Storm Forecast
Valid: Wed 31 Aug 2011 06:00 to Thu 01 Sep 2011 06:00 UTC
Issued: Wed 31 Aug 2011 01:08
Forecaster: PUCIK

A level 1 was issued for Northeastern Spain and Southern France mainly for large hail and severe wind gusts.

SYNOPSIS

At mid and upper levels of troposphere, two cyclonic vortices will be influence the conditions, one of them being centered just west of the Iberian penninsula and the other stagnating over Scandinavia, with several vort maxima rotating around each of them. Between these two features, a ridge is simulated, being connected to the high geopotentials over the Central Mediterranean. An insignificant trough stretches from the Scandinavian cyclonic vortex all the way into Southeastern Europe. Closer to the surface, situation is similar with two lows placed under the vortices mentioned above. The main frontal zone will be placed over the Iberia, stretching then across Central France into Southern Germany and further eastwards, where, from Ukraine will curve northwards.

DISCUSSION

... Northeastern Spain ...

Both simulations of ECMWF, GFS and rawinsonde measurements from Tuesday 12 UTC from the region suggest the presence of moderate to strong latent instability, resulting from high mid-level lapse rates. The formation of shallow surface low over the Balearic Sea, will likely result in easterly flow component in the region, enhancing the uplift and low level convergence. Albeit CIN, especially close to the coastline, might be high (exceeding -100 J/kg), at least isolated convection might be observed further inland. With moderate windshear, reaching 15-20 m/s in 0-6 km layer, and steep lapse rates, well organised convection including supercells posing threat of isolated large hail or downbursts is possible. Due to the overall threat that might be posed by the convection a low-end Level 1 is introduced despite the initiation uncertainities.


... Parts of Southern France ...

A combination of moderate degree of instability and moderate values of wind shear (with the highest degree of both in the extreme southeastern part of France) might prove to be conducive for a few well organised multicells / brief supercells posing a threat of isolated large hail or marginally severe wind gusts also thanks to the high lapse rates. The conditions should be less favorable towards northwest of the Level 1 area, but at this location, forcing should prove to be most prominent and the storm coverage likely the highest. Threat will be most significant during the late afternoon and evening hours.