Storm Forecast
Valid: Wed 08 Dec 2010 06:00 to Thu 09 Dec 2010 06:00 UTC
Issued: Wed 08 Dec 2010 03:54
Forecaster: PUCIK

A level 1 was issued for Southwest Iberia mainly for excessive precipitation, tornadoes and to the lesser extent for marginally severe wind gusts.

SYNOPSIS

At midlevels, the dominant feature will be a positively tilted trough, stretching from Scandinavia to Western Iberia, with strong flow surrounding the trough. As a ridge stretches from Atlantic eastwards during the day, cut off process will begin west of the Iberian penninsula. The trough over Scandinavia will start amplifying towards south and then southeast, under the influence of a northerly 45 m/s jet-streak in the rear flank of the trough. Otherwise, most of Europe will be under a prevailing southwesterly flow.

Closer to the surface, under steering southwesterly flow, a deep surface low will cross the Bay of Biscai and start slowly filling over France, moving northeastwards. By the evening hours, when the center of low will have reached Poland, cyclogenesis is forecast again, in the right entrance region of a jet-streak at 300 hPa. This low will be accompanied by a strong frontal system, namely a rapidly advancing cold front that will cross most of Western Europe, Germany and western Poland by Thursday morning. Behind the front, an advection of arctic airmass will begin, on the forward flank of the large high pressure system which will be placed over Atlantic. Further cyclogenesis is forecast on the southern part of the Alpine range. Southwestern, Southern and Southeastern Europe will be under a warmer airmass.

DISCUSSION

... Southwestern Iberia ...

The situation from the previous day will persist with a gradual decrease in the degree of instability over the inland areas. Nevertheless, more than 500 J/kg of CAPE are possible over the southwestern parts of the penninsula and this in a strong unidirectional wind shear regime. DLS values will likely reach 25 m/s while shear in the 0-3 km will range from 15-20 m/s. Moreover, with stronger ageostrophic components of the flow inland in the boundary layer, enhanced SREH values are simulated. No obvious synoptic scale forcing is apparent from the mid and upper levels of troposphere but abundant storm initiation is simulated, probably due to the low level convergence. Well organised storms are possible, including some low-topped supercells. Small hail, marginally severe wind gusts will be possible with the stronger cells. Moreover a "training" cell pattern can arise, especially in the coastal areas with threat of locally excessive rainfall. With low LCLs and enhanced wind shear at the lowest 1 km, tornadoes are not ruled out either, especially if some isolated rotating storms manage to persist.