Storm Forecast
Valid: Tue 27 Jul 2010 06:00 to Wed 28 Jul 2010 06:00 UTC
Issued: Tue 27 Jul 2010 06:14
Forecaster: KOROSEC

A level 1 was issued for parts of Finland, extreme NE Sweden and Norway mainly for large hail, strong to severe wind gusts and an isolated tornado threat.

A level 1 was issued for Estonia, Latvia, Lithuania, western Ukraine and E-Poland mainly for excessive rainfall and large hail.

A level 1 was issued for Moldova and E-Romania mainly for large hail, an isolated significant event possible, and locally excessive rainfall.

SYNOPSIS

A quasi-stationary upper trough remains in place over central/SE Europe with its negative tilt. This configuration still forces an unseasonably hot/moist air mass well to the north, also affecting parts of Finland and N-Norway during the forecast period. Another short-wave trough/upper low moves across NW Europe.

The life-threatening strong heat wave keeps going over most parts of W-Russia and another heat wave also affects Spain. In-between, unsettled conditions occur with numerous showers and thunderstorms.

DISCUSSION

... Finland, extreme NE Sweden and N-Norway ...

An anomalous warm and moist air mass is about to spread north/northeastwards over Lappland and Finnmark, creating a favorable environment for organized convection. Yesterday's 20Z surface data printed the 20°C-surface isotherm already over central Finland so no concern exists, that surface based convection will evolve in the warm sector. As impressive EML plume also grazes the highlighted area during peak time heating, roughly 1000 J/kg weakly capped MLCAPE envolve. Forcing is present in the shape of a rapidly northeastward translating short wave. This wave also enhances the wind field at all levels, but strongest winds probably exit the warm sector already around noon, which may leave the strongest storm activity (afternoon hours) in a somewhat weaker shear environment. However, an organized deep moist convection is expected, developing along the warm/cold front structures and later on also in the warm sector with large hail and strong wind gusts. LL shear will be modest along the fronts itself, where LCLs are forecast to remain at or below 1 km, so an isolated tornado event is possible. This threat is also fostered by strong LL CAPE build-up in latest model data. Given the overall weak shear, storms tend to cluster betimes with a decreasing severe risk and an increasing risk for heavy rainfall, especially with this very moist airmass in the warm sector.

... Estonia to E-Poland and western Ukraine ...

Upper level flow features a slightly diffluent, anticyclonically curved streamline pattern during the forecast with a persistent influx of warm/moist air at mid-/lower levels from the SE. During the afternoon hours, a weak surface low forms in NW Ukraine and slowly moving into eastern Poland. This pattern is prime for rather slow moving thunderstorms with locally excessive rainfall amounts, especially where training or orographic enhancement occurs. During the initiation stage of those thunderstorms (probably already during the late morning hours and regionally ongoing from the nighttime activity) large hail will be possible, given the modest CAPE amounts in hail growth zone with some veering present. However, rapid upscale growth into numerous clusters is forecast with intense convective rainfall threat. Those clusters will gradually spread to the west and also affect E-Poland after sunset.

... Moldova and E-Romania ...

Placed beneath the eastern fringe of the extensive upper-level trough, overlap of modest shear and 1000 J/kg MLCAPE create a favorable environment for organized multicells/isolated supercells around noon / during the afternoon hours. Large hail will be the primary threat and even an isolated significant large hail event looks probable, as 20 m/s speed max at 500 hPa approaches from the south. Also, Moldova seem to be in the spot for some severe storms due to the favorable connection to the very moist air mass over the W-Black Sea. As storms repeatedly evolve over Moldova all day long, locally excessive rainfall amounts may result.

... British Isles...

British Isles will be afftected by the short-wave trough crossing during the daytime hours. Weak signals for instability are found just ahead of the moving cold front. With some deep-layer shear in place, convection could envolve into a multicells with mainly some strong wind gusts locally. However, both high uncertainities and only marginal severe threat preclude a level 1, so only 15% probability thunderline was placed where convection could occur.

... northern Marocco and Gibraltar area ...

A weak upper-level low remains quasi-stationary just WNW of Marocco and continues with strong WAA into SW Europe. It seems that convection will be mostly aligned the the complex terrain of Atlas mountains. Moderate instability builds up overlaping with 15-20 m/s of deep-layer shear which will provide sufficent conditions for some organized severe storms. The main threat will be strong to severe wind gusts and large hail given the high LCLs. Storms will probably tend to cluster and gradually move towards Gibraltar area later in the forecast period.


No threat exists for the rest of the thunderstorm areas, including Germany, Italy, the Balkan States and Bulgaria. Weak to modest SBCAPE and deep-layer shear below 10 m/s preclude any more organized updraft to evolve, so mostly pulsating convective storms are forecast. Marginal hail and strong wind gusts may accompany the stronger storms.