Storm Forecast
Valid: Wed 18 Jul 2007 06:00 to Thu 19 Jul 2007 06:00 UTC
Issued: Wed 18 Jul 2007 07:18
Forecaster: GATZEN

SYNOPSIS

Rather intense trough present over northwestern Europe. Axis of low geopotential extends from Sweden to southern Norway, and Ireland today. While the Scandinavian low geopotential moves northeastward, the southwestern part shows a tendency to cut off southwest of the British Isles. A strong southwesterly flow is present ahead of the low geopotential from southwestern Europe to the Baltic States. Embedded 300 hPa 40 m/s jet streak now located from Bay of Biscay to northern Germany, Baltic Sea, and southern Finland spreads south-eastward and is expected to extend from southern Baltic Sea to central Baltic States, and western Russia on Thursday morning. Corresponding frontal boundary remains quasi-stationary from southern Spain to southern Germany ahead of the developing cut-off low and associated ridge axis over Germany, and North Sea. Further northeast, cold front accelerates eastward across Czech Rep, most of Poland, northern Belarus, and north-western Russia during the period.

DISCUSSION

Southern Germany, northern Austria, Czech Rep., southeastern Poland

Warm air mass east of the main frontal boundary is characterized by well mixed low- to mid-levels. Low-level moisture has weakly improved from northern Alpine region to southeastern Poland as indiacted by latest observations, resulting in CAPE. Thunderstorms have already formed overnight to the northwest of the surface front from southwestern Germany to Czech Rep and southwestern Poland. During the day, convection is likely to go on/develop furthern south/east along the outflow-boundaries of actual convection as well as due to orographic features. Limiting factor is rather weak QG forcing at the anticyclonic flank of strong upper jet streak. Given strong (around 20 m/s) deep layer vertical wind shear, thunderstorms will likely organize. Isolated supercells will have a potential to produce very large hail and severe wind gusts, while chance for tornadoes is not expected to be high given poor low-level moisture and rather weak low-level vertical wind shear. With supercells, an isolated event is not ruled out, though. An upgrade to level 2 due to the possibility of very large hail may be warranted when it turns out that low-level moisture recovers strongly. Thunderstorms will likely weaken during the night as QG forcing remains weak.

Southern France to western Alps

Rather weak QG forcing starts to increase over southwestern Europe as another strong jet streak moves northeastward ahead of developing cut-off low. In the range of the frontal boundary, latest model outout indicates developing instability due to increasing low-level moisture and upper height falls ahead of the approaching trough/jet streak. Actual soundings do not indicate instability at this time, and latest GFS CAPE values seems to be rather high given weak low-level moisture. Initiation is xpected in the range of the frontal boundary, though, given low-level convergence and diabatic heating during the day. Thunderstorms are expected to develop in the afternoon/evening hours and may go on during the night. Rather strong DLS is forecast, and a few mesocyclones are not ruled out, capable of producing large hail/isolated strong wind gusts.

Finland

Strong upper jet streak over Baltic States will affect southern/central Finland today, where DCVA is forecast underneath the cyclonic flank of the jet axis. Ahead of approaching trough, mid-level height falls are expected. At lower levels, slightly backing winds are forecast ahead of the surface trough, and latest model guidance shows that rather moist air mass spreads northeastward into Finland. Expect that some low-level instability will indeed form, showers and thunderstorms are possible. Given strong low-level vertical wind shear as well as favorable veering profiles, a few severe events are likely with the stronger storms, especially tornadoes. Best potential seems to exist in the afternoon hours, when low-level moisture has recovered and cold air advection has stopped. Although given great uncertainty about low-level moisture development and associated convection, decide to place a level 1 at this time.

Russia

Along the cold front, thunderstorms will likely go on given strong forcing. Limiting factor is weak low-level moisture/instability and latest GFS CAPE calculations seem to be too optimistic. Current thunking is that severe wind gusts and isolated large hail is possible especially where strong diabatic heating is present during the following hours.

British Isles

Low-level convergence and rich moisture as well as weak vertical wind shear in the range of the upper trough center will pose a potential for waterspouts/tornadoes across the British Isles during the day again. Limiting factor is rather weak low-level instability that has to improve during the day.