Storm Forecast
Valid: Fri 06 Mar 2009 06:00 to Sat 07 Mar 2009 06:00 UTC
Issued: Thu 05 Mar 2009 20:05
Forecaster: TUSCHY

SYNOPSIS

A cold cyclonic vortex is centered over the central Mediterranean and steers the weather over most parts of Europe. Conditions over the warm Mediterranean and SE-Europe are supportive for scattered thunderstorm activity. A dry/cold continental airmass over far NE-Europe suppresses any convection, while an eastward building high pressure area over SW-Europe brings warmer/stable conditions to Portugal and Spain.

DISCUSSION

... Tyrrhenian Sea, Adriatic Sea and Ionian Sea ...

An active thunderstorm day is in store for parts of the central Mediterranean as the upper low and attendant pool of cold mid-levels remains parked over that area. The quality of the surface airmass is passable as airmass already made a complete loop around the cyclonic vortex and had time to ingest moisture from the warm Mediterranean. However, largest temperature spread between mid- and lower levels is present along the southern fringe of the vortex, and that's where strongest thunderstorm activity is forecast. A few isolated thunderstorms are well possible around Corsica or the N-Adriatic Sea but we decided to highlight the area with the strongest activity. This is a winter-like moderate CAPE/weak shear environment, so scattered thunderstorms are likely to occur with just a marginal hail/strong wind gust threat. Isolated large hail is possible over SE-Italy and the S-Adriatic Sea, where DLS is somewhat stronger.

SE-Italy and the central/southern Adriatic Sea are also the regions, where waterspout development could be favored by quite steep LL lapse rates/some LL CAPE release and more than 500 J/kg MLCAPE, so rapid vortex stretching would be possible. A marginal level-1 was issued to reflect the combined hail/spout risk in this area.

... W/extreme NW-Turkey, eastern Greece, S-Greece and Bulgaria (until 21Z) ...

A sharp surface trough swings northwards from Malta (06Z) to the N-Ionian Sea (~12Z ) and is responsible for the temporarily speed up of the eastward moving cold front. This front will affect S/W-Greece at roughly 12Z and the west coast of Turkey at 18-21Z. Along and ahead of the cold front, storm relative helicity is augmented at both 1km and 3km. Due to the combination of a robust moisture surge NNE-wards and ongoing mid-level CAA, a broad sector becomes quite unstable with readings of 400-800 J/kg MLCAPE. Obviously the main risk is a severe wind gust risk during the passage of the active cold front with winds of 20-30m/s at 850hPa and well mixed lower levels. Steep mid-level lapse rates, strong veering at mid-levels, 30-40m/s DLS and a lowering WBZ all point to a quite widespread large hail risk with each more persistent thunderstorm, especially over NW-Turkey, extreme S-Bulgaria and E-Greece. The main limiting factor is the missing forcing, which arrives quite late / during the evening hours /, when instability is on a rapid decrease. Initiation is possible as atmosphere remains just weakly capped, but how widespread this will be is still quite uncertain, especially over NW-Turkey.

Regarding the tornado risk an isolated event can't be ruled out over S-Greece as LL directional shear is augmented along and just ahead of the front. However a more robust tornado risk arises at 12Z onwards over NE-Greece and W/NW-Turkey, as 20m/s LL shear, 200-400 J/kg SRH1 and 500J/kg MLCAPE overlap. The LCL rises from roughly 600-800m over NE-Greece to more than 1500m over NW Turkey. Even a strong tornado event is possible with those values. Thunderstorms over the extreme NE Aegean Sea will keep going through the night although it is questionable if thunderstorms can persist in an environment with constantly decreasing instability and extreme shear of 50-55m/s DLS.

A level-2 was issued for the area, where confidence in more widespread initiation is the highest and where all kind of severe weather modi are possible.

... NE Greece, Bulgaria and Romania at 21Z onwards ...

A constantly intensifying surface depression over W-Bulgaria and SW-Romania will be the main feature of interest during the following 9h. A warm front will be situated around the border Bulgaria/Romania at 21Z. At the same time aforementioned cold front finally enters SW-Bulgaria from the SW and moves rapidly northwards. The cold front will reach S-Romania around midnight and will finally catch up the warm front, so an occlusion process is forecast somewhere over S-central Romania around midnight. The quality of the warm sector airmass looks good with forecast dewpoints at the surface running just below 10°C and this looks reasonable, given latest readings of 6°C over S-Romania and 8-9°C over Bulgaria (18Z). This is not yet enough for surface based convection in respect to latest GFS forecast soundings, but we have to monitor that possibility. Nevertheless MUCAPE values of up to 400J/kg indicate that elevated thunderstorms are possible under the left exit of an extreme 55m/s mid-level jet. Strong to severe wind gusts and isolated large hail are likely over central/E-Bulgaria, S/E-Romania and the tornado threat has to be monitored in upcoming model runs.
Just west of the jet axis (W-Bulgaria and SW-Romania), very cold mid-levels and still appreciable surface moisture generate a swath of some modest instability release in a weakly shared environment and thunderstorms are possible, too. Weaker shear should limit the severe weather risk.

Altogether the main concern right now arises along the occlusion point, which will be around Bucharest at 00Z-03Z, where strongest shear/forcing/instability overlap. However, some run-to-run inconsistency in latest model outputs is still present, so we will stick with a broad level-1. Parts of Moldova and NE-Romania may need to see a level-1 for 00Z onwards but right now I fear that the ongoing occlusion process over central Romania could weaken the moisture surge northwards and hence limit instability release.