: Sat 07 May 2011 05:35
Forecaster: VAN DER VELDE

A level 1 was issued for UK and western France mainly for some chance of tornadoes.
A level 1 was issued for eastern Spain mainly for some chance of tornadoes, large hail, and excessive rain.
A level 1 was issued for northern Algeria mainly for large hail.

SYNOPSIS

A large low pressure area centered west of Ireland covers Iberian peninsula, France, British isles and Benelux. A warm and unstable airmass extends northward beyond Scotland with here and there embedded thunderstorms. A satellite upper shortwave trough swings over southeastern Spain, Balearic islands, and southern Mediterranean Sea and has an associated strong dynamic tropopause anomaly and QG lifting signature. Another major low is situated over Romania and surroundings, with the strongest lifting and instability on its south side.

DISCUSSION

...UK and western France...

While some areas will be capped, cloudy, or having only elevated CAPE, some parcels will likely be rooted in the boundary layer and experience quite reasonable vertical shear and helicity values. GFS predicts about 20 m/s 0-6km shear vectors, 200 m²/s² 0-3 km SREH and in the central UK around 12 m/s 0-1 km shear, which combined with a few hundred J/kg CAPE and mixed layer LCLs of 600-800 m hints at a chance of tornadogenesis. Two PV maxima pass over the southern UK with ascending motions, during morning and evening. During the evening pass the shear conditions seems best and there is a significant wind maximum near the dynamic tropopause level. As LCLs and CAPE are low, large hail isn't as likely, but cannot be ruled out given the chance of mesocyclones.

...eastern Spain...

Deep layer shear of 20-30 m/s and SREH of 200 but perhaps locally 400 m²/s² may overlap with some CAPE, though it is likely that most CAPE is outside the highest shear conditions. The southernmost section features more than 1000 J/kg MLCAPE and higher LCLs. Large hail is there the primary threat. More to the north, around the Ebro delta, near the surface vorticity maximum, tornado chances are somewhat elevated. In fact GFS 00Z very locally predicts overlap of CAPE and SREH in that area for the late afternoon/evening.
Large onshore moisture transport and lifting in Catalonia can lead to large rain sums, but since CAPE isn't really predicted so far north, it will be mostly non-convective. Strong, perhaps severe convective gusts may occur in southern Catalonia. Everything considering CAPE is rather weak, though, but forcing and shear strong.

...northern Algeria...

Strong convergence, some CAPE and 200-300 m²/s² SREH and 20 m/s DLS can result in rotating storms here, although the dominant convective mode may become linear easily, due to shear vectors parallel to the convergence zone.

...eastern Bulgaria...

This is another area with very slight CAPE which around 18Z can see elevated shear values (20 m/s DLS, 200 m²/s² SREH) with shear vectors perpendicular to the linear forcing. Marginally large hail may result locally. No threat level issued.