Storm Forecast
Valid: Thu 05 Nov 2009 06:00 to Fri 06 Nov 2009 06:00 UTC
Issued: Wed 04 Nov 2009 23:39
Forecaster: PUCIK

A level 1 was issued for a belt from SE Italy to Montenegro, Albania mainly for tornadoes and excessive precipitation.

SYNOPSIS

A broad, cyclonic vortex over most of Western Europe will persist also during this forecast period. With strong mid-level jet streak at its rear flank with speeds over 40 m/s at 500 hPa level, an impulse will result in the significant trough amplification and a subsequent cyclogensis over the Mediterranean. Strong flow will surround this trough, which is expected to stretch from Scandinavia to Algiers by Friday morning. Most of it will be filled with cool, maritime airmass with cold front strenghtening over the Ionian and Adriatic sea.
Most of Europe will be under a extensive surface low pressure system with two centers - one over the Northern Sea and another one over the Central Mediterranean.

DISCUSSION

...Northern Spain...

Due to the frictional reduction of the windspeeds close to the complex terrain of this region, enhanced values of LLS are simulated by models. Non-zero CAPE values are expected over the sea and the major issue becomes the amount of overlap between the strong shear and the instability. At the moment, overlap does not seems to be of much significance and at the same time, strong mid-level impulse will have passed the region by the Thursday morning. Therefore, forcing will probably not play a major role in this case. A very small tornado risk will exist in the narrow coastal zone as showers or thunderstorms might be advected into the zone of enhanced LLS. But with rapid depletion of unstable air with ther arrival onshore, they should decay rapidly and no Level is introduced at the moment.

... Belt from SE Italy to Montenegro, Albania...

Ahead of the slowly advancing cold front, parallel to the frontal boundary, a low level jet is forecast to form. Its strenght is resolved differently by different models, with GFS showing more than 25 m/s by Friday morning while ECMWF produces only 15-20 m/s at 850 hPa level. Strenght of the jet will probably depend on its interaction with the upper-level flow (jet-stream surrounding the trough) and the degree of frontogenesis. Nevertheless, backing flow ahead of the surface low in combination with this jet might yield high values of LLS ( 10 m/s and locally even more) and SREH ( over 250 J/kg in 0-3 km layer). Strong flow at mid and upper levels should keep the deep layer shear values quite high, around 20 m/s, so organised convection is probable. From the perspective of strong low level shear and low LCLs, slight tornado threat seems to be warranted for this region. High Theta-E airmass advected from the south is expected to destabilise, yielding marginal to moderate MLCAPE values.

Towards Montenegro, another threat will become more distinct with the development of low level jet, which should have its delta over the coastal regions, providing moist and unstable air to the thunderstorms, which might keep reforming over the region, drifting to the northeast. Excessive precipitation threat depends very much on the strenght of the low level jet and its timing - it seems that the highest threat and precipitation amounts will occur in the next forecast period, but it is mentioned also in this forecast.