Storm Forecast
Valid: Thu 21 Sep 2006 07:00 to Fri 22 Sep 2006 06:00 UTC
Issued: Thu 21 Sep 2006 07:25
Forecaster: GATZEN

SYNOPSIS

Western part of central European omega is a strong amplified long-wave trough that affects western Europe. An intense short-wave trough curving around the tip of the main trough is forecast to travel northward into British Isles during the period ... providing strong DCVA in the range of strong upper jet streak reaching around 50 m/s at the 300 hPa level. At lower levels ... very warm airmass originating from Hurrikan Gordon is involved in the system ... while unstable airmass over western Mediterranean advects into Bay of Biscay ahead of the system. Models show that cyclogenesis will occur during the next hours ... but there are still inconsistencys about the strenght of the developing low. To the west ... convectively mixed air mass present in the range of the main trough spreads eastward during the day ... affecting Bay of Biscay region.

DISCUSSION

North-eastern Spain

Over western Mediterranean/eastern Spain ... latest soundings indicate an EML originating from the Atlas mountains ... that is advected into Bay of Biscay ahead of the approaching trough. At lower levels ... especially over western Mediterranean substantial boundary-layer moisture has formed as indicated by Palma de Mallorca sounding ... where low-level mixing ratio reaches about 13 g/kg. Quite strong capping inversion was present over most of the region ... while well mixing occurred at the northern coast of Spain where EML reached the surface locally during the night. Today ... mid-level height falls are expected over Iberian Peninsula in the range of approaching trough ... while insolation is should occur over eastern Spain. Both effects should lead to weakening of capping inversion ... and increasing southerly flow ahead of approaching surface low over western Spain should lead to upslope flow over north-eastern Spain. Although DCVA will be weak/negative east of the main vort-max ... models do indicate WAA ... and UVM over western Iberian Peninsula should spread into the eastern portions during the afternoon ... and it is expected that initiation will take place during the afternoon/evening hours. Thunderstorms that form will have a potential to organize given strong vertical wind shear in the range of upper jet ... and strong SRH values forecast by latest GFS model run ... showing weak surface low forming over south-western Mediterranean with backing winds over eastern Spain ... should be favorable for mesocyclones. Expect that a few storms will form ... at least isolated large hail and severe wind gusts should be likely. Given strong LLS ... isolated supercells should also pose a threat of tornadoes. Thunderstorms are forecast to weaken during the night hours.

Western/central Iberian Peninsula, Bay of Biscay

Tropical airmass advects into western Iberian Peninsula today. Latest satellite image shows a narrow band of convection reaching into SWrn Spain. In the range of this warm airmass ... models indicate very strong vertical wind shear and low-level helicity. Although nearly neutral lapse rates are present ... weak CAPE should be possible during the next hours ... and non-supercell tornadoes are forecast to form over western Iberian Peninsula. A couple of events is not ruled out ... but as there is no lightning observed ATTM ... amount of instability seems to be rather weak ... indicating that strong convection will not form during the next hours ... and a level 2 seems to be not warranted. During the day ... instability in the range of the warm airmass will be rather unlikely over central Iberian Peninsula and Bay of Biscay ... but convection that forms and roots to the boundary-layer may produce tornadoes as well given favorable low-level wind shear. Further north and east ... chance for thunderstorm once decreases ... and severe convection seems to be not likely ATTM near British Isles/France.

Eastern Mediterranean, Black Sea

In the range of weak upper trough ... steep low-level lapse rates and moist low-level airmass as well as weak vertical wind shear is present. As on Wednesday ... a few waterspouts are expected to form ... with the greatest chance in the range of the trough center over Aegean. However ... it is not ruled out that a event will occur outside the level 1 area.