Storm Forecast
Valid: Thu 27 Jan 2011 06:00 to Fri 28 Jan 2011 06:00 UTC
Issued: Wed 26 Jan 2011 22:37
Forecaster: GATZEN

A level 1 was issued for southern Italy to southern Greece mainly for tornadoes.

SYNOPSIS

Low geopotential is present over Europe to the east of a high that is centred over the northern British Isles. The high expands into the North Sea and Baltic Sea region whereas a cut-off low moves into Iberia at its southern flank. With the westerly jet stream, a short-wave trough will travel over the south Mediterranean Sea eastwards. It will be associated with a surface cyclone that will advect a warm and moist air mass into the area to the south of Greece and Turkey. Cool and dry boundary-layer air is expected over most of Europe.

DISCUSSION

Southern Italy to southern Greece

A quite intense surface low pressure system is expected is association with the mid-level short-wave trough that travels eastward on Thursday. In the afternoon and evening hours, a strong low-level jet is forecast in the warm sector advecting quite moist low-level air north-eastwards. The strong warm air advection will be accompanied by strong lift and models agree about excessive and mostly stratiform precipitation.

However, the air mass in the warm sector is expected to become unstable late in the period as the mid-level trough approaches. Strong DCVA is likely at the cyclonic flank of a strong mid-level jet streak that travels eastwards. Weak and mostly uncapped instability is expected to develop, and given the synoptic forcing and low level convergence indicated by latest models, storms are forecast to form.

Although the weak instability will likely be a limiting factor, the strong vertical wind shear will assist for organized convection. Current scenario is a couple of low-topped mesocyclones that may be embedded in elevated convection. The main threat will be tornadoes given the strong low-level vertical wind shear. Large hail and damaging wind gusts are not ruled out, though. Excessive convective rain is not forecast, but together with the stratiform rain along the warm front, excessive rain may also occur.

Late in the period the storms are forecast to cluster and will likely weaken as the mid-level trough weakens and the low-level forcing decreases.

A more robust scenario may turn out when instability will be stronger than anticipated. Given the strong forcing and vertical wind shear, a MCS may develop capable of producing more widespread severe wind gusts.