Storm Forecast
Valid: Sat 09 May 2009 06:00 to Sun 10 May 2009 06:00 UTC
Issued: Fri 08 May 2009 23:34
Forecaster: GROENEMEIJER

A level 1 was issued for central Germany mainly for the severe wind, large hail and tornado threat.

A level 1 was issued for SE Germany, Austria, Slovenia, W Hungary and N Croatia mainly for large hail.

A level 1 was issued for central and N Spain, and for SW France mainly for large hail.

A level 1 was issued for the central Ukraine mainly for large hail and heavy rainfall.

SYNOPSIS

Between a mid/upper-cyclone over the Norwegian Sea and a broad ridge stretching from the central Mediterranean to western Russia, the flow over most of the continent is from the southwest. Another closed low, to the southeast of the ridge, is located over the Sea of Azov and moves to the southeast. A frontal zone is located from western Finland across the Baltic States to central France to NW Spain. A frontal wave develops in the left exit region of a south-westerly speed max over Iberia, and translates to Germany during the afternoon and evening.

DISCUSSION

Central Germany...

Northward warm air advection will occur ahead of the surface low at the tip of wave, and there will be an associated destabilization. GFS/ECMWF forecast that 500-1000 J/kg of CAPE will develop in its warm sector, and that, near the wave's centre, strong wind shear will be present. In fact, GFS simulates 0-3 km shear in excess of 15 m/s across quite a large area. Furthermore, along the warm-front section of the wave, rather high storm-relative helicity of 200-300 m2/s2 should occur. Some storms will likely develop in the warm sector, although their coverage remains a big question. Storms that do form in this environment could well turn into supercells, capable of producing large hail, severe winds, and possibly a few tornadoes. Some tornado threat is froecast because of the sufficient low-level wind shear, and relatively moist boundary layer. Tornadoes would be most likely directly near the warm frontal zone, where storm-relative helicity would be the highest. At this time it is quite unceratin when and where storms develop near the system: this could be over NE France, but also over central Germany or not at all. If more than a few isolated storms form, the severe weather coverage could require a level 2, but we will refrain from doing so at this moment. An upgrade could be needed.

SE Germany, Austria, Slovenia, W Hungary and N Croatia...

Shear is forecast to be moderate across most of the area with about 10 m/s of bulk shear forecast in the 0-3 km layer. However, decent amounts of CAPE are forecast, on the order of 1000 J/kg both north and south of the Alps. An attendant threat of large hail is forecast. Model guidance suggests that the storms will be rather isolated and will weaken during the evening.

Central and northern Spain, and SW France...

Scattered storms are expected to develop within an environment with moderate shear. Weak CAPE limits the severe threat, so that a level 1 is not issued.

Central Ukraine...

Withn a zone of moderate shear, considerable cape is expected to develop, i.e. 1000-1500 J/kg, suggesting a large hail threat withany storm that forms. Models are in agreement in developing large amounts of convective precipitation, too. This, in combination with a rather weak suggest that a heavy rain threat exists here as well.