Storm Forecast
Valid: Sun 06 Dec 2015 06:00 to Mon 07 Dec 2015 06:00 UTC
Issued: Sat 05 Dec 2015 22:40
Forecaster: TASZAREK

A level 1 was issued for parts of United Kingdom, North Sea, Denmark, parts of S Norway, S Sweden, S Baltic Sea, Lithuania, Latvia, Estonia, N parts of N Belarus and parts of W Russia mainly for the severe wind gusts and in lesser extent for the damaging wind gusts.

SYNOPSIS and DISCUSSION

Europe is divided into two significant pressure systems. Most of the SE and S Europe is covered with broad high where MSLP exceeds 1030 hPa and DMC is unlikely. In opposition, most of the N Europe is covered with a large and deep cyclocne with 960 hPa in the center over the Norwegian Sea. This setup results in a powerful thermal wind with upper level wind speeds exceeding 70 m/s. Mentioned jet streak extends from British Isles up to W Russia.

A dangerous situation takes place at the beginning of the forecast period when a shortwave on the geopotential will take place over United Kingdom. Mentioned wave will move eastwardly with an impressive speed and is expected in the end of the forecast period to reach W Russia. CAPE along the wave's axis will be marginal (50-100 J/kg, locally 200 J/kg) but the presence of the powerful QG-lift and a very dynamic wind field (MLS ~ 20-25 m/s) may result in a low-topped convective line (and/or distinctive bowing lines) capable of producing severe to damaging wind gusts of more than 33 m/s. The best signal from PVA starts in the afternoon hours in the S Sweden and remains supportive for the line until the end of the forecast period (up to W Russia). In the low levels a wave will significantly amplify over the North Sea around 12 UTC and with well-pronounced cold front will move eastwardly to fade over the Baltic Countries. Increased lightning activity is forecast in the S edge of Finland behind the cold front where an advection of steep lapse rates will take place along with the convective cloud tops reaching the graupel zone. In the rest of the area the overall lightning threat remains marginal since the convective cloud tops along the cold front are likely to be near 0C. The overall threat for the severe convective wind gusts may locally meet the requirements for the level 2 threat, but due to the questionable presence of the instability along the wave's axis only a level 1 is issued. Apart from the convective activity, a widespread severe wind gusts exceeding 25 m/s (locally 33 m/s) due to non-convective mechanisms are also expected in the level 1 threat. However, these are outside the scope of the ESTOFEX.

Isolated thunderstorms are also forecast on the southern and western side of the low placed over Norwegian Sea. Within the advection of the cold polar air masses a steepening lapse rates, QG and orographic lift along the Norwegian coast will provide mostly a low-topped convective cells capable of producing lightning.

Thunderstorms are also expected in the W part of the Mediterranean where a favorable overlap of 7 C/km lapse rates and moist boundary layer (mixing ratio 10-11 g/kg) will result in a CAPE up to 500-600 J/kg. A nearly stationary mid-level cold drop (cut-off) near Corsica and Sardinia will support a CAPE to be released. Since no significant overlap of instability with a wind shear is expected by the NWP models and PW values will be moderate (~ 20mm), severe convective storms are not expected in this region.