Storm Forecast
Valid: Wed 04 Aug 2010 06:00 to Thu 05 Aug 2010 06:00 UTC
Issued: Tue 03 Aug 2010 22:55
Forecaster: PUCIK

A level 2 was issued for Finland mainly for severe wind gusts, tornadoes and to the lesser extent for large hail and excessive precipitation.

A level 1 was issued for Kola Penninsula and Karelian Republic mainly for severe wind gusts and to the lesser extent for tornadoes.

A level 1 was issued for Western Russia, Eastern part of Baltic States, Western Belarus and Ukraine mainly for large hail and severe wind gusts.

SYNOPSIS

Main macro-synoptic feature at mid-levels will be a complex trough with one axis stretching over Western Europe and another one stretching from Central Europe to the Southeastern Mediterranean. An embedded short-wave is expected to develop on its forward flank over Baltic States during the day and will quickly progress northward in 20-25 m/s 500 hPa flow. A 30 m/s jet-streak at 300 hPa will be observed on the fringes of this short-wave. To the east, a high pressure system will stall over Russia.

Closer to the surface, cyclogenesis over Baltic States, related to the short-wave trough, is forecast. Surface cyclone and its attendant frontal system will quickly move northward towards Scandinavia. A cold front, stretching from this low southwards, will separate unseasonably hot airmass over Eastern Europe from cooler, polar airmass over Western and Central Europe.

DISCUSSION

... Finland, Karelian Republic, Kola penninsula ...

With the dynamic effects of the approaching short wave trough at mid and upper levels of troposphere, in the exit region of the jet-streak, surface low is expected to deepen with central pressure predicted to drop below 1000 hPa (ECMWF even simulates the pressure to drop below 994 hPa). This surface low will travel northwards and in its warm sector, very warm and humid airmass will be advected from south/southeast. MLCAPE values should reach 1000 J/kg in the warm sector, locally even more, especially in the southern parts of the region.

Due to the strong flow at the lower levels, with windspeeds over 20 m/s both at 850 and 700 hPa, especially towards afternoon hours, wind shear will be strong and confined mostly to the lower levels of troposphere (with 20 m/s flow at 500 hPa and 25-30 m/s at 300 hPa level). Wind shear in the lowest 3 km will attain values up to 25 m/s and up to 15 m/s in the lowest 1 km. Furthermore, due to the pressure falls, ageostrophic component of the flow in the lower levels will likely contribute to the moderate values of SREH, with the highest values close to the warm front (reaching 300 J/kg in the 3 km layer). Initiation of the storms should not be problematic with the presence of strong frontal system and the position in the exit region of the jet-streak. Convection will be likely influenced by an overnight MCS, which might become part of the warm front precipitation shield of the developing low.

In such configuration, well organised convection is predicted, very probably involving strong, rapidly progressing MCS in form of a bow-echo. Isolated supercells are not ruled out either, especially closer to the warm front, but due to the weaker shear values in the deep layer of troposphere, their persistence is questionable. Severe (and possibly even isolated extremely severe) wind gusts are expected to accompany the passage of such MCS. With MCS and/or supercells, tornadoes can occur as well, given strong low level shear and small dewpoint depressions, with F2 or stronger intensity not ruled out. Marginally large hail might be observed with stronger, more isolated cells and an excessive rainfall could be observed at the northwestern part of the system.

Level 2 is issued for the region with the highest threat of severe (extremely severe) wind gusts and tornadoes. MCS might continue also overnight and progress towards Northern Finland, Kola penninsula and possibly also Karelian Republic, but with dimnishing boundary-layer instability, coverage and intensity of wind gusts will become less pronounced. For these regions, Level 1 seems to suffice at the moment.

...Western Russia, Eastern Baltic States, Western Belarus and Ukraine...

Ahead of the stagnating surface cold front, a belt of moderate instability with MLCAPEs ranging from 1000 to 2000 J/kg is forecast. Moreover, moderate wind shear, with DLS around 15 m/s is also simulated and with such configuration, well organised, strong thunderstorms are possible. Due to the combination of moderate wind shear and some low-end SREH, some brief supercells are not ruled out. Thanks to the high lapse-rates, moderate CAPE and wind shear, large hail will be a primary threat. Dry boundary layer and Delta Theta-E values over 20 K also point to the threat of local downbursts, involving severe wind gusts.

Main uncertainity is the initiation and the coverage of the storms, as some models suggest rather spotty initiation in the evening hours, as instability will be slowly diminisihing. Due to this fact, broad Level 1 is issued with the possibility of an update during the day, if storms prove to be more plentiful.