Storm Forecast
Valid: Thu 16 Jan 2014 06:00 to Fri 17 Jan 2014 06:00 UTC
Issued: Wed 15 Jan 2014 23:37
Forecaster: GATZEN

A level 1 was issued for the southern British Isles and western France mainly for severe wind gusts and tornadoes.

A level 1 was issued for western Portugal mainly for tornadoes.

SYNOPSIS

The mid-level flow across Europe is influenced by a high-over-low blocking across Scandinavia and a quasi-stationary long-wave trough over western Europe. A couple of vort-maxima will travel around the long-wave trough during the period. Rather moist maritime air masses can be found across western Europe near the base of the trough. A tongue of rich low-level moisture is also spreading into the North Sea region with the south-western flow ahead of the trough. The east Mediterranean is affected by a deep trough moving east. Rich low-level moisture and steep lapse rates will result in CAPE over the warm sea surface. From the west, subsidence is expected to become dominant due to an advancing mid-level ridge axis. Northern and eastern Europe are affected by dry air masses.

DISCUSSION

Southern British Isles to western France

The west-European mid-level trough expands east during the day. Cold air advection is expected especially at mid-levels, while low-level maritime air masses are advected into the forecast region. Due to strong DCVA, QG lift is expected, and differential cold air advection results in steepening lapse rates from the west. At low levels, a moist south-westerly flow will persist, and weak CAPE will be possible during the day.

Due to the large-scale favourable forcing, showers and thunderstorms forming in the maritime air mass are expected. Especially over the land, strong 0-3 km vertical wind shear will allow for multicells and mesocyclones (favourably low-level veering and 0-3 km SRH around 150 m²/s²). Given a large overlap of strong low-level vertical wind shear (0-1 km bulk shear 12 m/s), mesocyclones and tornadoes are not ruled out. Limiting factors are the rather weak low-level buoyancy and the lack of pronounced low-level warm air advection. Additionally, persistent multicells with bowing segments will be capable of producing severe wind gusts at the leading edge of fast moving cold pools (850 hPa winds around 20 m/s).

Convective activity will likely spread east but is expected to weaken given limited low-level moisture. Additionally, advection of drier low-level air will reduce the instability in the evening hours across most of France. Convective activity is therefore forecast to weaken.

East Mediterranean

Steep lapse rates are indicated by latest soundings in the range of the base of the mid-level trough across the east Mediterranean Sea. Some thunderstorms are currently going on along the coasts of Greece. On Thursday, isolated to scattered thunderstorms are forecast as well. Weak vertical wind shear will be present, and waterspouts may develop especially along coastal fronts in the morning hours. Convective activity will spread east during the day and chance of waterspouts is expected to decrease.

Western Portugal

Late in the period, deepening Atlantic short-wave trough will affect western Portugal. A moist low-level south-westerly flow and increasing lapse rates from the west will lead to some instability reaching the coasts in the morning hours. Given the strong low-level vertical wind shear, mesocyclones capable of producing tornadoes are not ruled out. Convective activity will continue on Friday given low-level warm air advection.