Storm Forecast
Valid: Sun 09 Mar 2008 06:00 to Mon 10 Mar 2008 06:00 UTC
Issued: Sat 08 Mar 2008 19:12
Forecaster: SCHLENCZEK

SYNOPSIS

The southwestern part of a large low pressure complex will be the origin for a rapid cyclogenesis. On the cold side of a 100 m/s upper level jet streak south of Greenland, an amplifying upper shortwave trough is forecast to move eastward, affecting the British Isles on early Monday morning. Associated baroclinic wave will intensify rapidly, leading to a surface low with less than 945 hPa SLP in the centre west of Ireland. The cold front is expected to cross Ireland and western UK around midnight and should stretch from central UK via northwestern France towards extreme northwestern Spain on Monday morning.

During the first half of the forecast period, an upper trough over western Europe, filled with cold polar airmass, will spread into western Mediterranean in the evening.

A weakening upper trough over eastern Mediterranean will move northeastward.

DISCUSSION

extreme western UK and northwestern France

Near the cold front of the surface low west of Ireland, vertical shear as well as strong QG forcing should support linear convection but instability will play a critical role. Vertical cross sections show an airmass with moist adiabatic stratification near the cold front. At 700hPa, more than 35 m/s should be present and severe gusts due to downward momentum transport may occur. Any isolated cell ahead of or within the line might develop a rotating updraft and an isolated tornado cannot be ruled out as very high SRH and 15 to 20 m/s low level shear will be present. Some uncertainties with respect to the instability exist and we decided to issue a level-one threat. An update may become necessary if instability is much greater / less than expected.

Northwestern Ireland will experience numerous non-convective gusts between 25 and 40 m/s due to a strong pressure gradient near the trough axis.

Western Mediterranean

As the cold upper airmass overspreads western Mediterranean, a few hundred J/kg of CAPE should be created. Vertical shear and forcing do not look that impressive and severe weather should not occur. Another region with cold air convection should be the Norwegian Sea but lightning is expected to be too sporadic for a thunderstorm area.

Central Mediterranean and Aegean Sea

Some few hundred J/kg of CAPE should be present over the Aegean Sea but vertical shear as well as low level buoyancy are rather weak. Better conditions are expected over southern Greece where some organized multicellular storms with isolated large hail and marginally severe gusts may evolve but overall threat should stay below the level-one threshold.