Storm Forecast
Valid: Sat 03 Jul 2010 06:00 to Sun 04 Jul 2010 06:00 UTC
Issued: Sat 03 Jul 2010 04:58
Forecaster: VAN DER VELDE
A level 1 was issued for N Spain, France, E Belgium, E Netherlands, W Germany for large hail, severe wind gusts and excessive convective precipitation.
A level 1 was issued for N Morocco mainly for large hail and gusts.
A level 1 was issued for S Belarus, N Ukraine mainly for excessive convective precipitation.
SYNOPSIS
A frontal zone and plume of warm, moist and very unstable airmass is stalling over a region from the Pyrenees to western Germany, into Sweden. Other systems of interest are the upper cut-off low over W Ukraine/Romania area, and a low pressure area over Morocco.
DISCUSSION
...N Spain, SW France...
In this area more than 1000 J/kg MLCAPE is present, some QG lifting from a small mid level trough, slightly enhanced SREH3 > 100 mē/sē, and moderate 1-4 and 1-8 km shear vectors. Well-organized multicells and some rotating updrafts are possible and the primary threat is large hail.
Convection can continue into the night on the north side of the Pyrenees where NW winds blow the moist air back upslope. Storm motion should be slow and a local flash flood is possible.
...central France...
This region should have 1000-2000 J/kg CAPE, but relatively weak shear (<10 m/s 0-6 km). However, storm motion will be the slowest, with some clustering, and LCL heights the lowest (<1000m), with an enhanced threat of local excessive convective rainfall (flash floods). A large difference between the maximum and minimum theta-e in the column (delta-theta-e), of 20-25° indicates enhanced potential for severe convective gusts via evaporational cooling and large precipitation mass.
...NE France, E Belgium, W Germany...
As previous area with respect to large delta-theta-e, but here deep layer shear is 10-15 m/s. This could enhance potential for a squall line or bowing segments with severe gusts, as well as large hail. The multicell storm motion seems parallel to the stationary front and shear vector direction, which is not too favorable for an intense squall line, though. Hodographs are more circular in western and northern parts of this area, which should enhance large hail from rotating updrafts. But the area is weakly capped and the western half is in low level cold air advection, a chance exists that after early storms new convection struggles, but the mixing ratio (and dewpoints) are high, 14-15 g/kg averaged over 1000 m in GFS.
...S Belarus, N Ukraine...
A shortwave trough passes through an area with unstable air, reasonable deep layer shear, and very slow storm motions, which with a LCL under 1000m could lead to efficient precipitation production and local flash floods.
...Morocco...
Indications in GFS are that the instability is elevated over a large area, but weakly capped. In the western section also surface-based. However, very good wind shear and circular hodographs are in place in the area. It is suspected that the elevated convection cannot profit from the helicity (400 mē/sē) over NE Morocco... but if it happens supercells or MCSs can occur with large hail and severe gusts.
