Storm Forecast
Valid: Fri 12 Jun 2009 06:00 to Sat 13 Jun 2009 06:00 UTC
Issued: Thu 11 Jun 2009 15:34
Forecaster: TUSCHY

A level 2 was issued for N-Belarus, E- Latvia and Estonia, extreme S-Finland and parts of Russia mainly for tornadoes, large hail and severe wind gusts.

A level 1 was issued for the areas surrounding the level-2, Ukraine, Moldova and N-Romania mainly for tornadoes, large hail and severe wind gusts.

A level 1 was issued for the areas E/NE Czech Republic, E/NE Hungary, Slovakia, S-Poland and W-Ukraine mainly for large hail.

SYNOPSIS

The progressive weather pattern continues with one trough over E-Europe moving slowly eastwards whereas another one approaches Ireland from the west. Ridging is present in-between backed by the northward advection of very hot African air over the Iberian Peninsula. Downstream of the E-Europe trough, strong WAA also results in hot and unsettled conditions well to the north.

DISCUSSION

... Parts of Romania, Moldova, Ukraine, Belarus, parts of Lithuania, Latvia, Estonia, Finland and Russia ...

This kind of pattern with a gradually eastward building trough and an extensive warm sector/flat ridging ahead, fosters a broad area, where strong to severe thunderstorms are possible. To highlight at least the most serious spots, we split the complete region into subsections, discussed below:

... N-Belarus, E-Latvia and Estonia and extreme S-Finland 06Z until roughly 21Z ...

A significant tornado risk exists in an area, marked by Kyiv, Moskow, St.Petersburg and Minsk during the forecast period as the first surface depression moves to the north. EZMWF and GFS agree in a closed feature with pressure drop bottoms out shy above 1000hPa although model fluctuations are still present. Strong moisture advection from the east persists during the day beneath far westward reaching tongue with steeper lapse rates, which allows for moderate instability release of up to 1.5kJ/kg MLCAPE. It has to be noted that ECMWF is less aggressive in respect of westward sprawling instability tongue but EZMWF was not as persistent during the past few runs as GFS was, so we follow the idea of GFS/WRF/AFWA. The area of most concern is the intersection of the warm/cold front, where backing of surface winds is maximized as 20-25m/s low/mid-level jet draws near from the south. In addition, LL-CAPE release along this intersection increases during the day and all that with LCLs between 500-800m. Thunderstorms will have a high chance to become tornadic in this environment with a significant tornado event not ruled out. This activity is also accompanied by severe to damaging wind gusts and large hail. The level-2 was expanded somewhat more to the west as models advertise, but despite diminishing instability, shear remains very strong, so tornadic supercells are still possible. As daytime heating weakens during the evening hours, surface based instability release is on the decrease. However, GFS has near 1kJ/kg MUCAPE over S-Finland with 300-400m^2/s^2 SRH3 next to the occlusion/warm front, so large hail and strong wind gust are still possible.

...E-Belarus, Ukraine and regions SW/W of Moscow ...

The focus for widespread thunderstorm activity is the eastward sliding cold front. Slow forward propagation characterizes this boundary, separating a warm/muggy airmass ahead from cooler and more stable air to its west. As prefrontal airmass destabilizes constantly with MLCAPE values between 1-1.5kJ/kg (locally higher peaks) and DLS near 20m/s, thunderstorms ought to gain organisation rapidly. It is the current thought that numerous northward traveling short waves periodically enhance thunderstorm development along this front with numerous northward moving thunderstorm clusters all day long. Severe wind gusts and large hail are all possible with discrete supercells but the prevailing hazard is flooding rain beneath those clusters, especially when numerous clusters affect the same region due to the slow forward propagation of the cold front.

During th evening hours, surface pressure drops ahead/along of the cold front as another, elongated and weakly organized surface depression evolves over N-Ukraine and E-Belarus. Despite gradually decreasing instability values, shear especially at lower levels ramps up and hence the tornado risk increases next to severe wind gusts and large hail. Latest GFS runs would bring a set-up to the Moscow are, supportive for tornadic thunderstorms, but we want to see more consistence also in the rest of the models in respect of the strength/position of the surface depression, before acting.

... Rest of the Russian regions, which are highlighted ...

Steep mid-level lapse rates and surface dewpoints in the mid-/upper tens result in widespread moderate to strong instability release in the order of 1000-2000 J/kg MLCAPE. DLS is more than adequate for those thermodynamic profiles with 10-20m/s, maximized along the warm front east of St.Petersburg. The main uncertainty is where, when and how strong initiation will be in this broad warm sector due to missing mid-/upper forcings, various surface convergence zones and local support by the topography. As the cold front draws near from the west and boundary layer heating becomes maximized during the evening hours, scattered thunderstorms are forecast, probably organizing into small clusters, which travel to the north. The highest large to very large hail risk was highlighted by the level-2, where tornadoes and severe wind gusts are also possible along the warm front. Further to the south, instability release is weaker, but large hail/severe wind gusts are still possible. The thunderstorm activity keeps going well into the late night/early morning hours.

... E/NE Czech Republic, E/NE Hungary, Slovakia, S-Poland and W-Ukraine ...

Cold mid-levels beneath eastward building upper trough and residual moisture in the postfrontal airmass clear the way for scattered, partially diurnal driven thunderstorms in the left exit of a 40m/s mid-level jet. Low WBZ and 10-15m/s DLS point to a marginal to isolated large hail risk, likely maximized over E/NE Hungary, extreme S-Poland and W/SW-Ukraine, where instability is the best (roughly 500J/kg MLCAPE). Thunderstorms decrease after sunset.

... Spain ...

A spanish plume is on its way to the north with very steep mid-level lapse rates due to strong WAA . Models hint on some elevated instability release over N-Spain with most of it remaining capped all day long. However, a short wave rounds the ridge from the west and touches N-Portugal and N-Spain during the afternoon and night hours. Fow now, we stick with the models, which don't show initiation aside from UKMO with slim chances. Kinematic/thermodynamics would be favorable for large hail so we will monitor the area.