Storm Forecast
Valid: Fri 20 Mar 2009 06:00 to Sat 21 Mar 2009 06:00 UTC
Issued: Thu 19 Mar 2009 17:29
Forecaster: TUSCHY

SYNOPSIS

A temporarily retrograde moving upper trough over S-central Europe starts to cut off with a rapidly intensifying surface depression over Sicily, S-Italy and W-Ionian Sea. Strong ridging persists over W-Europe while an intense cold-core vortex opens the floodgates to another outbreak of very cold air, affecting extreme N/NE-Europe.

DISCUSSION

... Adriatic Sea, Tyrrhenian Sea and Ionian Sea ...

Cold upper low enters the central Mediterranean with widespread moderate instability release expected over the warm waters. The environment is supportive for scattered thunderstorms under the weakly sheared base of the trough with marginal hail/gusty winds. LL CAPE release is not impressive, but synoptic patterns is not unkown for spout reports, so we include the possibility of a few spout events, especially from Sicily NE-wards, where best timing/strongest convection will likely occur.

Another focus is the southward racing cold front, crossing Sicily during the late afternoon hours. Main concern arises as the front finally enters the Ionian Sea from the west ( ~ midnight ) and continues to shift eastwards. Better LL moisture ahead and placed under the left exit of a strong mid-level streak, thunderstorm coverage and intensity will be on the increase. Only the southern fringe of the line will see strongest deep-layer shear, also quite strong at lowest 3km (15-20m/s). We went with a level-1 to highlight the chance of a few severe wind gusts, isolated large hail events.

The immediate W-coast of Greece was included, too for the 03Z-06Z time-frame as the squalline approaches from the west. Directional shear at lowest levels increases markedly and it's not out of question that thunderstorms take profit of that environment, developing LL mesocyclones with a very isolated tornado risk. Hail could also locally reach 2cm with strong wind gusts possible.

Rapidly developing surface depression over S-Italy and attendant teight gradient augment the severe wind gust risk, but best downward transport of strongest impuls occurs in the dry/cold postfrontal airmass, where deep convection is unlikley. Hence this risk is not reflected on the map.

... Slovenia,Croatia and Hungary ...

A swath of marginal better LL moisture covers those areas as some moisture from the Black Sea depression advects SW-wards under very cold mid-levels (readings at 500hPa below -35°C). A diurnal dirven increase in convection is forecast and a few thunderstorms are possible although degree of electrification of expected convection is somewhat uncertain. However confidence is high enough for issuing a general thunderstorm area. Marginal hail and strong wind gusts are possible.

... Extreme S-Portugal ...

Past few GFS runs had persistent signals of roughly 500J/kg SBCAPE over extreme S-Portugal during the afternoon/early evening hours. Strong diabatic heating will push surface temperatures at or above 20°C and forecast sounding show a well mixed layer below ~ 800hPa. The attention turns to a weak mid-level negative temperature anomaly, associated with a SE-ward sliding upper low. Latest runs keep this feature with coldest mid-levels well offshore and at 700hPa a belt of marginal warmer temperatures runs from Portugal SE-wards. No forcing is present during peak heating. However geopotential heights decrease somewhat and we see some weak upper divergence so a very isolated and short lived thunderstorm can't be ruled out. 12Z run (GFS) and NOGAPS/GEM support that idea and a thunderstorm area may be needed, if new model/synop data confirm that.