Storm Forecast
Valid: Sat 14 May 2011 06:00 to Sun 15 May 2011 06:00 UTC
Issued: Sat 14 May 2011 05:25
Forecaster: VAN DER VELDE
A level 1 was issued for parts of Portugal, Spain and the Alpine region mainly for large hail.
SYNOPSIS
The dominant system of interest on the map is a strong cold front, most pronounced at mid levels. It belongs to a depression near southern Norway and is pushed by a steady northwesterly flow into western Europe. Atlantic high pressure is strong and builds behind the cold front. A jetstream runs parallel over the frontal region and creates 15-25 m/s deep layer shear in the region with typically 500 J/kg MLCAPE in most areas before the front. Almost everywhere along the front CAPE density is actually predicted weaker in the mixed phase region than at lower levels.
DISCUSSION
...southern/eastern France...
At the moment storms are ongoing over southern France in GFS-indicated strong mid level cold air advection area. As more surface-based CAPE becomes available during the day, storms can profit better from the shear, but shear is aligned with the frontal convergence zone and this provokes more linear convection than isolated cells. The most curvature in the hodographs is found over (north)eastern France, where SREH is also calculated to be more than 250 mē/sē. However, the poor shear angle might destroy the environment, where LCL is also calculated to be low, yielding too much warm-cloud water content. So large hail seems only marginally possible. Near the Alps, upslope moisture lifting combined with convection could result in flash floods.
...southeastern Germany, Austria/Slovenia....
In this area SREH and LCL are higher and the frontal zone less defined with shear vectors also under a larger angle than in France. Large hail is the primary threat. To the south, shear is weaker, but combined with orographic effects, an isolated rotating updraft may occur and produce hail.
...northwestern Italy...
GFS predicts for the night after 00Z when the front and massive QG lifting arrive a CAPE and precip signal in the lee of the Alps. A Genua low pressure area will develop. Bulk shear can reach 30 m/s or higher locally and 1-4 km shear vectors appear unusually large. A supercell may develop and dump large hail, and may also grow into an MCS. Low-level shear is not simulated to be large enough for tornadoes.
...Portugal and Spain...
A rich overlap between 15-25 m/s 0-6 km shear and MLCAPE over northern Portugal and central and northeastern Spain and somewhat helical hodographs (SREH around 150-200 mē/sē) show good potential for supercells, with large hail as largest threat. The shear vectors are aligned with the west-east oriented frontal gradient, so this is not the favorable setup for discrete cells but rather a tendency for linear mode. In northeastern Spain the shear is weaker but strong and deep convergence in the lee of the Pyrenees will be present, and orography could further enhance shear locally. In this area strong lifting from the PV anomaly will arrive during the evening and GFS simulates a remarkable warm theta-e island in the wake of the Pyrenees during the night, collocated with strong QG lift and elevated instability signals. Large hail may again occur.
The southwestern peninsula has over 1000 J/kg MLCAPE in GFS but less shear and forcing, nevertheless a chance of large hail and a local severe wind gust exists.
