Storm Forecast
Valid: Sat 20 Aug 2011 06:00 to Sun 21 Aug 2011 06:00 UTC
Issued: Sat 20 Aug 2011 00:06
Forecaster: VAN DER VELDE
A level 1 was issued for the western Iberian Peninsula mainly for isolated large hail and severe wind gusts.
SYNOPSIS
The low pressure area responsible for an outbreak of thunderstorms along its cold front over central parts of Europe the past two days is occluding and all CAPE seems to have disappeared except near its center over the eastern Baltics/N Belarus area. A rather intense wind maximum (without convective storms) develops at the back-bent occlusion over/near Lithuania. Meanwhile, over southwestern Europe, a southerly flow between a Mediterranean ridge and an Atlantic trough advects hot air into the Iberian Peninsula and France. Mid level lapse rates (2-4 km) have become very steep over a large area, creating CAPE almost anywhere in SW and S Europe, but in many places to the east also provides a cap.
Warm air elevated convection may develop along a front around the UK, mostly over the North Sea in the next early morning.
DISCUSSION
...western Iberian Peninsula...
There is around 10-15 m/s bulk vertical shear predicted between 0-6 km, which is not so much, but sufficient for multicell organization with somwhat enhanced potential for large hail. This potential is further enhanced by shallow warm cloud depth (a cold LCL around 2500-3000m). Lifting factors look fine today with advection of potential vorticity, although quasi-geostrophic factors can be limiting: the real QG lifting should occur on the trough which remains offshore from Portugal for most of the period. This is also the area where shear (both deep and low-level) and helicity increase rapidly and eventually are predicted in coexistence with CAPE over W/SW Portugal. After 21Z, Portugal may see a break of the strong cap or a rooting of initially elevated CAPE to surface-based CAPE, however, GFS and WRF models predict storms here with a lot of difficulty. If however storms do develop, they may produce severe wind gusts due to fast storm motion and still rather high LCL (1500-2000m) while the strong low level shear (10-15 m/s) and SREH (250 mē/sē) can enhance both bow echo and tornado potential. The relatively high LCL would work against tornadoes though, and the overall threat is very conditional given the low confidence level at this point regarding initiation.
