Storm Forecast
Valid: Thu 02 Oct 2008 06:00 to Fri 03 Oct 2008 06:00 UTC
Issued: Thu 02 Oct 2008 05:59
Forecaster: GATZEN

SYNOPSIS

Intense polar trough over northern Europe expands to the east during the period as strong mid-level jet spreads into Russia. At the same time another short-wave trough ejects at the western flank of the long-wave trough and amplifies into France and Iberian Peninsula. As a consequence, very strong upper level westerly jet streak over Europe gets a more south-westerly orientation from the Pyrenees to the Alps, Baltic region and western Russia. Cold air advection will go on the wake of the trough over western Europe and spread into northern Iberian Peninsula behind a cold front. To the east, frontal boundary becomes quasi-stationary over eastern Europe. Warm and unstable air over east Mediterranean is expected to spread north-east ahead of the main mid-level long-wave trough.

DISCUSSION

East Mediterranean including Aegean

A 35 m/s mid-level jet streak moves north-eastward into southern Greece and Aegean during the period. It affects a frontal boundary over south-east Mediterranean that forms a weak frontal wave. Associated warm air advection is forecast over southern Aegean during the period. To the north of the upper jet streak, mid-level height falls are expected to spread into Greece and Aegean. Soundings show indications that rather steep mid-level lapse rates have likely developed from southern Italy to western Greece that will spread eastward during the period. At low levels, rich boundary-layer moisture is indicated by latest observations and ascends, and this will likely result in moderate instability during the period.

Given quite robust forcing in the range and north to the frontal wave, initiation is forecast and has already begun to the south of Italy. Thunderstorms will spread into Greece during the next hours and will likely affect the Aegean in the afternoon and evening hours. Convection will likely organize into well-developed multicells as 0-3 km vertical wind shear will increase to 15 to 20 m/s in the range of the frontal wave, where south-westerly mid-level jet will strengthen in the next hours. Embedded supercells are also not ruled out as low-level veering profiles and backing boundary-layer winds are forecast ahead and north of the frontal wave from western Greece to Aegean during the period. Isolated large hail and severe wind gusts are forecast. Tornadoes are also expected given moderate to strong low-level vertical wind shear in the range of 7.5 m/s in the lowest kilometre, as well as quite buoyant low-level air mass due to rich moisture. Best potential for tornadoes will extend from western Greece in the morning hours to eastern Greece by noon/afternoon. Thunderstorms may evolve into clusters with embedded bowing lines that will propagate eastward into Turkey in the evening and night hours. Local hail and tornadoes are forecast as well as severe wind gusts along the leading gust fronts. Intense precipitation may lead to local flash flooding during most of the period.

Balearic Islands to central Mediterranean

Cold front will enter the west Mediterranean Sea in the afternoon and evening hours and will move further south during the night. Latest Palma de Mallorca sounding indicates that moist low-level air mass is still present over the Mediterranean Sea. Ahead of the cold front, strong mid-level jet will spread southward, and upper height falls will likely lead to increasing instability. Expect a convective line along the cold front and also some convection initiating to the south of it that will likely intensify during the night hours as instability increases. Given strong vertical wind shear, storms will likely be well-organized capable of producing severe wind gusts and isolated large hail. Best potential exists near the Balearic Islands where instability is forecast to be strongest, while it will gradually weaken to the east. There is also a potential of tornadoes given strong low-level forcing and increasing low-level vertical wind shear of about 10 m/s in the range of the cold front. Storms will likely go on in the morning hours.

Western Russia

Ahead of the strong upper jet streak, a strongly forced convective line travels into Russia. Weakening instability is forecast and lightning was not reported. However, strong low-level vertical wind shear is present, and a tornado is not ruled out initially where some low-level buoyancy may develop. Severe wind gusts are more likely, but severe threat will further decrease as low-level moisture is poor.

North-western Europe

Especially over the Sea, some waterspouts may occur during the period underneath the trough axis due to quite strong low-level buoyancy and weak vertical wind shear.