Storm Forecast
Valid: Mon 26 Apr 2010 06:00 to Tue 27 Apr 2010 06:00 UTC
Issued: Sun 25 Apr 2010 20:39
Forecaster: TUSCHY

A level 1 was issued for parts of W-Poland and extreme N-Czech Republic mainly for large hail.

SYNOPSIS

The pattern can be characterized by ridging, which builds in over W-Europe and a vast upper trough further east with numerous smaller-scale disturbances. This configuration features an active day in respect of diurnal driven thunderstorms over most parts of central Europe. The main focus for somewhat more organized convection arises over Poland, as a weakening upper trough draws near from the west.

DISCUSSION

... E-/NE-Germany, N-Czech Republic and parts of Poland...

The models gradually converge to a solution, where scattered thunderstorms overspread the highlighted area from west to east during the day. This area may be quite expansive in space due to the strong tilt of this trough, as surface pressure channel/embedded convergence outruns (12-15Z over W-Poland) the mid-level cold pool (12-15Z over N-Germany).

During the morning hours, the slowly eastward moving vorticity lobe may already spark a few thunderstorms over central/east Germany, but coverage and intensity increases around noon and thereafter, as ongoing diabatic heating assists in hitting the convective temperature over a broad area. Thunderstorms therefore spread eastwards during the afternoon hours, eventually affecting west Poland during the late afternoon hours and central Poland until midnight.

Final degree of instability is still a bit uncertain as BL recovery is forecast to be quite impressive. However, a favorable pressure pattern for ample LL theta-e advection from the south, moisture pooling along eastward sliding convergence zone and constantly steepening mid-level lapse rates support the idea of WRF/GFS with at least up to 500J/kg MLCAPE over the highlighted region, maximized over W-Poland with up to 800 J/kg.

A weak and mostly unidirectional wind field beneath 5km dominates the picture throughout the day, so organized storms will be hard to materialize. Nevertheless, a 300hPa speed max curves in from the west, enhancing anvil-layer shear vectors. This may help to evacuate precipitation from the updraft, so storms may last longer with some degree of organization, especially in the level 1 area. A few large hail/strong wind gust reports are possible due to favorable timing/ovelap of strongest CAPE/shear fields. Thunderstorms tend to weaken over Poland after sunset, as shear and CAPE relax. Slow storm motion may also result in a few location, having some good rain amounts, but no focus yet for anything more widespread.

Numerous showers/thunderstorms also evolve over NE-Germany during the afternoon hours, as mid-level cold pool crosses the area from west to east. A 50%-thunderstorm area was issued but with no appreciable severe risk. Also, general low-prob. thunderstorm area was expanded well to the NW, including NW-Germany, due to the negative tilt of the mid-/upper trough axis atop some postfrontal BL moisture.

... Czech Republic, the Alps, Italy, NE-coast of the Adriatic Sea and extreme E-France ...

All areas beside Czech Republic:

Moist LL beneath cold mid-levels, no significant forcing/speed max and good insolation all indicate a diurnal cycle of thunderstorm development. Pulsating storms may pose an isolated large hail/strong wind gust risk, especially over N-Italy, where DLS approaches 10-15m/s, but that's not enough support for a level 1. Again, slow storm motion and good inflow moisture support may cause a few heavy convecive rainfall reports, but again, no focus is seen for a concentrated swath with higher probabilities. Thunderstorms decrease in coverage/intensity after sunset.

East Czech Republic:

Thunderstorm coverage may become quite widespread after 12Z, as some high-level divergence and the right entrance region of a mid-level streak further north affect the area. The past GFS runs hint on a cluster of showers/storms, spreading eastwards during the day and overall synoptic set-up could support such a scenario. Best forcing grazes the area to the north, the same for the CAPE and shear is weak to modest at best, so expect only strong wind gust/marginal hail with this activity.

...Pyrenees...

An active NW flow sets up during the forecast, as a strong mid-/high level jet approaches from the NW. Placement beneath the jet axis is not the best, so initiation will be more isolated in nature. However, GFS indicates an eastward moving cyclonic surface low over E-Spain, which moves/re-develops offshore after 12Z and therefore supports some weak theta-e advection towards the Pyrenees. A developing moisture convergence zone or the mountains itself will be the foci for a few showers/thunderstorms. Mid-level lapse rates steepen somewhat, so at least modest CAPE may be realized. Kinematics improve throughout the day, so any deeper thunderstorm may organize, while moving rapidly to the SE into better LL moisture. Expected thunderstorm coverage is too marginal for a level 1, but any thunderstorm may pose a large hail and/or strong wind gust risk. The threat diminishes rapidly after sunset. An upgrade may become necessary if thermodynamics become better than currently anticipated.