Storm Forecast
Valid: Wed 21 Apr 2010 06:00 to Thu 22 Apr 2010 06:00 UTC
Issued: Tue 20 Apr 2010 22:25
Forecaster: PUCIK

A level 1 was issued for Slovenia, Croatia, Bosnia and Herzegovina mainly for large hail and to the lesser extent for marginally severe wind gusts.

A level 1 was issued for Portugal mainly for large hail.

A level 1 was issued for Morocco and Western Algiers mainly for large hail, severe wind gusts and to the lesser extent for excessive precipitation.

SYNOPSIS

Strongly disturbed flow pattern has established over Europe with a deep trough stretching from Scandinavia to the Eastern Mediterranean. An embedded disturbance filled with arctic air will translate slowly southeastwards within the trough. Strong flow at mid and upper levels is surrounding the trough with a cyclonically curved mid-level jet-streak expected over Germany and the Czech Republic by Wednesday 12 UTC. A ridge is expected to cover the Western Mediterranean and a shallow trough is expected to approach Iberian penninsula during Wednesday, weakening gradually.

At the surface a system of shallow lows will cover a vast area from Turkey to Eastern and Southern Scandinavia. To the west a ridge will stretch from the Atlantic to France. Behind the cold front, an arctic airmass will have covered a significant portion of Central Europe.


DISCUSSION

... Slovenia, Croatia, Bosnia and Herzegovina...

Ahead of the approaching cold front, a slightly unstable environment is simulated over this region during diurnal heating, with CAPE values simulated to reach values in range from 400 to 1000 J/kg. Strenghtening mid and especially upper level flow will keep high values of wind shear in 0-6 and 0-8 km layer (15 to 20 and 20 to 30 m/s respectively) so that well organised convection is possible, including supercells. Detrimental factor for this situation is a lack of synoptic-scale forcing, which would contribute to the destabilisation. Local convergence zones should initiate scattered convection and especially in case of stronger multicells or supercells, large hail is possible or marginally severe wind gusts (latter threat in case that supercells manage to develop). Coverage of storms will rapidly dimnish after 18 UTC.

... Portugal ...

A window for severe weather threat will exist approximately from 12 to 18 UTC with strong mid and upper level flow ( up to 20 m/s at 500 hPa and 30 m/s at 300 hPa level). Thanks to the approaching trough, storms might be initiated in the slightly unstable environment, with CAPE values probably below 1000 J/kg in most of the area. High wind shear values, especially in 0-6 km layer, reaching 20 m/s might result in well organised storms, including a slight possiblity of low topped supercells. A Level 1 is issued due to the possibility of marginally severe hail in the stronger cells.

... Morocco, Western Algiers ...

A good overlap between the instability and strong wind shear will exist over the cerrtain parts of the country, especially more inland than can be depicted in the map. Wind shear values above 20 m/s in 0-6 km will support well organised convection, including supercells. Large hail and severe wind gusts might occur, especially during the afternoon hours. Towards the evening, strenghtening low level shear and clustering of storms might lead to the development of MCS, capable of producing severe wind gusts and on its northern side, also excessive rainfall. Level 1 is issued as the region with greatest threat (with a borderline Level 1/2 situation) will lie outside the map.