Storm Forecast
Valid: Wed 13 May 2009 06:00 to Thu 14 May 2009 06:00 UTC
Issued: Wed 13 May 2009 00:40
Forecaster: PUCIK

A level 1 was issued for E Spain and S and W France mainly for large hail and excessive precipitation.

A level 1 was issued for SW Germany and W Austria mainly for large hail.

SYNOPSIS

A significant short wave trough with has established over E Europe ( its axis stretching from E Scandinavia into Ukraine) whilst undergoing a cut-off process. Strong mid-level flow is surrounding it, reaching 30 m/s at 500 hPa level. To the west, a large ridge covering the area from the Central Meditteranean / Central Europe to the Norway Sea will slowly weaken with time. Meanwhile, another trough will affect the weather across W Europe, slowly moving eastwards. At the surface, a low pressure system will deepen over E Russia with a strong baroclinic zone associated with it. Looking to west a high will have settled over the Norway Zone and a shallow trough is forecast from E Atlantic across France, Spain to N Africa. Slight cyclogenesis is anticipated over N France by Thursday morning. The main frontal zone will be running from E Spain to France, S Germany, Austria, Bulgaria, Black Sea and then curving northwards toward the deep low over E Russia.

DISCUSSION

... Spain / France...

Ahead of the frontal zone a well mixed airmass airmass with steep lapse rates will quickly destabilize with the commence of the day time heating. GFS is quite aggressive with the instability release, having more than 1600 J/kg of MLCAPE for S France region . ECMWF exhibits lower values so that if both of the model results are considered , local values of 1000 J/kg might realise, but MLCAPE > 1500 J/kg seems to be unlikely. With aid of the QG forcing ahead of the trough, thundestorms will easily initiate. As of Wednesday 0Z, satellite and radar measurements are indicating that thunderstorms are already forming, so we can expect some of them to persist till the morning and their outflow boundaries might serve as a focus for the redevelopment of thunderstorm activity. Storm coverage is expected to greatly increase by afternoon hours and will probably persist into the night hours.

Region enclosed by a Level 1 line will lie under the belt of enhanced wind shear, reaching 15 - 20 m/s in 0-6 km layer and more than 20 m/s in 1-8 km layer. Shear in the lower troposphere will be less pronounced, but 10 m/s in 0-3 km layer are still simulated by the models and a well organised multicells can be expected with the threat of isolated large hail. Spain exhibits slightly higher values of shear than France but lower instability in the model runs ( generally over 500 J/kg of MLCAPE), so large hail threat will be probably less than across S France but still present, warranting Level 1 also for this area.

Moreover, high values of Effective precipitable water are simulated over S France along with a slow movement of thundestorms. Strong convergence is expected and MCS might form during the late afternoon hours, moving northwards. On top of that, several rounds of thunderstorms are quite possible during the forecast period and an excessive precipitation amounts with some local flash flooding are possible.

.... SW Germany / W Austria...

A very similar situation as in case of France will occur over this region with moderate values of MLCAPE expected to form as result to the day time heating. Wind shear will be quite high with more than 15 m/s of DLS over the whole area, but enhanced values are simulated over Austria - more than 20 m/s so chance of a supercell will be higher here. GFS has in more subsequent runs shown the belt of enhanced SREH, which could aid in the overall storm organisation. Therefore, isolated large hail might occur with the stronger ( or rotating) cells. In case of mountaineous areas, especially over Austria, excessive amounts of precipitation might be recorded, especially if thundestorms train over the same spot. Nevertheless, with quicker storm propagation speeds, expected forward building of the potential complex and only moderate precipitable water values, this threat will not be very pronounced.

....Bulgaria...

Moderate values of instability ( MLCAPE over 800 J/kg) are simulated also here, but the overlap with at least 15 m/s of DLS is only meager and the best potential for the large hail will exist to the south ( towards Greece), where, however, no storm initiation is predicted. Therefore, we will stay with Level 0 for this area, but it will be closely monitored during the day and a Level 1 might become necessary due to the large hail threat.