Storm Forecast
Valid: Mon 25 Sep 2006 06:00 to Tue 26 Sep 2006 06:00 UTC
Issued: Mon 25 Sep 2006 00:11
Forecaster: GROENEMEIJER

Monday at 06:00 UTC... an elongated mid-level trough stretches from the northern Atlantic over Ireland to the Wrn Mediterranean. A low pressure area splits off this system and reaches central Italy Tuesday morning. Quasigeostrophic forcing for upward vertical motion is expected to provide ongoing destabilisation of the air-mass ahead of the system, sustaining multiple mesoscale convective systems.

Wrn Mediterreanean, Italy...

Ahead of a relatively cold air-mass flowing SEwd over the Wrn Mediterranean, a weak frontal zone is expected to form. Significant instability should become available as lift affects the potentially unstable air-mass, that is moist at low levels. Neither the cut-off low system nor its associated zones of lift are moving Ewd very quickly, so that the associated convective systems are likely to back-build over the same areas for a long time. This will lead to flooding problems in several places, especially where ascent is orographically enhanced.

Initially, the main activity is expected over Sardinia and the Tyrrhenean Sea, and during the morning hours, the first convection is forecast to affect the Italian mainland as well.

A few storms may develop mesocyclones, as 0-6 km bulk shear on the order of 20-25 m/s is forecast across the southern and eastern portions of the level 1 area. This will create some threat of large hail. Given that low-level shear values of about 10 m/s are expected, the formation of a tornado is not ruled out.

Most convection, however, will likely be clustered in large MCS's, that prefer to form and linger on the upwind side of mountain chains such as those on Sardinia, Sicily, mainland Italy, and later the western Balkan coast. They may produce brief severe winds approaching or marginally exceeding 25 m/s. This appears most likely across southern Italy and Sicily where low-level winds are expected to be strongest.

During the evening and night, the convection is expected to spread eastward to the central and southern Adriatic Sea, while diminishing over Sardinia and Corsica. However, storms will likely continue near the mid-level low pressure centre over central Italy untile the end of the forecast period.

Ireland, UK...

The arrival of a cold mid-level low during the day is expected to give an impulse to convective activity. Given that Ireland and the Wrn and Srn UK will see weak low-level winds and the condensation level will be low, it is possible that one or two weak tornadoes manage to form. Overall severe threat is expected to be low.