Storm Forecast
Valid: Thu 24 Oct 2013 06:00 to Fri 25 Oct 2013 06:00 UTC
Issued: Wed 23 Oct 2013 22:44
Forecaster: GATZEN

A level 1 was issued for Portugal, western Spain, Bay of Biscay, and south-western British Isles mainly for tornadoes and excessive rain.

SYNOPSIS

South-westerly flow continues across Europe given rather weak geopotential across the Atlantic Ocean. An embedded short-wave trough will move from southern Scandinavia to the Baltic States, affecting Germany and even the Adriatic Sea with its trough axis. Cold air will spread into central Europe in the wake of this trough. From the west, a weak ridge will follow that affects especially France during the daytime. A new Atlantic trough will enter western Europe until the end of the period.

Ahead of the central European short-wave trough, a tongue of moist and locally unstable air spreads into eastern Europe. Low-level moisture will also increase across western Europe. Steep lapse rates are expected from the Mediterranean to the central Balkans. The central European trough will provide DCVA across the Mediterranean and northern/eastern Europe, whereas warm air advection will increase across Iberia and France late in the period.

DISCUSSION

Portugal and western Spain, Bay o Biscay, south-western British Isles

Ahead of the new approaching trough, another tongue of warm subtropical air spreads into western Europe. Low-level moisture will reach more than 10 g/kg. Whereas rather weak lift is expected near the ridge axis from eastern Spain to southern France, a couple of vort-maxima will travel north-east from Portugal across the Bay of Biscay into south-western Ireland and England. Late in the period, a strong 30 m/s mid-level jet streak will spread across the Bay of Biscay.

Given this strong DCVA and WAA over the region, large ageostrophic flow and low-level convergence is expected. Excessive rain seems to be quite likely from Portugal into the British Isles. In the wake of the leading warm front, lapse rates will increase due to lift along the cyclonically-sheared flank of the jet, and CAPE is forecast.

Current thinking is that thunderstorms will first affect Portugal and will spread north-east, reaching south-west England at the end of the period. Storms will be capable of producing excessive rain. Tornadoes will be an additional threat due to the strong low-level vertical wind shear and low LCL heights. A strong tornado is not ruled out. Tornado potential is expected to spread into the central British Isles on Friday (after the forecast period).

North Mediterranean to southern Austria and Slovakia

The central European trough will pass by leaving a cold front that becomes quasi-stationary at the flank of the south-east European ridge. To the south of the front, a warm and well-mixed airmass with steep lapse rates will affect the Balkans. Along the frontal boundary, a tongue of rich low-level moisture will spread into eastern Europe with 0-1 km mixing ratio exceeding 9 g/kg from northern Italy to Hungary and Slovakia. This moisture will partly overlap with the good lapse rates, resulting in CAPE.

Main uncertainty is the low-level buoyancy due to diurnal heating. The highest potential of storms clearly exists across the Mediterranean Sea in the morning hours, and numerous storms are expected to go on from the Ligurian Sea to the northern Adriatic. Due to the slowly eastward movement of the upper trough, storms will gradually spread south-east into the Tyrrhenian Sea and central Adriatic. Some storms may also evolve across northern and central Italy in the noon/afternoon hours, but low-level stable air masses will limit the potential over most places.

The storms that form across southern Europe will be affected by weak shear. Therefore, well organized storms are not forecast. However, an isolated excessive rain event or a waterspout is not ruled out.

Further north-east, storms are expected to develop along the frontal boundary in the noon and afternoon hours. Main limiting factor will be again the weak low-level buoyancy and CIN may be too large for initiation. Storms that my form can become severe given 15 to 20 m/s 0-3 km vertical wind shear. Especially mesocyclones may produce large hail and severe wind gusts. The potential decreases from Austria to Slovakia, but a few events are expected. Storms will rapidly weaken after sunset.