Storm Forecast
Valid: Thu 19 Aug 2010 06:00 to Fri 20 Aug 2010 06:00 UTC
Issued: Wed 18 Aug 2010 18:38
Forecaster: GATZEN

A level 2 was issued for portions of the west Mediterranean Sea mainly for severe wind gusts and large hail.

A level 1 was issued for the west Mediterranean Sea and eastern Iberia mainly for severe wind gusts and large hail.

A level 1 was issued for western Russia mainly for large hail.

SYNOPSIS

A westerly flow dominates Europe. A mid-level jet streak spreads into eastern Europe, while a broad ridge starts to form over western Europe. A trough that is initially centred over Iberia will weaken and spreads north-eastward together with a mid-level jet streak at its eastern flank. At the 850 hPa level, a broad frontal zone has formed across Europe. While stable conditions are present over most regions due to rather dry low-level air and warm mid-levels, some moisture and steeper lapse rates in the range of the troughs are expected to lead to instability during the period.

DISCUSSION

West Mediterranean

A cut-off low centred over Iberia on Thursday morning will accelerate northward and is expected to weaken in the wake of the stronger northern trough axis. However, a strong mid-level jet streak exceeding 30 m/s at the 500 hPa level will continue and travels across the west Mediterranean until Friday morning. At lower levels, an EML is expected to spread northward ahead of this trough and is expected to affect also the westernmost portions of the Mediterranean to some extend. Cool mid-level air in the range of the trough centre will also lead to increasing lapse rates over the west Mediterranean during the day.

The low-level air mass is characterized by ample moisture indicate by latest observations, and GFS simulates a low-level mixing ratio of about 14 g/kg. Given an easterly low-level flow ahead of the approaching low, strong moisture flux convergence can be expected to the east of Iberia, and diurnal upslope flow is expected to support initiation.

Current thinking is that storms do develop especially along the eastern coasts of Iberia. Additional storms may also survive along of outflow-boundaries of convection over northern Algeria during the night hours. Storms that form over the Iberian Peninsula may become severe given favourable veering profiles. The strong vertical wind shear will be placed over the Mediterranean Sea, however. Storms that move into this region where rich low-level moisture and instability with CAPE in the order of 1500 J/kg will overlap with strong vertical wind shear of 30 m/s in the lowest 3 km are forecast to have a rather high severe potential.

Along the frontal boundary placed over the west Mediterranean, one or two bow echoes may develop and are expected to travel rapidly north-eastward. Severe wind gusts will be the main threat. The high moisture is expected to support an excessive rain threat and local flash flooding, while large hail and tornadoes are not ruled out given the favourably veering profiles.

Although instability will decrease during the period as the mid-level trough weakens, and the low-level frontal boundary will also weaken, the resulting MCS may spread into southern France or north-western Italy during the night hours. Severe wind gusts are not ruled out given the rather strong vertical wind shear.

Russia

A trough travels into Russia and overspreads a moist low-level air mass present to the south of a polar cold front. Latest models expect some CAPE and rather strong westerly winds will be associated with moderate vertical wind shear. Thunderstorms are forecast to produce locally large hail. A brief tornado is not ruled out. Storms will rapidly weaken after sunset.

Eastern Europe

In the range of a mid-level trough, models indicate some instability overlapping with a strong westerly flow and increasing vertical wind shear in the afternoon and evening hours. A brief tornado and hail threat is expected, while the chance seems to be too marginal for a level 1.