Storm Forecast
Valid: Fri 26 Jun 2015 06:00 to Sat 27 Jun 2015 06:00 UTC
Issued: Thu 25 Jun 2015 20:12
Forecaster: TUSCHY

A level 2 was issued for CNTRL/E Ukraine and SW Russia mainly for large to very large hail, severe wind gusts, excessive rain and a few tornadoes. A significant tornado is possible.

A level 2 was issued for CNTRL Romania mainly for excessive rain. An isolated tornado event is possible.

Two level 1 areas surround both level 2 areas for a similar risk with less coverage.

A level 1 was issued for Belgium and W-Germany mainly for an isolated tornado risk.

SYNOPSIS

NEward arching ridge over SW Europe and two cyclonic vortices over S-Scandinavia and SE Europe support an ongoing NW-/N-erly advection regime of cool and dry air far south. Different trajectories (either mainly over onshore areas or offshore over the CNTRL Mediterranean) result in different depths of dry layers in latest soundings. Weak mid-level lapse rates aloft support broad area with rather stable conditions. Orographic lift and/or regionally enhanced diabatic heating or enhanced BL moisture (e.g. over parts of the Mediterranean) result in pockets of somewhat enhanced thunderstorm probabilities over CNTRL Europe.

A strong extratropical cyclone SW of Iceland approaches NW Europe during the forecast and decreasing mid-level heights with a gradual cool-down result in enhanced chances for a few overnight storms from the North Sea to Germany.

Latest data show a rather interesting temperature anomaly of a depression's core, which is placed just north of the NW Black Sea. Phase diagrams and forecast strength of LL vortex support the idea of at least a shallow warm-core feature with strongest signals in the lowest 700 hPa. This vortex steers a very moist air mass west with moderate 800-600 hPa lapse rates aloft, so high MLCAPE is forecast. Heating/forcing and a pronounced N-S aligned convergence zone along the western fringe of that depression all serve as foci for CI....organized DMC is forecast.

DISCUSSION

... CNTRL/E-Ukraine to far SW Russia ...

Interaction of a potent LL vortex with a very unstable air mass result in an overlap of more than 2kJ/kg MLCAPE with 15-20 m/s DLS. As mentioned above, of concern is the strength of the lower part of that vortex with 20 m/s 0-3 km shear and substantially enhanced LL speed/directional shear.

Convection from the last night continues during the start of the forecast with numerous large clusters moving in general to the W/SW or NE (dependant on where they will be placed in respect to the low). Temporal weakening during the morning hours is forecast, but ongoing WAA regime with constant advection of a very moist/unstable air mass to the west should support rapid CI or re-intensification of storms. Main focus for CI will be a pronounced N-S aligned convergence zone, which runs roughly from CNTRL Ukraine to the NE. Further east, outflow boundaries from overnight's convection, diabatic heating and lift of the depression also insert high probabilities for scattered CI. During initiation, more discrete storms pose a large to very large hail, severe wind gust, excessive rain and tornado threat. A significant tornado is possible especially next to the convergence zone and along outflow boundaries, where directional shear will be enhanced. Betimes, upscale growth into numerous intense clusters is forecast. Large hail, severe wind gusts and especially excessive rain will be the main risk with a decreasing tornado threat. Cold pool driven storms may pose a regionally enhanced significant wind gust threat.
The main activity shifts north during the night towards SW Russia with ongoing thunderstorms.

... Romania ...

A compact cold-core low is placed over the border of Romania/Bulgaria. This vortex sags south during the forecast. This vortex features strong winds at mid-levels which decrease a bit towards lower levels. Some models also show strong LL convergence signals north of that vortex (over CNTRL Romania), which tend to shift gradually south betimes. PPWs of 30 mm and quais-stationary nature of the synoptic feature and the convergence zone indicate a chance for numerous slow moving thunderstorm clusters, which move from E to W. Excessive rain will be the main hazard, although an isolated tornado event can't de discounted with LCLs of less than 800 m and enhanced LL shear (forecast hodographs are strongly looped over CNTRL Romania). A few spots can see more than 150 l/qm of rain in less than 24 h, so we issued a level 2 due to a significant flash flood risk.Thunderstorm activity gradually weakens until the evening hours.

... Belgium, Luxembourg and W-Germany ...

During the evening/overnight hours, diffluent upper flow regime affects Benelux/W-Germany. Of interest is a wave in 800-600 hPa, crossing W-Germany during the night from W to E. Influx of rather rich BL moisture ahead of that feature is forecast with mixed-layer mixing ratios in excess of 10 g/kg. Forecast soundings indicate an interesting setup with strongly looped hodographs and SRH 1/3 km in excess of 200 m^2/s^2 with 15 m/s LL shear. Forecast soundings show a good chance for mainly elevated thunderstorm activity. However, not much BL modification is needed for thunderstorms to become near surface based. This would increase the chance for an isolated tornado event over W-Germany mainly between 00-06Z and beyond our forecast period. Despite still ongoing uncertainties regarding final nature of thunderstorms (elevated or surface based), we decided to issue a level 1 for an isolated severe wind gust/tornado threat.

...Rest of lightning activity ...

Numerous areas with isolated thunderstorm activity are forecast but nothing severe is anticipated.