Storm Forecast
Valid: Thu 08 Dec 2011 06:00 to Fri 09 Dec 2011 06:00 UTC
Issued: Wed 07 Dec 2011 20:54
Forecaster: GATZEN

A level 1 was issued for the UK and North Sea region mainly for tornadoes and severe wind gusts.

SYNOPSIS

Two strong mid-level jet streaks will affect the forecast region. One will be present at the southern tip of a deepening trough that expands from the Balkans to western Turkey during the period. The second will cross the UK and is expected to extend across northern Germany and Poland at the end of the period. Both jet streaks are expected to be the main foci of severe convection.

DISCUSSION

UK and North Sea region

A intense low will move across the northern British Isles and the North Sea. Ahead of the cold front that is expected over the central UK at noon and over the eastern North Sea in the evening, a rather moist low-level air mass is predicted by latest numerical models. Low-level mixing ratios of 6-7 g/kg may be advected into most of the UK and the North Sea region. The cold front will move south-eastwards rather quickly and will mostly stay ahead of the strong jet streak axis. Nevertheless, lift will likely enable weak/shallow CAPE in the moist air mass. The strong lift along the cold front will likely lead to a narrow cold-frontal rain band. Thunderstorms seem to be rather unlikely, but are not ruled out, especially along the northern portions of the cold front, where the overlap of strong lift near the jet axis and low-level moisture will be most likely.

Given the very strong vertical wind shear and favourably veering profiles ahead of the cold front, tornadoes are not ruled out. Additionally, severe wind gusts are likely with 35 m/s winds at 850 hPa.

In the wake of the cold front, a strong sting jet is indicated by latest models. Although showers and thunderstorms may develop in the vicinity of this jet, their contribution to severe wind gusts near the surface is not expected to be significant. Wind gusts in excess of 40 m/s will be well possible even when deep moist convection is absent.

Balkans

The second jet streak will move southward along the Balkans. At low-levels, models indicate that some moisture from the Mediterranean Sea will spread into the Balkans ahead of a cold front. The lift along the jet streak axis may be sufficient to produce steep lapse rates and CAPE. Main uncertainty will be the transport of the moisture. Current thinking is that only marginal CAPE will develop. That will likely reduce the potential of severe storms, although strong vertical wind shear (20 m/s 0-3km bulk shear) will overlap with the instability. Observations may indicate a better moisture transport and may require an upgrade to level 1 over this region.