Storm Forecast
Valid: Thu 31 May 2007 06:00 to Fri 01 Jun 2007 06:00 UTC
Issued: Wed 30 May 2007 21:55
Forecaster: GATZEN

SYNOPSIS

Low geopotential over west Europe weakens, while a high pressure center develops over Scandinavia. Over south-west Europe, a strong upper jet is present at the southern flank of European trough. At lower levels, relatively cool air mass is present over Europe west of a line from the Baltic States to Turkey.

DISCUSSION

Southern France

Maritime air mass spreads into France in the range of a surface low that develops ahead of the axis of approaching western European trough. This air mass will likely have poor lapse rates, almost no CIN and quite rich moisture. Given some diabatic heating during the day as well as low-level convergence and QG forcing, showers and thunderstorms are expected to form. Given rather strong deep layer vertical wind shear underneath the westerly jet stream, thunderstorms may organize into multicells given unfavorable veering winds. Strong wind gusts are forecast to be the main threat, and isolated severe gusts are not ruled out. Given relatively strong low-level buoyancy and weak CIN over most of southern and central France, weak tornadoes are also expected in the afternoon.

Western Russia, eastern Balkans, northern Turkey

Warm and unstable air mass remains over west Europe. In the range of the frontal boundary mentioned above, thunderstorms initiation is most likely due to low-level convergence. Given weak to moderate deep layer vertical wind shear in the range of upper jet that curves around the European trough as well as rather strong instability, a few supercells are forecast to develop, capable of producing (very) large hail and isolated severe wind gusts. Chance for tornadoes is forecast to be relatively weak given weak low-level vertical wind shear. Thunderstorms may merge into clusters during the afternoon hours and are expected to weaken during the night hours. Further east, deep layer vertical wind shear will be even weaker, and severe thunderstorms are not expected. However, an isolated large hail report is not excluded.

Northern Russia

A very intense cold air outbreak starts over northern Russia today. Associated strong jet streak is forecast over northern Russia. Embedded short-wave trough and surface low are expected to move eastwards south of the main frontal boundary. Strong WAA is expected ahead of developing surface low over northern Russia. Advected air mass is characterized by strong instability given well-mixed low and mid levels over central Russia and rich boundary layer moisture over parts of northern Russia. Expected scenario is that upper height falls will lead to strong instability in the warm sector of the surface low, and that thunderstorms will initiate during the day. Very favorable vertical wind shear of 15 m/s LLS, 25 m/s DLS and 500 J/kg 0-3 km SRH will be present by latest GFS model output, and thunderstorms will rapidly evolve into supercells, capable of producing very large hail and severe wind gusts. Especially isolated supercells will also pose a significant threat for tornadoes that may be strong or even violent given strong low-level wind shear in the warm sector. Thunderstorms will likely organize during the evening ahead of approaching cold front. Severe wind gusts will be the main threat in the evening/night hours.