Storm Forecast
Valid: Thu 12 Nov 2009 06:00 to Fri 13 Nov 2009 06:00 UTC
Issued: Wed 11 Nov 2009 20:17
Forecaster: GATZEN

A level 1 was issued for portions of England and Wales mainly for severe wind gusts and for a lower extend for tornadoes.

SYNOPSIS

A broad trough over Europe moves north-eastward, while warm air advection sets in over western Europe ahead of an Atlantic trough. This is associated with a strong mid-level jet streak spreading into western Europe. At lower level, a strong jet will advect moist air across the Bay of Biscay and the British Isles late in the period. Further east, most of Europe is dominated by dry air masses. Over the Turkey region, rather moist low-level air mass is present in the range of the trough.

DISCUSSION

Turkey
Ahead of the trough axis of the approaching long-wave trough, a rather strong mid-level jet streak will spread into Turkey in the morning hours, leading to some QG forcing. At lower-levels, a rather moist south-westerly flow will be present. Latest model output suggests rather stable conditions ahead of the trough axis, but QG forcing will likely be strong enough to lead to moist adiabatic lapse rates at low-levels. Especially along the southern coasts of Turkey, on-shore lifting may lead to some embedded convection in the morning hours. Near the coasts, low-level vertical wind shear will be around 10 m/s and convection that root to the boundary-layer may evolve low-level rotation and therefore pose a risk of tornadoes. Limiting factors will be rather poor low-level buoyancy, and the probability is expected to be too weak for a categorical risk. Furthermore, strong precipitation is possible along the coastal regions initially. As the trough moves on, the strong on-shore lift will likely end, while low-level mixing is likely near the coasts. To the west, numerous showers and thunderstorms are likely in the range of the trough axis. Given weak vertical wind shear, weak low-level convergence and the advection of rather dry low-level air masses, severe thunderstorms are not forecast.

Portions of England and Wales

Models agree on the development of a strong mid-level jet streak spreading into the northern Bay of Biscay in the evening hours, and strong QG forcing will be present in the warm air advection regime ahead of the surface cold front of low pressure over the Atlantic Ocean. A 25 m/s low-level jet is forecast by latest numerical models leading to strong low-level vertical wind shear, while the low-level mixing ration may reach more than 8 g/kg in the south-western portions of the British Isles. Weak instability is forecast due to differential warm air advection in an air mass characterized by nearly moist-neutral lapse rates. Together with strong linear forcing along the cold front, a shallow line of convection is forecast to develop in the evening hours. North of the mid-level jet, deeper convection will likely evolve cold pools due to melting of graupel and snow, helping the convection to organize. Bowing elements are expected, associated with severe wind gusts and possibly tornadoes in the range of embedded mesocyclones. The convective line will rapidly spread into the North Sea late in the period, where the potential of deep convection decreases due to weaker low-level moisture.