Storm Forecast
Valid: Thu 15 Nov 2007 06:00 to Fri 16 Nov 2007 06:00 UTC
Issued: Wed 14 Nov 2007 19:26
Forecaster: GATZEN

SYNOPSIS

Amplifying trough remains over Europe with an axis stretching south-westward into west Mediterranean, where advection of cold polar air mass is expected. Given strong QG forcing as the trough starts to cut-off into Mediterranean Sea, a storm is forecast to develop that will likely affect W Mediterranean Sea with severe winds. In the range of the low pressure centre, cold mid-level air mass will likely lead to unstable conditions, given rather warm sea surface.

DISCUSSION

North-west Mediterranean

A rather dangerous situation builds over west Mediterranean, as strong Mistral winds will spread across the area west of a surface low over Sardinia. While rather dry low levels are forecast near the French coast, polar air mass will rapidly become unstable over the sea surface due to increasing low-level moisture and steepening lapse rates. This will result in CAPE as indicated by latest GFS model output. Some QG forcing and low-level convergence will help for initiation, and shallow convection is expected in the northern area, while deeper convection and thunderstorms are forecast to the south. Although vertical wind shear will be not favorable for organized convection, and most thunderstorms will be rather short-lived and weak, strong winds at the 850 hPa level may support some tornadoes in the range of the coasts, where low-level vertical wind shear may be significantly higher. Allover threat seems to be too low for a categorical risk, but will draw a small level 1 near the low pressure centre, where strong low-level buoyancy and vertical wind shear may overlap. More independent on the convection will be severe wind gusts that will affect a widespread area during the period.

In the range of the low pressure centre, weak vertical wind shear, strong low-level buoyancy, and low-level convergence may be favorable for a few waterspouts.

Greece, Aegean, Bosporus

Ahead of the European trough, WAA will go on over east Mediterranean, where latest soundings indicate some 100s J/kg CAPE. On Thursday, expect that CAPE will further increase as boundary-layer moisture also increases over the warm sea surface. Some QG forcing can be expected in the range of the propagating trough, as can low-level forcing along a cold front that moves eastward into Greece. Additional forcing is forecast at the western coasts due to upslope flow. Thunderstorms are forecast that will go on during the period. Given very strong vertical wind shear around 15 m/s 0-3 km LLS, strong thunderstorms are likely, capable of producing isolated large hail and severe wind gusts. Given very strong low-level winds exceeding 25 m/s at the 850 hPa level, tornadoes are also not ruled out, especially along the coasts, where low-level vertical wind shear is expected to be strongest. Severe threat will shift towards the north into the Bosporus during the period, as a weak frontal wave will form along the cold front, moving northward. This will lead to increasing low-level convergence and storm-relative helicity over northern Aegean, while cold air advection sets in over the southern parts.