Storm Forecast
Valid: Sun 09 Aug 2009 06:00 to Mon 10 Aug 2009 06:00 UTC
Issued: Sat 08 Aug 2009 22:14
Forecaster: SCHLENCZEK
A level 1 was issued for eastern Spain mainly for large hail and severe gusts and to a lesser extent for excessive rainfall.
A level 1 was issued for eastern France and southwestern Germany mainly for large hail and excessive rainfall.
SYNOPSIS
On Sunday, most parts of Europe will remain in an environment with weak pressure gradients. A weakening upper trough over W / SW Europe will slowly shift eastward during the period. Ahead of this trough, a 30 m/s upper jet streak stretches from N Morocco towards NE Spain. Further north, a shortwave trough over N-central France is forecast to move northeastward into the Netherlands / NW Germany during the period. A ridge over the C Mediterranean stretches northward into E-central Europe and Scandinavia. Severe thunderstorms should be most likely over E Spain.
DISCUSSION
...E Spain...
In the vicinity of the upper jet streak, very warm and moist air is advected northward and steepening mid level lapse rates as well as LL moisture advection will result in about 1500 J/kg MLCAPE. Deep layer shear is forecast to strengthen during the afternoon and should be in order of 25 m/s in most portions of the level 1 area. In the southern part of this area, the airmass should be strongly capped but the approaching shortwave trough may provide enough lift for initiation. Most of the isolated storms that develop should be well-organized multicells and a few supercells that may produce large or even very large hail. In the northern part of the level 1 area, the cap is much weaker and convection can become more widespread - discrete multicells may merge into one or two MCSes with isolated large hail and severe gusts as main threat. Where storms persist for some hours, torrential rainfall may lead to flash floods. Although very large hail is possible in the southern part of the level 1 area, storm coverage is probably too low to issue a level 2.
...E France, SW Germany...
Diurnal heating may create up to 1500 J/kg MUCAPE in this region during the day. According to recent satellite imagery and GFS, a belt from S France into SW Germany and S Belgium should be almost cloud-free in the early morning. Thunderstorm activity should depend much on insolation and the region mentioned above will provide good conditions for some organized multicells and perhaps also a few supercells as deep layer shear will increase to about 15 - 20 m/s. Main threat should be large hail with the strongest storms. Thunderstorm activity should decrease rapidly in number and severity during the evening / night hours.
...Central Germany...
Although there is much of mid and high level cloudiness, there should be up to 1 kJ/kg CAPE over central Germany in an environent with deep layer shear at or below 10 m/s. Near the convergence line that stretches from Hamburg towards Stuttgart, thunderstorms should develop in the early afternoon. Most storms should only be briefly organized but some multicells may become almost stationary, leading to excessive rainfall. At this moment, predictability is too low to issue a level 1.
