Storm Forecast
Valid: Wed 18 Aug 2010 06:00 to Thu 19 Aug 2010 06:00 UTC
Issued: Tue 17 Aug 2010 21:12
Forecaster: KOROSEC

A level 1 was issued for central Ukraine, eastern Belarus, Baltic states and NW Russia mainly for large hail, strong winds, excessive convective rainfall and tornadoes.

A level 1 was issued for NE Marocco, northern Algeria, SW Mediterranean into extreme SE Spain mainly for large hail, strong winds and excessive convective rainfall.

A level 1 was issued for Denmark mainly for excessive (convective) rainfall.

SYNOPSIS

A large trough centered over north-western Europe remains quasi-stationary. Embedded in this trough, an elongated upper low from British Isles moves towards the North sea. A weak upper low over extreme SW Europe makes only slow progress towards NE where it is expected to merge with the main trough on Thursday.
At surface, a frontal system across eastern Europe pushes towards western Russia, where both cold and northwards advecting warm front are focus for convective activity. Another (strong) cold front enters NW Russia from the north and merges with the southern frontal system overnight to Thursday.

DISCUSSION

... central Ukraine, eastern Belarus, Baltic states and NW Russia ...

With the eastern side of the large trough extending towards NE Europe/NW Russia, surface front continues moving eastwards during the day. Strong surface heating ahead of it should release moderate/high instability within the moist airmass. Deep layer shear of around 15 m/s should support organized storms, numerous multicells and a couple of supercells can be expected. Storms will tend to cluster into MCS or two, especially over Ukraine later in the afternoon. A threat level 1 was also extented NW-wards towards the Baltic states given the expected convective activity with slower moving storms, providing threat for excessive rainfall, as well as a few large hail/strong wind events. It seems that high-end level 1 fits the best for our criteria, though especially Ukraine might require a level upgrade if the severity/coverage will be higher that is currently anticipated.

Another focus for organized severe storms is expected along the northwards advecting warm front which merges with the slowly southwards moving strong cold front across NW Russia, connected with the deep trough/low to its north. South of this trough, a strong mid-level jet streak will provide strongly sheared environment, which will be only partly overlaping with the instability along/just ahead of the surface front. However, it seems that MLCAPE of around 1000 J/kg, around 15-20 m/s of deep-layer shear and QG forcing will be sufficient for numerous severe storms to occur along the front. Those will bring threat for intense rainfalls, large hail and strong winds. Near 200 m^2/^2 of SREH3 and around 10 m/s of LL shear seems supportive for a couple of tornadoes as well.

... NE Marocco, northern Algeria, SW Mediterranean into extreme SE Spain ...

A rather strong mid-level jet ahead on the upper low and large scale ascent will again support widespread convective activity. Marginal to moderate instability will be available, while jet provides 20-25 m/s of deep layer shear. Organized storms with numerous multicells/clusters are expected, while a couple of supercells cannot be excluded either, easpecially over the southern part of the level 1 area. Training effect of the cells and high PWAT suggests that intense rainfall/flash flood threat is likely as well. The more isolated storms/strong clusters should bring large hail and strong wind gusts threat.

... Denmark and UK ...

Placed beneath the remnants of the upper low and stationary surface low near Denmark, while another deepening vortex approaches from the west, Denmark will be in the position for widespread rainfall event. Jet streak will be to the north/south of vorticity maxima placed here, so weak wind field should support slow moving cells/storms and therefore excessive rainfall/flooding threat. With only marginal instability in place, convective storms are limited, but will enhance flooding threat where they occur.

Across central UK where models show some signals for some hundreds of instability, shallow convective cells could occur. Cold mid-levels and rather weak wind field could support some funnel clouds given the steep LL lapse rates. Similar temperature gradient in the lowest levels could support a few funnel clouds around Denmark as well.