Storm Forecast
Valid: Sat 07 Mar 2009 06:00 to Sun 08 Mar 2009 06:00 UTC
Issued: Fri 06 Mar 2009 18:07
Forecaster: DAHL

SYNOPSIS

Deep and intense upper low, centered over the central Mediterranenan on Friday early evening, is simulated to slowly move eastwards during the period, being centered over Romania by Sunday 06Z. Meanwhile ... an intense Atlantic trough digs towards Europe, reaching the NW coast of continental Europe early Sunday morning. At low levels, a rather intense -- but weakening -- SFC low is accompanying the Mediterranean upper system. The other SFC low, associated with the Atlantic trough, should be centered over the northernmost tip of the UK by the end of the period.

DISCUSSION

... Ionian Sea, Aegean Sea, Greece, Bulgaria and S-Romania ...

Broad upper trough is centered over the central Mediterranean and keeps up its slow progress towards the east. Filled with cold air, the environment remains supportive of scattered showers/thunderstorms over the warm Mediterranean whose SSTs are at roughly 15°C. The airmass is weakly capped, EL temperatures cold enough for deep convection and the airmass at lower levels well mixed, so an active convective day will be in store for those areas. DLS is weak with values around 10m/s and strong wind gusts/marginal hail will be the main hazard with those storms. An isolated large hail event is possible over the N-Aegean Sea, where up to 1kJ/kg MLCAPE release looks reasonable due to colder mid-levels. The waterspout risk should be limited by missing LL convergence zones and weak LL lapse rates. Probably the best chances will be over the N/E Aegean Sea, where instability release will be quite strong, especially along the west coast of Turkey, where thunderstorms could ingest somehow stronger helical flow.

It is slightly more complicated onshore over Greece, Bulgaria and Romania. Latest GFS run has pockets of better boundary layer moisture scattered over a broad area. Placed under the base of the upper trough steep mid-level lapse rates are present and deep convection is possible, probably concentrated on local features like orographically favored areas or local convergence zones. Thunderstorms will evolve in a weakly shared environment and hence no severe weather threat is forecast despite marginal hail and strong wind gusts.

... W-Turkey ...

Around noon a tongue of some low-end instability runs from Antalya northwards to the Black Sea just along the eastward moving cold front. A short wave enters the area from the south at the same time, providing enough forcing for a few thunderstorms. DLS in the order of 40-45m/s, 0-3km shear in excess of 25m/s and strong LL speed/directional shear all favor a rapid organization of existing thunderstorms with a large hail and severe wind gust threat. The rough terrain could locally increase SHR values significantly and an isolated tornado event is possible. A level-1 seems to be enough as instability vanishes during the early afternoon hours.

... British Isles ... Channel Region ...NW Benelux ... NW France ...

An intense cold front will be accompanying the approach of the Atlantic upper trough over the British Isles and the Channel region late on Sunday/early on Monday. Current indications are that strongly-forced, comparatively shallow, linearly-organized convection may develop, but GFS suggests EL temperatures of only about -10°C, which is very marginal for cloud electrification. However, the shear profiles will be quite strong, and it is likely that vertical momentum transport within the convective downdrafts may augment the already strong background gradient flow. However, convective weather events should remain below severe limits, and highlighting the area does not seem to be necessary.

In the wake of the front, cellular convection is anticipated in the deeply mixed polar air. However, these cells should still be quite shallow and weak, so that lightning should be too sporadic for a lightning area.
As the cells make landfall, they will briefly be exposed to strong low-level shear, and given low LCL heights as well as practically absent capping, a small chance of a brief tornado or two exists. However, the cool land surface should result in rapid decay of the cells, so that the allover threat is too weak for a risk area.