Extended Forecast
Valid: Mon 24 Mar 2008 06:00 to Tue 25 Mar 2008 06:00 UTC
Issued: Sat 22 Mar 2008 22:41
Forecaster: TUSCHY

SYNOPSIS

A glance on the streamline maps at 300hPa still reveals an unusal massive longwave trough, running from N-Greenland all the way down to N-Africa , covering most parts of western, central and southern Europe. The main difference compared to the past few days is the infiltration of a more energetic airmass as numerous depressions rotate around this trough. This increases the severe weather risk over parts of southern / eastern Europe.

DISCUSSION

...Western Ukraine and eastern Belarus....

There is a risk for a concentrated swath of severe thunderstorms mainly during the daytime hours.

A strong depression is forecast to move out of the base of the intense trough generally in a northeasterly direction. The model pool agrees quite well in this solution and even the strength of this feature seems to be well handled so confidence is growing regarding organized thunderstorms. As warm front lifts northward during the morning hours, reaching S-Belarus during midday, airmass recovery will be underway as moisture advection helps to push dewpoints to at or above 5°C, which looks plausible given the current (22.03.08 ) airmass over the Black Sea region. Strong WAA and anticipated diabatic heating should bring surface temperatures to at or above 15°C . Time of the day and maybe limited diabatic heating could suppress convective activity in the warm sector as airmass should stay capped. Main focus will be a cold front, pushing through the area from the SW between 09Z and 18Z. GFS has a persistent signal of modest instability release and also quite significant LL CAPE over NW Ukraine and SE-Belarus . Current thinking is that convective activity gets stronger over NW Ukraine and S-Belarus and a lot points to an intense squalline, racing NE-wards.
Shear is impressive and latest forecast soundings of Kiev show an uncapped and strongly sheared environment. LL shear is off the map with 0-1km helicity values at or above 200 m^2/s^2. Despite those impressive values, the tornado threat could be limited by a strong T-Td spread over NW-Ukarine and a cold / stable boundary layer over S-Belarus.
However there is a considerable severe wind gust threat, especially if a squalline can develop as forecast soundings indicate the chance for 30m/s at 700hPa to mix down to the surface due to a well mixed boundary layer. The risk for large hail will be also high mainly over S / SE Belarus, where elevated convection in combination with impressive 0-3km helicity could support numerous large hail reports.

Strength of diabatic heating ( hence quality of warm sector airmass ) and final position and strength of the surface depression will determine the degree of severity of this event and an upgrade to a level-2 could become necessary in subsequent outlooks!

...Eastern Romania, Moldova and S-Ukraine...

Aforementioned cold front continues to race eastwards, affecting the area of interest during the evening hours. Both the cold front and the prefrontal airmass have to be monitored closely for deep convection. Current thinking is that airmass ahead of the cold front will stay capped but GFS tries to develop some instability over the coastal areas of the Black Sea during the afternoon and evening hours. Those are of elevated nature and hence severe weather risk would be low . If confidence for thunderstorm initiation grows, higher probabilities may become necessary.


... Greece, Aegean Sea and western Turkey...

A strong cold front blows through those areas from the west, crossing Greece during the morning and afternoon hours, reaching W-Turkey during the early evening hours. A combination of not completed airmass recovery behind a departing surface depression the day before and very warm mid-levels could prevent a more widespread severe weather event.

Greece:

GFS has decent signals of modest instability just along the cold front in a highly sheared environment over Greece, but biggest CAPE area can be found between 700 and 400hPa. Right now we think that deep convection will be present along the cold front as it crosses Greece, but this convection will be of elevated nature with an enhanced large hail risk. If dewpoints are higher than currently anticipated we also have to include a tornado / severe wind gust threat.

Another story during the night hours as well mixed post frontal airmass overspreads central and southern Greece. SRH-1km of 150-200 m^2/s^2 and some instability release will overlap and strong veering at lowest levels could support a tornado and severe wind gust risk after midnight .

W-Turkey:

A quite potent set-up evolves at about 21Z onwards for western Turkey as cold front approaches from the west. LL are moist and in combination with rapidly cooling mid-/ upper-levels 500 - 800 J/kg SBCAPE will be available. Shear will be more than adequate for rotating storms with an attendant tornado / severe wind gust and large hail risk. Parts of this level-1 area may see an upgrade in later outlooks.


A broad area of enhanced convection develops in the polar airmass over the North Sea, UK, Belgium, the Netherlands and most parts of Germany. Daytime driven thunderstorms are forecast with a risk for gusty winds and sleet. A thunderstorm area may be issued in subsequent outlooks.

Numerous convergence lines, weak shear and locally enhanced 0-3km instability points to an isolated waterspout risk over the central Mediterranean , but threat is too low for issuing a huge level-1 area.