Storm Forecast
Valid: Fri 25 Apr 2008 06:00 to Sat 26 Apr 2008 06:00 UTC
Issued: Thu 24 Apr 2008 16:51
Forecaster: TUSCHY

SYNOPSIS

Strong ridging continues to arch upward over western / northwestern Europe resulting in strong WAA over those areas. Downstream of this feature, upper-level trough builds southward and as a result most parts of central Europe experience CAA which is most active in the mid-/upper-levels. This constellation results in an extensive area of enhanced thunderstorm activity. Quite cold air overspreads N-Sweden / Norway and Finland but limited moisture in the BL should suppress deep convection.

DISCUSSION

... A broad belt running from Denmark all the way down to Greece / W - Turkey ...

As a result of CAA at upper levels, mid-level lapse rates continue to steepen during the forecast period and in combination with a well mixed BL an extensive area with low - end to moderate instability release is forecast. The main storm mode will be pulsating thunderstorms with strong wind gusts and marginal hail.
There are a few regions where we could see a better environment for severe thunderstorms:

... NE Italy, W - Croatia and Slovenia ...

A very tight shear gradient establishes during the day with DLS around 15 m/s over W - Slovenia, increasing to near 40 m/s over NE Italy. Despite some instability release we do not like the strong cross-mountain component wich would bring those areas in a more downward directed branch ( lee - effects ), but global models hint on initiation. GFS 06Z ( 24th April ) was the most aggressive run during the past few model cycles which had initiation all the way down to central Italy but at the moment this one is thought to be an outlier. We therefore want to wait for more model information / new surface and sounding data before increasing probabilities over NE Italy, W - Slovenia and W - Croatia . There would be a large hail risk if thunderstorms could develop.

... SE Germany and NW Austria ...

The same for those areas. A tight shear gradient sets up with near zero to 15 m/s DLS over NW Austria, increasing to 35 m/s over SE Germany. Regarding parameters neither instability release nor parameters for enhanced storm rotation are convincing but complex terrain and locally stronger instability release could support a more vigorous updraft with hail, reaching our criterion. Intensity and expected coverage do not justify any level area right now but the regions have to be monitored.
Otherwise pulsating thunderstorms will bring a marginal hail / sub-severe wind gust threat.

... NW Spain ...

Despite subsidence and strong capping there are some signals for initiation along the mountains N / NW of Madrid due to strong diabatic heating but confidence is too low that more than an isolated thunderstorm will develop so no area was highlighted. There exists no severe thunderstorm risk.





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