Storm Forecast
Valid: Sun 04 Oct 2009 06:00 to Mon 05 Oct 2009 06:00 UTC
Issued: Sat 03 Oct 2009 18:15
Forecaster: TUSCHY

A level 1 was issued for extreme NW-Spain mainly for excessive rainfall.

A level 1 was issued for Latvia, Estonia, Lithuania, parts of Belarus and western Russia mainly for severe wind gusts.

SYNOPSIS

No changes in the steering weather pattern for Europe. Persistent branch of the polar vortex is placed over N-Europe with another weak impulse over the central/east Mediterranean. Downstream of a quasi-stationary low over the Azores, a warm and moist airmass spreads northeastwards with the strongest WAA affecting Spain and Portugal.

DISCUSSION

... Extreme eastern Atlantic ...

Downstream of the stationary upper low over the Azores, tropical airmass spreads northeastwards. Sounding data from Funchal and Lajes sample the airmass well with weak lapse rates throughout the troposphere and high PWAT values. A weak impulse is embedded in the SW-erly flow and affects extreme NW-Spain and the highlighted areas over the Atlantic during the night hours. This disturbance outruns the somewhat more favorable tropical airmass and therefore, thunderstorm chances are not high. Instability is slim but shear augmented, so an isolated strong thunderstorm is expected although limited coverage does not justify a level area. A level 1 was introduced for locally excessive rainfall, especially, where repeated showers/thunderstorms cross the same area.

A few elevated thunderstorms could also occur over the Bay of Biscay during the night hours, but instability is on the low-end side, so no thunderstorm areas are included for now.

... SW of Greece, parts of the Aegean Sea and adjacent areas and the western Black Sea ...

Another upper trough crosses the area from the west and lifts out to the NE. The main difference compared to the previous events is the limited BL moisture access in the boundary layer due to frontal intrusions. The airmass had no time yet to modify, so limited instability will be the main issue. The best chances for thunderstorms will be confined below the coldest part of the trough and along the W/NW-coast of Turkey. Shear is weak, so multicells will be the main storm mode with strong wind gusts and locally heavy rain. No level area is needed for this event and thunderstorm coverage decreases during the night hours from west to east.

... Latvia, Estonia and Lithuania ...

Another cold-core trough crosses the Baltic Sea during the forecast. Airmass is not very susceptible for enhanced convection, given limited moisture content and somewhat warmer mid-levels compared to the past days. Only a 15% thunderstorm area was issued due to the very limited thunderstorm activity during the past days in similar environments. Severe wind gusts are likely with 850hPa wind speeds of 20-25 m/s and a well mixed airmass, so a level 1 for this risk was issued. Otherwise, marginal hail is possible, given lowered WBZ-levels.

The level was expanded far eastwards, as convective activity - not necessarily electrified - is possible well inland beneath 20m/s 850hPa wind field.