Storm Forecast
Valid: Wed 21 Oct 2009 06:00 to Thu 22 Oct 2009 06:00 UTC
Issued: Tue 20 Oct 2009 21:38
Forecaster: PUCIK

A level 1 was issued for Portugal mainly for severe wind gusts and tornadoes.

A level 1 was issued for Southern coasts of France and Northwestern Italy mainly for excessive rainfall and to the lesser extent for tornadoes.

A level 1 was issued for Western Italy mainly for tornadoes and to the lesser extent for severe wind gusts.

SYNOPSIS

A deep mid-level trough over the eastern Atlantic will be a major factor in the weather development during the day. Its southern part will move quickly to the east with strong jet surrounding it. Jet-streak with speeds over 70 m/s at 300 hPa embedded in the northwesterly flow should result in the strong synoptic impulse, travelling across Portugal and Spain in the evening hours. A cut-off low situated over the Central Europe should progress to the northeast, slowly filling.

At the surface, two main pressure systems will dominate - a high over Russia and a large low centered just west of Ireland coast. This low has occluded and filled with moist maritime polar airmass so that widespread bands of showers or weak thunderstorms are anticipated in the environment of weak shear and marginal instability. Strong cold front will cross the Western Mediterranean during the day and will affect Italy in the late evening and night hours. The aforementioned developing impulse will result in cyclogenesis and another frontal system will cross Iberian Penninsula.

DISCUSSION

... Portugal and Spain...

In the left-exit region of the jet-streak cyclogenesis is forecast with strong synoptic upward motion along the newly developed frontal system. In the warm sector of the surface low, a moist maritime airmass will reside with very marginal values of CAPE, as shown by GFS and ECMWF models. This instability should be most prominent close to the western coasts of Iberian penninsula. Very strong low level flow, over 25 m/s at 850 hPa should result in the LLS values locally over 15 m/s. With DLS over 30 m/s and high SREH values, well organised thunderstorms might form, possibly a strongly forced convective line on the leading edge of the quickly moving cold front. Embedded circulations might form with slight risk of tornadoes. Due to the strong flow, marginally severe wind gusts might occur as well. Level 1 is issued for Portugal where the threat of tornadoes and severe wind gusts will be the highest, but due to the many uncertainities, mostly regarding the instability release, an update might become necessary during the day, e.g. shifting the area more to the east.

... Southern coast of France to Northwestern and Western Italy...

As the cold front progresses eastward, a plume of high Theta-E values will spread northwards, ahead of the significant frontal trough. Also, high lapse-rate airmass with origin from Northern Africa, overlying the warm and moist airmass over the Mediterranean. This combination will yield a tongue of instability with MLCAPE values around 1000 J/kg, with the highest values around the western coast of Italy towards the late evening and night hours.

Low level jet is expected to form ahead of the front, with windspeeds between 20-25 m/s at 925 and 850 hPa level and this feature will have a delta mostly over the southern coast of Spain and northwester coast of Italy. At mid-levels, strong flow around the trough will yield moderate to high values of DLS, between 15 and 25 m/s. Ageostrophic flow induced by the pressure falls ahead of the trough and strong winds at lower levels should result in very high SREH values, in 0-3 km models generate locally more than 400 J/kg. Low level shear will be strong as well, its values reaching more than 15 m/s. Such environment is conducive to well organised, rotating thunderstorms.

One of the detrimental factors should be the lack of synoptic-scale forcing and thunderstorms will mostly initiate around the front or close to the coasts, where southerly flow will impinge on the land. With high moisture content and a possible "thunderstorm training", excessive rainfall is possible, especially concerning the southern coast of France and northwestern coast of Italy. Low LCLs, high SREH values in 0-1 km and strong LLS well above 10 m/s point to the threat of tornadoes, especially in the case should the unstable parcels stay rooted in the boundary layer. The tornado threat depends on the fact, whether the instability, confined mostly to the coastal areas, will overlap with strong shear, but at this time models simulate the overalp (mainly over western coast of Italy), so this risk is included as well. Around the western coast of Italy, marginally severe gusts might occur as well, due to the strong flow at 850 level ( 20 m/s) with strong component perpendicular to the boundary and quite high lapse rates over the region.

A high end Level 1 is issued and an update might become later on, when the overall threat proves to be higher than previously anticipated.