Storm Forecast
Valid: Tue 21 Jun 2011 06:00 to Wed 22 Jun 2011 06:00 UTC
Issued: Mon 20 Jun 2011 22:00
Forecaster: SCHLENCZEK
A threat level one was issued for the eastern / northeastern parts of Spain and S-central France mainly for large hail, severe wind gusts and excessive precipitation.
A threat level one was issued for the Alpine region mainly for large hail and isolated severe wind gusts.
SYNOPSIS
A southwesterly upper streamline pattern with a 30 - 45 m/s jet stream at 300 hPa has established over western Europe with a cyclonic vortex southwest of Ireland. Another low pressure system over the N Baltics moves towards NW Norway. Unstable air is present ahead of both systems and also along / in the wake of the very long frontal boundary which stretches from the Azores via extreme NW Iberia and central Germany towards SE Romania and all the way up towards Finland. Some diurnal convection may occur over the Mediterranean region, e.g. central and southern Italy. As the subtropical jet stream is shifted very far towards the southeast, anything that will develop on the warm side of the frontal boundary will stay in a rather weakly sheared environment.
DISCUSSION
GFS shows some signals for low to moderate instability in a strongly sheared environment. Those regions with the greatest potential of severe convective weather are discussed below:
...E Iberia, France...
Although there is little QG forcing to expect within the whole level 1 region, low to moderate instability in connection with 10 - 15 m/s of 0-3 km shear, respectively 15 - 20 m/s of 0-6 km shear should provide enough potential for some severe convective storms. The front-normal component of the upper level winds are very marginal and therefore it is likely that organised multicells tend to merge into linear systems. Locally enhanced 150 - 250 mē/sē of SRH3 may support a few mesocyclonic storms. Isolated large hail is possible as well as severe wind gusts due to locally strong vertical gradients of Theta-E and 10 - 15 m/s background flow at 700 hPa. This pattern is not supportive for tornadoes as LCL / LFC heights are around 1,5 km or higher in most places and LL shear as well as 0-1 km SRH are low. Locally, an isolated heavy rain / flash flood event is not ruled out but effective PWAT from GFS does not give much support for a significant heavy rainfall threat. The probability of severe weather should be maximised shortly after initiation when storms are mostly discrete.
...Alpine region...
Some hundred J/kg CAPE and 15 - 20 m/s deep layer shear partly overlap over the Alpine region. Towards the Czech Rep., instability decreases but shear increases to about 25 m/s which should be supportive for well-organised multicells and / or a few short-lived supercells with marginally severe hail as main threat. Overall threat should be lower than in the regions further upstream, but a level 1 should be warranted.
...Other regions...
Isolated to scattered thunderstorms are expected in the other thunderstorm regions. A marginally severe hail event is not ruled out but overall potential of severe weather is too low for a threat level.
