Storm Forecast
Valid: Tue 01 Sep 2009 06:00 to Wed 02 Sep 2009 06:00 UTC
Issued: Tue 01 Sep 2009 05:32
Forecaster: VAN DER VELDE
A level 1 was issued for NE Spain mainly for large hail and isolated severe wind gusts
A level 1 was issued for France mainly for a chance of excessive precipitation, and hail in S/E parts
A level 1 was issued for W Germany for a chance of excessive precipitation.
SYNOPSIS
Today we see a large Atlantic trough moving over Ireland and UK to Scandinavia. Steep height gradients reach as far outward as central France and NW Germany. The center of the trough will see a lot of QG lift and slightly unstable airmass, likely causing thundery convection. A cold front stretches between N Spain and S Scandinavia, with a moisture tongue of some 12 g/kg overspread by steep mid level lapse rates of 7-8°/km generating CAPE over 1000 J/kg, especially in the N Spain, S France area. The jet resides mostly on the cool and stable side of the front. Initially, the flow and shear vectors will be rather parallel over the front, but GFS predicts eastward acceleration during the early evening. The front then will be found over Poland by 6Z the next day, and remains dragging over southern France and the Alps.
The Black Sea and S Ukraine deal with an upper cut-off low and unstable conditions.
DISCUSSION
...N Spain....
Weakly capped, high-based (>2000m) convection will likely start early over Sistema Ibérico and Pyrenees and will subsequently cross the Ebro valley, possibly clustering into 1-2 MCSs. 15-20 m/s DLS and 100-150 m˛/s˛ SREH make long-lived multicell and also supercells possible, with a good chance of large hail. Any supercell should move due east by Bunkers storm motion. Delta-theta-e enhanced to more than 20K increases chances of strong or locally severe downburst winds, but weak synoptic forcing should limit that threat.
...S and E France...
Supply of warm moist air into the frontal convergence zone creates a CAPE maximum (up to 2000 J/kg) and with some influences from the trough a relatively early MCS or repeated storm developments could move along the front. This should result in locally large precipitation sums / flash floods and repeated convection can significantly moisten and neutralize the profiles. Effective PW is large, around 35 kg/m˛, and wind profiles may support backbuilding storms along the front. To the southeast of the front, the CAPE-shear-SREH combination allows for supercells with primary threat of large hail, if storms develop there. This is a point of uncertainty, because in the unmodified airmass GFS predicts a rather steep transition from moist and more neutral to capped conditions, not surprising with such steep lapse rates on top. Over E France and W Switzerland, LLS is predicted over 12 m/s along with decent DLS, and one could start thinking of a tornado or MCS bow echo chance. Line-parallel shear and -advection of unstable air, weak cold pool and linear forcing could cause a potentially long-lived PS-MCS with large rain sums over E/SE France during the night.
...Germany...
A narrow strip of unstable air with very low LCL over the west, while the rest is quite dry and capped. Slow passage of the front with parallel shear and storm motion with low LCLs are favorable for some storms with high preciptitation efficiency.
SREH is >250 m˛/s˛ over N Germany, CAPE weaker, and a strong deep convergence is predicted by GFS. It may imply a wave in the frontal zone perhaps with associated MCS. However, a narrow tongue of weak CAPE seems not too likely to sustain this for long, limiting its wind threat.
