Storm Forecast
Valid: Wed 07 Oct 2009 06:00 to Thu 08 Oct 2009 06:00 UTC
Issued: Tue 06 Oct 2009 22:33
Forecaster: PUCIK

A level 1 was issued for Portugal and Western Spain mainly for tornadoes, excessive precipitation and to lesser extent severe wind gusts.

A level 1 was issued for Northwestern France and Belgium mainly for tornadoes.

SYNOPSIS

A complex scenario will evolve during the forecast period with a trough over the Eastern Atlantic and a broad cyclonic vortex over northern Europe. Around this vortex, strong mid-level jet is rotating with two jet-streaks developing embedded in it. The first one will result in a cyclogenesis over Finland and far NW Russia, namely Kola penninsula. The second jet-streak will curve around a short wave trough which will affect northern part of the British Isles during Wednesday evening and night hours. In the meantime a ridge stretching over the Central Mediterranean will move eastwards, weakening.

At the surface a very moist and warm airmass is spreading over Western and Central Europe behind the warm front, which will reach Ukraine and Belarus by Wednesday morning. Deep low pressure system will track across Central Scandinavia to Kola penninsula by the end of the forecast period with surface pressure at its center below 976 hPa. Another surface low is tied to the cold front, stretching from Northern Europe to Iberia penninsula, providing the front a wavy structure.

DISCUSSION

... Portugal, Western Spain...

A moist airmass has advected over Portugal and extreme W Spain per surface observations at Tuesday 20 UTC. Ahead of the progressing cold front, airmass is expected to destabilize with MLCAPE values above 500 J/kg. On the forward flank of the low a strenghtening flow will enhance the wind shear values, which should reach 15-20 m/s in 0-6 km layer and approximately the same in 0-3 km layer, so well-organised storms can be anticipated. The mid-level trough and a cold front will aid in the storm initiation with the highest coverage along the front itself. During the previous forecast period a risk of severe weather has been adressed for this area. We expect that this risk will persist at least very early in the period with the highest threat around Wednesday 06Z. Around this time, 20 m/s flow at 850 hPa will yield LLS values up to 15 m/s and with low LCLs, tornadoes might occur. Also with quite strong flow at 850 hPa and 700 hPa ( around 20 m/s) marginally severe wind gusts might occur - but this threat will be limited due to the high relative humidity of low and mid-levels. The threat will spread eastwards with the surface front, probably dimnishing by Wednesday 18Z as wind shear will gradually weaken as well as instability. Futhermore a slight risk of heavy convective rainfall is being considered as numerous rounds of showers and thunderstorms are possible especially over the region of N Portugal and NW Spain, combined with quite high moisture content in the troposphere.

... NW France, Belgium...

Surface observations show that very humid airmass with dewpoint depressions less than 5°C has overspread the area as the surface low and associated cold front approach. A low-end instability release is expected with GFS and ECMWF expecting several hundred J/kg of MLCAPE over the region. Close to the cold front, storms are expected to initiate. This front should attain a wavy characterstic with one of the waves reaching the area with shallow surface low. This will cause backing low-level winds and enhance the SREH values. At the same time, LLS values will increase to about 10 m/s. Detrimental factor should be only weak to moderate DLS, perhaps limiting the chance of storms to become supercellular. Another questionable issue is the track of the frontal wave and a degree of instability. If storms manage to form in the region,
with moist airmass - low LCLs and enhanced shear at lower levels, tornado threat can not be ruled out and a Level 1 seems to be warranted. Copious amounts of precipitation might fall, especially to the northwest of the frontal wave, but most of it should be of stratiform nature and therefore the threat is not included in this forecast.