Storm Forecast
Valid: Wed 21 Jul 2010 06:00 to Thu 22 Jul 2010 06:00 UTC
Issued: Tue 20 Jul 2010 23:12
Forecaster: PUCIK

A level 1 was issued for Northern Spain mainly for large hail and severe wind gusts.

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

A level 1 was issued for Central France to Northwestern Germany mainly for severe wind gusts.

SYNOPSIS

At midlevels, a large trough will reside over the Eastern Atlantic. On the forward flank of this feature, strong flow, reaching 20-25 m/s at 500 hPa will be observed. To the east, mostly insignificant pool of higher geopotentials will be present.

At the surface, a shallow occluded surface low will be centered over the British Isles by Wednesday morning, with its center shifting towards Scandinavia. A cold front will progress across France, Western Germany and Northern Spain during the forecast period. Ahead of it, a modified warm airmass will lie over most of Europe. With abundant daytime heating, airmass will become conditionally unstable over much of Eastern, Central and parts of Southern Europe.

DISCUSSION

... Northern Spain ...

A well defined frontal boundary will develop over this region during the day with a hot airmass being advected from Northern Africa ahead of the cold front. This hot airmass is characterised by large CIN values, which will decrease towards the cooler airmass behind the front. Models resolve slight to moderate destabilisation and MLCAPE values should exceed 1000 J/kg locally. Weak synoptic scale forcing due to DCVA is also forecast. With strong flow at midlevels and backing surface winds ahead of the front, moderate to strong DLS is forecast. Values will, especially towards the evening, likely exceed 20 m/s. Moreover, enhanced SREH values, reaching 150 J/kg in the 0-3 km layer, might add to the degree of the storm organisation.

Even though, initiation is questionable, especially towards the east, where an airmass with high CIN values will be present, at least isolated, well organised storms are expected, possibly including some supercell. Large hail or severe wind gusts might occur with stronger cells.

... Southern France to Western Germany ....

A narrow belt of mostly weak instability is predicted by models ahead of the surface cold front, with ECMWF showing only marginal degree of destabilisation and GFS putting values of MLCAPE mostly below 1000 J/kg. Overlap with strong southwesterly flow will be likely and wind shear in the 0-3 km layer is expected to reach 15-20 m/s, with the highest values generally on the western fringe of the "instability belt". Flow will be mostly parallel to the surface front and only weak storm relative winds will exist at the anvil-level of the storms.

Another detrimental factor will be lack of synoptic scale forcing, except for the front itself. Moreover, a scenario with the precipitation being carried ahead of the progressing linear storm is possible. There is still a chance, however, that in 15-20 m/s flow at 700 hPa, quickly traveling bowing segment will form with a chance of producing severe wind gusts. Over Southern France and Southwestern Germany, marginally severe hail can not be completely ruled out, as higher degree of instability will occur over these regions. A low end Level 1 is introduced for these regions.

... Central to Eastern Europe ...

Moderate instability built-up is forecast with MLCAPEs locally exceeding 1500 J/kg. With lack of synoptic scale forcing and very weak winds at all levels of troposphere, mostly disorganised, diurnally driven thunderstorms are expected. Local flash flooding, marginally severe hail or wind gusts can not be ruled out from stronger pulse storms or multicell clusters, but coverage should not be high enough to warrant a Level 1 attm. Upgrade might follow, however, during the day.