Storm Forecast
Valid: Wed 17 May 2006 06:00 to Thu 18 May 2006 06:00 UTC
Issued: Tue 16 May 2006 23:13
Forecaster: GATZEN

SYNOPSIS

Between strong and amplified long-wave trough over northeastern Atlantic and subtropic ridge over northwestern Africa/western Mediterranean ... well-developed southwesterly jet is directed towards western Europe. An intense short-wave trough/vort-max indicated on latest WV satellite images is expected to travel eastward reaching from central England to northwestern Iberian Peninsula on Thursday, 00Z ... and winds will likely reach more than 80 kts @ 500 hPa in the jet axis stretching from Bay of Biscay to NWern France. To the east ... broad long-wave trough covers most of eastern, and southeastern Europe ... and rather strong upper jet curving around the Mediterranean ridge is expected to affect northern Mediterranean, Alpine region, and Balkans. At lower levels ... rather moist and unstable airmass present over central France in expected to spread northeastward into northern France, southern England, and Benelux during the period .. while plume of well-mixed warm airmass originating from Atlas mountains/Iberian altiplano advects northeastward into southern France, western Alps, and northern Mediterranean. Rather cool but unstable airmass remains over eastern Europe.


DISCUSSION

France, southern England, Benelux

Focus of expected severe convective development is central and western France. E of Atlantic trough ... well-mixed airmass from Iberian Peninsula spreads into western, and central France. At low levels ... east of propagating cold front of Atlantic surface low pressure system ... warm and moist boundary-layer airmass advects northeastward into northern central France ... where latest models suggest low-level mixing ratios of about 13 g/kg. Given actual dewpoints around 16 to 19 C over southwestern France ... GFS forecast does not seem to overestimate boundary-layer moisture significantly. During the day ... insolation is expected although high clouds should be present in the warm sector ... and surface temperature will likely reach 23..28 C over central France ... where latest GFS model suggests CAPE up to 1000 J/kg at 15Z.

As the upper jet approaches ... DCVA is expected over the Channel region ... while weak DCVA will be present over most of France. However ... due to strong WAA ... synoptic forcing should be present over northern and central France ... helping to break the cap in the afternoon/evening hours. Current thinking is that initiation will take place in the range of the nose of high theta-e plume over northwestern France ... propagating eastward in the range of the theta-e axis. Given southeasterly surface winds, rather strong (8..10 m/s LLS/20..25 m/s DLS) vertical wind shear ... thunderstorms that form will likely organize into multicells ... and supercells ... capable of producing large hail, severe wind gusts and a few tornadoes given quite low LCL heigts and high low-level helicity. Given rather weak synoptic forcing ... cells are expected to merge into clusters that spread eastward. During the evening hours ... developing LLJ and associated very strong (30 m/s) DLS expected over central France may support one or two MCS propagating E-ward over central France ... with straight-line winds the most signifcant threat. Dry low-level airmass over southeastern France will limit severe potential later on over western Alps ... and western France.

During the night ... cold front and associated upper vort-max are expected to cross northern and western France. Along the cold front ... thunderstorms are expected to form .. as models suggest some surface-based CAPE. Given very strong vertical wind shear ... organized convection is not ruled out ... and bowing lines and embedded mesocyclones will pose a threat of severe winds and large hail. Best chance for isolated mesocyclones should exist ocer southwestern France ... where one or two tornadoes ... as well as very large hail are not ruled out.

To the north ... WAA regime of unstable airmass will affect southern England, the Channel region, and Benelux. Strong synoptic forcing is likely ... but given weak instability ... chance for thunderstorms should be limited. Current thinking is that large stratiform rain band will form ... and embedded thunderstorms should be elevated. Thunderstorms that are surface-based should quickly become severe given strong vertical wind shear ... and large hail, severe winds and tornadoes should be quite likely. Given cool and initially rather dry boundary-layer ... surface-based convection seems to be unlikely ATTM.


Balkans

At the western flank of eastern European long-wave trough ... upper jet streak turns southeastward ... associated with synoptic forcing over Balkans. At lower levels ... warm and unstable airmass will lead to showers and thunderstorms during the day. Given strong DLS in the range of the upper jet ... thunderstorms should likely organize into multicells and mesocyclones ... capable of producing severe wind gusts, large hail, and probably a tornado. Severe activity is expected to weaken during the evening/night hours.