Storm Forecast
Valid: Thu 14 May 2009 06:00 to Fri 15 May 2009 06:00 UTC
Issued: Wed 13 May 2009 21:54
Forecaster: GATZEN

A level 1 was issued for the Balkans mainly for large hail.

A level 1 was issued for north-eastern Iberia and southern France mainly for excessive rain and large hail.

A level 1 was issued for northern France, Belgium, southern Germany, Switzerland mainly for non-supercell tornadoes.

A level 1 was issued for the northern Tunisia region mainly for large hail and severe wind gusts.

SYNOPSIS

A high west of Scandinavia dominates the northern European mid-level flow with a cold trough over eastern Europe and another short-wave trough moving westward across southern Scandinavia. Western Europe is affected by an Atlantic low that amplifies into Africa, leading to cold air advection into most of the Iberian Peninsula and western France. Warm and unstable air masses remain from the Alps to the Balkans and east Mediterranean.

DISCUSSION

The Balkans

The axis of a broad ridge over central Europe slowly moves eastward and will be situated over the Balkans late in the period. Warm air advection is expected to intensify during the period underneath the right entry region of a strong mid-level jet over the northern Black Sea. Latest upstream soundings indicate steep lapse rates over the Adriatic Sea that will likely advect over the southern and central Balkans on Thursday.

Limiting factor will be the weak boundary-layer moisture that is not expected to increase significantly, but strong daytime heating may be sufficient for latent instability. Some QG forcing and low-level convergence zones are forecast to lead to initiation of local thunderstorms. Given the low wet-bulb zero height as well as relatively strong vertical wind shear in the northern portions of the Balkans, large hail is forecast with the stronger storms. Isolated severe wind gusts are also not ruled out given dry profiles and rather steep low-level lapse rates.

Weak QG forcing and expected CIN may suppress deep moist convection over most places and convective activity will likely decease rapidly in the evening hours.

North-eastern Iberia and southern France

The axis of the west-European trough spread eastward into eastern Iberia during the period. A relatively strong southerly jet is situated just ahead of this trough axis. At the surface, a low is forecast to intensify over the Balearic Islands, associated with easterly winds and advection of rather warm air into the north-eastern Iberian Peninsula.

Given rather strong low-level convergence ahead of a cold front moving eastward over the central Iberian Peninsula as well as QG forcing ahead of the trough axis, widespread convection is likely on Thursday. Main severe threat seems to be excessive rain and local flash flooding given forecast slow propagation of mesoscale convective systems. Embedded mesocyclones may also pose an isolated threat of large hail. Storms are forecast to spread north-eastward into France. In the night hours, severe potential is forecast to decrease due to weaker low-level forcing and stabilization.

Northern France, Belgium, southern Germany, Switzerland

Remaining warm air masses will be unstable due to diurnal heating, and thunderstorms will likely develop. Low LCL heights, locally strong low-level buoyancy, relatively weak vertical wind shear, and low-level convergence lines and outflow-boundaries are forecast to lead to locally favourable conditions for non-supercell tornadoes, and a few events are not ruled out. Limiting factor may be rather cool boundary-layer due to clouds. Stronger storms may also be capable of producing isolated large hail and local flash flooding. Storms are forecast to spread northward and decease in the night hours.

Northern Tunisia region

At the south-eastern flank of the west-European trough, a jet streak travels across central Tunisia during the period, providing QG forcing at its northern flank. Easterly low-level winds are forecast to advect moist air masses into Tunisia, while an elevated mixed-layer is forecast to spread northward, and CAPE in excess of 1500 J/kg is forecast. Low-level convergence will likely be strong enough for initiation.

Given strong vertical wind shear and favourable veering profiles, supercells are forecast to develop, capable of producing large hail and severe wind gusts. A tornado is not ruled out, but weak low-level vertical wind shear and rather high cloud base are limiting factors.