Forecast Update
Valid: Mon 11 May 2009 16:00 to Tue 12 May 2009 06:00 UTC
Issued: Mon 11 May 2009 16:06
Forecaster: GROENEMEIJER / PUCIK

A level 2 was issued for SW and central France mainly for large hail, tornadoes, severe winds and extreme rainfall.

A level 1 was issued for an area surrounding the level 2, across W France, a larger part of S France, N Spain, NE France, central and S Germany mainly for large hail, and severe winds and to a lesser extent for tornadoes.

A level 1 was issued for N Switzerland, N Austria, parts of the Czech republic, Slovakia, S Poland and W Ukraine mainly for large hail and severe winds.

SYNOPSIS

Refer to the convective forecasts...

DISCUSSION

SW and central France...

SBCAPE values of 1500-2000 J/kg have developed across SW France, where strong deep-layer shear on the order of 25 m/s in the 0-6 km layer is present. Along the warm front, which stretches from Bordeaux to Dijon, some 200 m2/s2 of 0-3 km storm-relative helicity should be present, and possibly somewhat more just north of the front.

Convective storms are now intiating near the coast of SW France. It is expected that these will include a number of supercells. Those storms will probably produce large hail and possibly giant hail with sizes of 5 cm or more. Additionally, damaging wind gusts are expected. One or two tornadoes are also possible. It is expected that the storms cluster and will translate northeastward along the warm front, possibly in the shape of a bowing convective system. The main threat of the system should then shift away from hail to severe winds. The threat of a few tornadoes will however persist. Some back-building of the system may however also occur, so that a threat of extreme convective rain will also exist in and around the Regions of Poitou, Pays de la Loire, Centre, and Limousin.

NE France, central Germany

Although deep-layer shear (0-6) km is with about 15-20 m/s, most of this shear is concentrated in a shallower 0-3 km layer, which has proven to be sufficient for the development of a couple of supercells. Along and just north of the surface warm front, which stretches from near Saarbrucken to Nuremberg, shear in the 0-3 layer is strongest, and -- given that strong low-level buoyancy should be in place here -- one or two short-lived tornadoes cannot be excluded. Otherwise, the main threat will be large hail and perhaps a few severe wind gusts.

S Poland, Slovakia

A complex of thunderstorms is currently moving NE wards across S Poland and N Slovakia. The complex has a history of producing hail and sub-severe wind gusts. Moderate instability and moderate deep-layer wind shear (15 m/s), along with a belt of enhanced SREH on the convergence line suggest that large hail will continue to be possible for a couple of hours, before instability decreases after sunset.