Storm Forecast
Valid: Thu 21 Oct 2010 06:00 to Fri 22 Oct 2010 06:00 UTC
Issued: Wed 20 Oct 2010 19:01
Forecaster: GATZEN

A level 1 was issued for the Ukraine and western Russia mainly for severe wind gusts.

SYNOPSIS

The axis of an intense long-wave trough will move north-eastward on Thursday, crossing most of eastern Europe. A strong mid-level jet will curve around the negatively-tilted trough and will affect especially the Ukraine and western Russia. A cold front will extend from western Russia to the Black Sea and farther into the south Aegean. In the wake of the trough axis, another strong mid-level jet streak will travel across the North Sea into Poland. It will be associated with warm air advection and cyclogensis over southern Scandinavia.

DISCUSSION

Ukraine to western Russia

The nose of a strong jet streak spreads into the Ukraine and western Russia in the morning hours. Ahead of the negatively tilted trough axis of the European trough, a frontal wave may develop along the cold front. This would support an overlap of rather rich boundary layer moisture with the forcing at the cyclonic flank of the jet. Strong low-level south-westerly winds will additionally advect warm air masses to the east of the cold front. This warm air mass will likely be stable at most places with only 10 to 12°C low-level temperatures and 8°C at the 850 hPa level. Current thinking is that stratiform rain will spread north-eastwards in the warm air advection regime. Along the cold front, a narrow rain band will likely develop and move eastward from the western Ukraine to western Russia on Thursday.

Along the narrow cold frontal rain band, models indicate weak instability especially in the morning hours near the low pressure centre, where a mid-level temperature decrease below -20°C is indicated. Given equilibrium level temperatures below -10°C, some thunder may occur. The main threat of this convection will be severe wind gusts. Tornadoes that develop along the leading gust front are also not ruled out. In the afternoon hours, the jet nose will travel northward, where low-level moisture will be too weak for deep moist convection.

In the wake of the cold front, the trough axis is expected over the western Ukraine region in the afternoon hours. Although dry low-level air will be present west of the cold front, low mid-level temperatures are expected to result in weak instability. Showers that develop are forecast to produce some thunder given equilibrium-level temperatures around -20°C as forecast by latest GFS. The severe potential is rather weak, but given strong low-level vertical wind shear in excess of 12 m/s in the lowest kilometre a tornado is not completely excluded.

Aegean Sea

At the southern parts of the cold front, strong cold air advection and differential anticyclonic vorticity advection will limit the potential for initiation over the Black Sea region. Over the Aegean, weak mid-level forcing will not suppress some convection near the surface front that will become quasi-stationary. Rich low-level moisture and decreasing mid-level temperatures from the west will enhance the potential of showers that may also be thundery. The best potential exists in the evening and night hours over the Aegean Sea and to the south of Greece. Severe storms are not forecast given the weak vertical wind shear.

North- and Baltic Sea region

In the wake of the trough axis, another strong mid-level jet streak will travel eastwards. Explosive cyclogenesis is forecast over southern Scandinavia along the nose of this jet streak, and warm air advection will likely result in rather weak lapse rates. In the wake of the low the lapse rates of the maritime polar air mass will likely increase over the sea surface near southern Norway. Showers and thunderstorms are expected that will spread eastwards during the night hours and may also affect the southern Baltic Sea. Although a tornado is not completely ruled out near the coasts, where surface-based instability will overlap with 10 m/s low-level vertical wind shear, main threat will be severe wind gusts. However, these gusts are expected to be mostly related to the strong synoptic pressure gradient.