Storm Forecast
Valid: Thu 17 Aug 2006 06:00 to Fri 18 Aug 2006 06:00 UTC
Issued: Wed 16 Aug 2006 22:09
Forecaster: GATZEN

SYNOPSIS

Long-wave trough remains over northern/western Europe ... with a center over Bay of Biscay. Ahead of this feature ... a deep south-westerly flow is present from north-western Africa to northern Mediterranean ... advecting hot African airmass northward ... and 850 hPa temperature is expected to reach 25°C over southern Italy in the range of well-developed subtropical high at the end of the period. While maritime air mass in the range of the trough spreads into western Iberian Peninsula ... strong frontal boundary and very strong upper jet should form over south-western Europe during the next days. At lower levels ... capped boundary-layer moisture over central Mediterranean has increased rapidely ... and low-level dewpoints have reached around 20°C today. Further north ... rather moist air mass is present over France, Bay of Biscay, British Isles, Alpine region, Germany, and Baltic Sea region. Given relatively steep lapse rates over most of western Europe ... GFS expects CAPE over a broad region from central/northern Mediterranean to north-western and central Europe as well as Baltic Sea region.

DISCUSSION

Northwestern/northern Mediterranean, France into SW Germany

Latest soundings and surface observations indicate that low-level moisture has increased especially near Italy ... while moisture was rather shallow over western Mediterranean. It is expected that relatively dry low-level air mass will remain over western Mediterranean due to rather well-mixed low levels and westerly surface winds advecting dry air eastward. To the north-east ... moisture should increase during the day south of France due to south-easterly surface winds. WAA is also expected at the 850 hPa level ... and capping inversion is likely to suppress initiation. During the day ... intense upper vort-max/short-wave trough and associated strong jet streak over western Mediterranean is expected to race north-eastwards into northern Italy ... yielding strong QG forcing over northerwestern Mediterranean in the evening/night hours. It seems that initiation will likely occur in the range of the frontal boundary over central/southern France ... spreading north-eastward into western Alpine region. Additional thunderstorms should form over central/northern France in the range of unstable air mass. Focus of convective activity is unclear ATTM ... as low-level moisture is questionable over southern France as indiacted by latest GFS model run. Given southerly or even south-easterly surface winds ahead of the cold front that reaches southern France during the night hours from the west ... strong vertical wind shear could be expected ... and thunderstorms should rapidely become well-organized ... including supercells and bow echoes. Despite of large hail and severe wind gusts ... chance for tornadoes is also enhanced given strong LLS and low LCL heights over southern and central France. Given strong QG forcing ahead of the vort-max ... thunderstorms should merge into a few MCS moving north-eastward or eastward into northern Italy, western Alps, Benelux and western Germany. While chance for tornadoes should decrease given more stable low levels ... chance for severe wind gusts should remain quite high with well-organized convective systems.