Storm Forecast
Valid: Sat 22 Nov 2008 06:00 to Sun 23 Nov 2008 06:00 UTC
Issued: Fri 21 Nov 2008 23:28
Forecaster: PUCIK / GROENEMEIJER

SYNOPSIS

A massive arctic outbreak is underway across much of Europe as very cold air is pushed southward on the forward side of the high pressure system, which has settled over E Atlantic. Temperatures at 850 hPa level are approaching -10°C in the airmasss. Very strong jet sream is rotating around the deep trough at mid-levels. Under its left exit region, further cyclogenesis is expected and rapid deepening of surface low, now centered over the Ukraine will occur. A forementioned low will progress to NE, following SSWly steering flow at mid levels. Significant cold front will affect the weather in the belt from Italy to the Black sea region. Behind the front, very cold airmass is advected, characterized by steep lapse rates and low moisture content.

DISCUSSION

...Germany, Poland, Norway....

Some marginal development of conditional instability is predicted over the highlighted regions, in response to the interaction of relatively warm SSTs and very cold airmass. Storms should initiate mostly over the coastal areas, where the strongest convergence signals are generated. Strong northerly flow will likely carry some of the unstable airmass further inland, so few discharges might occur also there. Generally, only weak thunderstorm activity is predicted in given areas and with general lack of wind shear, no organised convection is expected.

... Ionian Sea, E Greece, Bulgaria, SW part of Black sea....

An impressive kinematic setup will develop ahead of the approaching cold front as strong flow forms in response to the surface low deepening. DLS should be in order of 30 m/s, with shear in the lowest 3 kms around 20 m/s. These values are more than enough to sustain very well organised convection. Instability should be relatively meager, with MLCAPE values under 800 J/kg, but very strong forcing is expected, both from front and mid level trough, so synoptic lift might compensate for the general lack of instability. Current thinking is, that strongly forced convective line will move across the region. As a substantial low level jet is predicted ahead of the front, with speeds over 25 m/s at 850 level, low level shear will exceed 15 m/s at places and SREH values should top at 400 J/kg. LEWP structures might develop within the line with some embedded rotating mesovortices due to the strong environmental helicity existence. With so strong wind profiles, damaging gusts should be the primary threat. Few tornadoes are expected too and they might reach strong intensity provided the strong kinematic setup will realize over the area. Hail should stay mostly sub-severe as very limited instability is expected, but nevertheless will commonly accompany the storms with cool airmass aloft.
Threat should be the greatest in the LVL 2 region, where widespread severe weather is anticipated. LVL 1 areas should see isolated severe gusts with one or two tornadoes not ruled out.

...Belt from Black Sea to W Russia...

Closer to the center of the deep low, this area will have yet even stronger kinematic fields than the one discussed above. DLS approximately 50 m/s and LLS around 20 m/s is expected just ahead of the accelerating cold front. Instability will be shallow, but nevertheless very strong forcing should yield a forced convective line, advancing to the NE. This line is expected to form in the exit region of LLJ. SREH values ahead of the front will climb to 800 J/kg, suggesting strong potential for rotating updrafts. With 850 hPa winds over 30 m/s, widespread severe gusts can be expected with the line and also tornadoes are anticipated due to the significant helical flow in the inflow layer of the storms. The greatest threat should exist across E Belarus / W Russia where the strongest convective signals are generated by GFS. Here, widespread severe gusts along with isolated extreme values are expected ( i.e. over 32 m/s). Moreover, chance of significant tornadoes exists with such strong LLS and SREH0-1 km values over 300 J/kg. Due to the above mentioned threats, Level 2 seems to be warranted.