Storm Forecast
Valid: Fri 02 Oct 2009 06:00 to Sat 03 Oct 2009 06:00 UTC
Issued: Thu 01 Oct 2009 20:00
Forecaster: TUSCHY

A level 2 was issued for SE-Italy, the S-Adriatic Sea and parts of the Ionian Sea mainly for excessive rainfall and tornadoes.

A level 1 was issued for the regions surrounding the level 2 mainly for excessive rainfall and tornadoes.

A level 1 was issued for parts of W-Russia mainly for severe wind gusts.

SYNOPSIS

A branch of the polar vortex affects N-Europe with cyclonic flow, overspreading all of Europe. Numerous disturbances circle around this branch with the most prominent one crossing the central Mediterranean from west to east/northeast. Stable conditions restrain to extreme SW-Europe.

--- Side note ---

Since the 28th Sept. 09, a strong cold-core vortex SE of Newfoundland dropped southeastwards for the past few days, straight towards the Azores. A prolonged time atop of increasingly warming SSTs gradually caused a warm-up of the cold low/mid-level cold core. Latest data (1st Oct., 17 UTC) have the center at 38N,29W above SSTs of 22-24°C (slightly positive anomaly compared to the climatology) and forecast track is somewhat erroneous in loops around the Azores during the followinf forecast and thereafter. IR/WV composite reveals an environment, already seen in other subtropical events with high-level (upper troposphere/lower stratosphere) airmass overspreading the center with oscillating convective activity along the center. For now, convection was too weak and short-lived for any classification but a tendency is seen to more persistent convection, wrapping around the center. 12Z sounding of Lajes samples the core of this feature well with a moist and warm troposphere and abundant CAPE release. Synop data has seasonably moist airmass covering the islands with low T-Td spread and 15-20kt southerly winds over the eastern part. This is in line with the QuikScat data, showing strongest winds of 30-35kt to the south of the center, but likely spreading northwards during the forecast period. GFS and ECMWF are in line with the development of a shallow warm core and 12 UTC Met9 data has another strong burst of convection wrapping nearly completely around the center with a thin cirrus canopy. A personal classification would be a ST2.5-3.0 (in respect to the Herbert-Poteat technique) due to the increasing organisation of the convection next to the center and hints on developing banding features next to the low-level circulation center. The National Hurricane Center, responsible for official warnings, recently highlighted the area, however no development of a subtropical cyclone is forecast ( chances less than 30%). Despite the more technical discrepancies, strong wind gusts will probably affect the Azores during the forecast.

DISCUSSION

... Sicily, S-Italy, the Ionian Sea and parts of the Adriatic Sea...

An upper trough crosses the central Mediterranean, before lifting northeastwards during the night hours. A strengthening cyclonic vortex between 1-5km AGL is forecast by all models, which agree in some modest strengthening throughout the forecast, crossing Sicily and S-Italy from the SW. Ahead of this depression, a persistent LLJ advects a moist and warm airmass northwards, affecting the Ionian Sea, extreme SE-Italy, the S-Adriatic Sea and the coastal areas of W-Greece.

0-6km / 0-3km bulk shear are weak but main focus is the strength of the LL field, dependent on how fast the cyclone intensifies. Tornadoes are possible SE of a line Napoli-Bari and along the west coast of Greece and Albania as SRH1 and speed shear increase. Otherwise the main risk is excessive rain as numerous thunderstorm clusters are forecast, maximized over SE-Italy, Albania and W-Greece. Decreasing instability onshore causes a rapid decrease of thunderstorm probabilities but an isolated embedded storm can't be ruled out. Therefore the level 1 was expanded well inland. We went far south with the thunderstorm line over the Mediterranean as yesterday's QPF forecast was already pessimistic and a backbuilding cluster could affect even the far southern Ionian Sea during the forecast.

...Baltic Sea and adjacent regions ...

Placed under an expansive trough, cold mid-levels cause enhanced and diurnal driven thunderstorms onshore with a weak diurnal signal offshore. Scarce LL moisture mixes out rapidly onshore, so main thunderstorm risk will be confined to the coastal areas and N-Poland, Lithuania, Latvia and Estonia. Despite an isolated spout risk offshore along the coasts, marginal hail and gusty winds, no organized thunderstorm risk is expected.

... W-Russia ...

An active cold front crosses the level 1 area from the west until the afternoon hours. Marginal CAPE ought to keep thunderstorms more isolated, but shear is adequate for severe wind gusts during the passage of the front, enhanced by embedded thunderstorms. The risk diminishes thereafter as prefrontal instability tongue vanishes.