Storm Forecast
Valid: Sat 03 Sep 2011 06:00 to Sun 04 Sep 2011 06:00 UTC
Issued: Sat 03 Sep 2011 01:13
Forecaster: VAN DER VELDE
A level 2 was issued for northern Algeria and Tunisia mainly for large hail and severe convective wind gusts.
A level 1 was issued for western Russia, parts of Finland, Estonia and Latvia mainly for local excessive convective precipitation and for spout-type tornadoes.
SYNOPSIS
Between a ridge over central Europe and a trough over western Europe a southerly flow advects warm and unstable airmass northward. Widespread thunderstorms can develop. A depression and upper cut-off low over Spain push a sharpening cold front into the western Mediterranean, where airmass contains 1000-2000 J/kg CAPE.
DISCUSSION
... northern Algeria and Tunisia ...
GFS simulates increasing CAPE here, but significantly capped for surface-based convection over sea. Over land, storms may become surface-based and tap also into the helical inflow in the lowest kilometers. Model hodographs are also curved in mid levels so even elevated convection may be well-organized or rotating. Near the North Africa coast, more than 350 mē/sē of 0-3 km SREH and 30 m/s 0-6 km shear should be present (of which not everything will be 'effective' in case of elevated storms). A potential vorticity lobe will push into the region and trigger storms (supercells and multicell/MCS) with threats of (possibly very) large hail and severe wind gusts.
... western Russia, parts of Finland, Estonia and Latvia ...
Very slight storm motion is forecast with as result the isolated occurrence of heavy or excessive convective precipitation. Spout-type tornadoes are likely in the weakly sheared, low LCL, moist environment allowing for undisturbed spin-up of vertical vorticity, mainly in convergence zones.
... Spain, France, Belgium/Netherlands ...
Heavy rain may occur near the coastlines of northeastern Spain and southern France (orographic lifting and convection), as well as in northern France, western Belgium/Netherlands (convection in very humid environment with large precipitable water content and relatively slow storm motion, particularly during the night). No levels were issued for these because persistent occurrence of storms seems low, forcing seems rather weak, and mesoscale models do not produce that much rain (more typically 20-50mm).
