Storm Forecast
Valid: Fri 02 May 2008 06:00 to Sat 03 May 2008 06:00 UTC
Issued: Thu 01 May 2008 20:06
Forecaster: TUSCHY

SYNOPSIS

Really not many changes to talk about as weak pressure gradients prevail over most parts of Europe. Retrograde southwestward shifting cyclone over the far NE Atlantic induces an intense surface depression on its SE quadrant which again strengthens WAA on the downstream periphery of the broad cyclonic vortex. The system is far west of Europe and so is the strongest axis of WAA but nevertheless, mid-levels warm up somewhat over western Europe ( e.g. France / S - UK ) so modifications to yesterday's broad and far westward extending general thunderstorm area become necessary.
Otherwise no changes for the rest of Europe with diurnal driven and sub-severe pulsating thunderstorm activity.
The focus for most intense initiation will be an eastward moving frontal boundary, slowly degenerating over Romania / Bulgaria into a loosely structured occlusion. Still, convergence along this boundary will serve as a focus for scattered to widespread thunderstorm development.

DISCUSSION

.... Bulgaria ...

Latest surface observations show dewpoints ranging from 10°C over W - Bulgaria to near 13°C over central / E - Bulgaria. Latest sounding reports from Bulgaria and SE Romania reveal a well mixed boundary layer with a strong T - Td spread and very steep LL lapse rates. Mid-levels show somewhat weaker lapse rates but this column should cool down 1 or 2 °C during the forecast period. Those parameters combined with a forecast increase of the BL moisture should yield moderate instability release of 500 - 1000 J/kg MLCAPE . DLS increases from the north ( 10 m/s ) to the south ( near 20 m/s ) and combined with locally intense updrafts, large hail will be a threat with hail stones of locally up to 4cm being a possibility. Coverage of storms should be quite high as both surface front and a weak UL impulse cross the area during maximized diurnal heating when atmosphere is only weakly capped.
Thunderstorms will cluster while moving eastwards and they will go on until 06Z over far eastern Bulgaria and offshore.

... Romania and Moldova ...

The same like described above but DLS is very weak with mostly sub - 10 m/s shear values forecast. BL is also not that humid with dewpoints just below 10°C but still 400 - 800 J/kg MLCAPE will be possible. Storm mode will be pulsating and upscale growth into one or more clusters is likely. Despite an isolated large hail / severe wind gust report during the mature phase of an intense thunderstorm, no real severe thunderstorm risk can be expected.
LL CAPE values are enhanced with very strong signals over Moldova as BL is well mixed and an isolated funnel report can't be excluded. We therefore decided to include those regions in a marginal level - 1 area as isolated large hail / funnel reports could justify one. However weak shear makes this a low-end level - 1 situation.
Thunderstorms will continue over E - Romania / Moldova and offshore until 06 Z.

The same for NE Poland, W - Belarus and S - Lithuania ( an isolated large hail risk ) but also for SW Finland ( impressive LL CAPE and therefore an augmented funnel / tornado threat ).

W - Romania, N - Serbia, S - Hungary, N / NW Austria, the Czech Republic and E - Germany are all placed along the cyclonic side of a strong UL jet with modest instability release and 10 - 20 m/s DLS. Shear could support some local storm organisation but overall environment does not support a level area as the main risk will be sub-severe hail / wind gusts. An isolated large hail report can't be excluded but confidence is too low to increase the probabilities.

N - Ireland / Scotland and parts of UK will see some low - end instability release combined with stronger DLS but warming from the south should help to decrease thunderstorm probabilities over S - UK during the day. The same story for the EL temperature which indicates an increasingly more hostile environment for deep convection. A few stronger thunderstorms could still develop over the area of interest but nothing severe expected.

... N / NE Algeria ...

Don't see a reason why to agree to the very optimistic instability forecast of latest GFS 12Z run, indicating more than 1 kJ/kg SBCAPE over NE Algeria. Temperatures at 500 hPa don't change a lot and LL get only warmer ( e.g. 850hPa temperatures warm up by ~ 5°C ). Impressive mid-level lapse rates, 15-20 m/s DLS would be favorable for large hail but at the moment we only expect a very isolated and short-lived thunderstorm in the highlighted area.



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