Storm Forecast
Valid: Mon 01 Feb 2010 06:00 to Tue 02 Feb 2010 06:00 UTC
Issued: Sun 31 Jan 2010 21:18
Forecaster: TUSCHY

A level 1 was issued for S-Greece mainly for severe wind gusts and isolated tornadoes.

A level 1 was issued for the Aegean Sea, W/NW Turkey and SE Bulgaria mainly for tornadoes, severe wind gusts, large hail and excessive rainfall.

SYNOPSIS

Cold air mass covers most parts of Europe. An active frontal zone affects SE-Europe with enhanced convection.

A sharp trough over the S-North Sea swings southwards over the Netherlands, Belgium and NW-Germany. Some LL moisture comes ashore beneath very cold tropospheric conditions (steep lapse rates). Diurnal heating between cellular convection may support enough input for a gradual strengthening trend in convective activity during the noon/afternoon hours. An isolated thunderstorm with sleet/marginal hail/snow is possible. The activity decreases rapidly after sunset, as BL stabilization occurs.

Latest satellite and model data support a moderate risk for subtropical development west of the Canary Islands. Phase diagrams now show a chance for a warm core evolution (even deep) and forecast track over increasingly warm SSTs and in a subtropical air mass hint on a good chance for subtropical development. Another indication is the persistent and deep convection, visualized in latest satellite data, which wraps around the circualtion's center. In any case, heavy rain and strong wind gusts affect the Canary Islands with flooding being a distinct possibility.

DISCUSSION

... S-Greece ...

A cold front drops to the south during the forecast, crossing the level 1 area during the late evening/early night hours from the north. Prefrontal 20-25m/s SW-erly flow in the lowest 1.5km advects moderately modified Mediterranean air towards the rough terrain of S-Greece. Forecast cross sections show a weakly structured quasi-stationary frontal zone over south-central Greece, which becomes more active during the evening hours, as a developing cyclonic vortex to the northeast sends the baroclinic zone as a strengthening cold front towards the south. Therefore, weakly organized/clustered showers/thunderstorms occur during the daytime hours over the level 1 with isolated severe wind gusts and enhanced rainfall, mainly in the orographically favored spots. An isolated tornado along the coast is possible, but the overall tornado risk increases between 15-00 UTC, as LL CAPE increases significantly and LL directional shear strengthens. Postfrontal air mass won't support deep convection, so the level 1 will expire after midnight.

... Aegean Sea and NW/W-Turkey ...

A complex forecast scenario unfolds for that region, especially for extreme NW/W-Turkey.

SE-Bulgaria and NW-Turkey, 15-21 UTC

During the morning and early afternoon hours, SW-erly flow dominates and advects moist air into the level 1 area. A quasi-stationary boundary is placed somewhere over E/SE-Bulgaria and NE-Greece during that period, but the structure of this boundary remains weakly organized. Nevertheless, prefrontal air mass becomes slightly unstable during the afternoon hours, as moisture advection keeps going, next to ongoing diurnal driven/advective warming trend. ECMWF and GFS agree in this scenario, so showers/thunderstorms increase in coverage and intensity during the afternoon hours, as numerous UVV maxima cross the area from the SW. In addition, 15 UTC onwards, the surface front increases its forward speed towards the E/SE (now as a cold front) temporarily and is another focus for initiation. Veering ahead of the front is strong, with good speed/directional shear, so any more discrete cell could acquire rotation with a severe wind gusts, large hail and tornado risk, especially as models support only a weakly capped air mass. However, the main uncertainty will be how thick the convective debris will be from convection to the SW, which could inhibit diabatic heating support and therefore could keep CAPE low. The cold front over NW-Turkey starts to slow down and again acquires quasi-stationary conditions during the evening hours, as a surface depression forms to the SW, so the overall risk may last well into the night hours.

NW/W Turkey and the Aegean Sea 15 UTC onwards

This surface depression is well forecast in model data and causes a gradual strengthening of the frontal zone itself. A lot depends on the exact strength and path of the depression, but the coarse picture of this scenario indicates a good chance for a developing warm sector over NW-Turkey, which reveals intense shear at all levels and moderate CAPE. All modi of severe would be possible, including damaging wind gusts (25-30m/s at 850hPa), and tornadoes. The thunderstorm risk spreads southeastwards with a large hail, tornado and severe wind gust risk. An upgrade may be performed over NW-Turkey, if new model data still indicate similar conditions.

Another concern is the frontal zone/cold front itself. The cold front reveals an anafront-type structure and therefore a messy shield of stratiform precipitation likely evolves north of this front/the occlusion with intense precipitation rates. Elevated convection is possible, given some MUCAPE release and isolated, elevated thunderstorm development is forecast. Next to this risk, rapidly cooling thermodynamic profilers throughout the lower troposphere increase the risk for freezing rain and very heavy snowfall, so elevated convection may locally increase the ice/snow amount significantly.