Storm Forecast
Valid: Wed 26 Aug 2009 06:00 to Thu 27 Aug 2009 06:00 UTC
Issued: Tue 25 Aug 2009 23:27
Forecaster: KOROSEC
A level 1 was issued for W Mediterranean, coastal areas of southern France and SE Spain mainly for excessive rainfalls and large hail.
SYNOPSIS
An upper trough over SW Europe slowly moves NE-wards while transforming into a weak upper level cut off low by the end of forecast period. The British Isles are affected by the remnants of extra tropical storm Bill in the morning hours as quite deep cyclone races through. Another short wave trough crosses Baltic sea during the afternoon hours. At surface, a weak front is extending from Scandinavia into central Europe and convective activity is expected along this boundary. A weak surface cyclone along the Spanish and French coast brings another day for severe weather threat.
DISCUSSION
...W Mediterranean, coastal areas of southern France and SE Spain...
Weakening surface low along the Spanish coast will slowly move NE-wards and continue with the risk for severe storms in the western Mediterranean. At least weak to moderate shear up to 15m/s in 0-6km layer will gain storm organization. In response to inflow of moist air from the Mediterranean, a considerable high amount of CAPE could become available and therefore a few large hail events may occur. Given the rather weak storm motion, an excessive rainfalls and locally flash flood risk exists. The threat level was extended a bit more to the east towards Corsica as a few severe storms could form with a risk for excessive rainfalls and large hail there as well.
...SE Germany, Czech Republic, Poland...
As the short wave trough crosses this area in the afternoon, convective activity is expected along the cold front. At least few hundreds of CAPE will be available and along with weak deep layer shear, some organized storms with threat for large hail and severe winds could form. Later on, GFS model simulates a cluster of storms in the evening hours/early night over east-central Poland, but that is only partly supported by other models. If this cluster/MCS manages to form, a local risk for flash flooding and some strong gusts will exist. But only weak shear and low probabilities do not warrant a threat level at the moment. A few storms are likely also across the southern Scandinavia around the vorticity maxima crossing Sweden and southern Finland during the day.
...British Isles...
Rather deep cyclone with a cold front rapidly crosses British Isles during the morning hours. Models simulate an impressive SREH and shear values along the warm front, but there are almost no signals of instability present. If at least some CAPE becomes available and convective cells form, a tornado event cannot be completely ruled out given the very strong LL shear (10-15m/s) and SR helicity (>500m^2/s^2) in place. A few convective storms are possible more to the south along the cold front where some CAPE could release.
... Alpine area, N italy, S Italy...
Mostly diurnal driven showers and thunderstorms are expected, as at least moderate CAPE will become available. Some storms will gain organization in the weakly sheared environment. Therefore a local large hail or severe gusts are possible. Slow moving storms could also support local flash flooding. Storms will mostly diminish after sunset, while some early night activity is possible around the northern Adriatic where models still holding high amounts of CAPE and precipitation.
