Storm Forecast
Valid: Thu 30 Oct 2008 06:00 to Fri 31 Oct 2008 06:00 UTC
Issued: Wed 29 Oct 2008 21:08
Forecaster: GATZEN

SYNOPSIS

Intense am amplified trough over western Europe extends into Iberian Peninsula. Strong upper jet streaks are located along the periphery of the trough. On Thursday, a 60 m/s 300 hPa jet streak spreads northward from central Italy to western Poland and into the Baltic Sea later in the period. Another jet streak curves around the troughs base and enters north-western Mediterranean. At lower levels, two lows are located over Bay of Biscay region and eastern Germany. A cold front will extend from southern Italy to central Balkans, Czech, and western Poland, and warm and moist air mass is advected northward in the range of a strong low-level jet pointing into southern Baltic Sea. Cold polar air masses over western Europe will be pushed to the north as warm maritime air mass spreads into Bay of Biscay.

DISCUSSION

Sicily to southern Adriatic, southern Balkan region

Ahead of the surface cold front, rich low-level moisture is indicated by latest observations. Soundings also show steep mid-level lapse rates extending over a broad area, where CAPE values up to 1000 J/kg are indicated. A rather weak inversion is present at low levels. On Thursday, strong QG forcing is forecast as axis of upper short-wave trough will move north-eastwards, and latest model out indicates increasing instability due to ageostrophic flow and upper height falls reaching 1500 to 2000 J/kg. Thunderstorms seem to be quite likely along and east of the cold front given low-level forcing and upslope flow near the coasts. Impressive vertical wind shear in the right entry region of the mid-level jet streak is expected to favor supercells and especially well-developed bow-echoes. Severe wind gusts seem to be quite likely along the cold front. Isolated tornadoes are also forecast given strong low-level vertical wind shear of 10 m/s in the lowest kilometer. Severe hail is not ruled out. Convection will likely spread north-eastwards into Balkans and Greece late in the period, where severe threat is forecast to decrease given weaker instability and forcing. However, isolated severe weather is not excluded along the cold front throughout the night over the Balkans and Greece.

Poland and southern Baltic States

Strong mid-level jet streak moves northward over Poland, associated with strong QG forcing. Along the well-developed frontal boundary extending over western Poland, models indicate that moist air mass will spread north-westward at the 850 hPa level, where strong low-level jet is forecast to reach 25 m/s. Ageostrophic flow and upper height falls are likely and models indicate increasing instability from Czech to western Poland ahead of the cold front. Given strong forcing, deep convection is expected to develop, and showers and thunderstorms will likely move northwards over eastern Germany and western Poland. Main question is where this convection will root to the relatively cool but moist boundary-layer. This seems to be most likely over central Poland, where temperatures will likely reach about 16 to 18°C with dewpoints around 12°C. Storms that root to the boundary-layer will have a potential of producing tornadoes given up to 20 m/s 0-1km vertical wind shear and strong low-level veering profiles in the warm sector, and a few events are forecast. Even strong tornadoes are not ruled out. Severe wind gusts are also possible. During the day, stable low-level air mass over western Poland is expected to spread eastward, limiting severe potential. However, isolated tornadoes are not ruled out in the warm sector over southern Baltic Sea, southern Finland, and Baltic States in the evening and night hours given rich low-level moisture and strong low-level vertical wind shear.

Southern Bay of Biscay region

South of low pressure system over Bay of Biscay, maritime air mass spreads into Iberian Peninsula and southern Bay of Biscay region. Strong QG forcing at the cyclonic flank of the mid-level jet streak will likely be associated with widespread rain and embedded showers given moist low-level air mass and rather cool mid-levels. Strong vertical wind shear just along the coasts may be favorable for a few tornadoes affecting northern Spain and south-western France during the period. Overall threat seems to be quite weak given rather poor low-level buoyancy and limited low-level forcing.