Extended Forecast
Valid: Mon 20 Jun 2016 06:00 to Tue 21 Jun 2016 06:00 UTC
Issued: Sat 18 Jun 2016 20:54
Forecaster: TUSCHY

A level 2 was issued for parts of Croatia, Bosnia and Herzegovina, Serbia, E-Hungary, W-Romania, E-Slovakia and W-Ukraine mainly for damaging hail, severe to damaging wind gusts, excessive rain and tornadoes.

A level 2 was issued for parts of Poland, parts of Belarus and SE Lithuania mainly for tornadoes - a significant event is possible -, large to very large hail, severe to damaging wind gusts and excessive rain.

A level 1 surrounds the level 2 with similar hazards below the "extreme" threshold.

A level 1 was issued for CNTRL Italy mainly for excessive rain and an isolated tornado event.

SYNOPSIS

Long-lived upper trough over CNTRL Europe weakens during the forecast with constantly rising geopotential heights. In addition, it starts to choke off of the westerlies but won't achive the status "cut-off" unitl 06 UTC. Meridional flow regime - reflected by ongoing neutral tilt of the trough - results in rising thickness values over SE/E-Europe per downstream development. This offsets the trough's weakening trend and ensures a steady state tight geopotential height gradient from the Ionian Sea to Belarus. Embedded in this brisk deep southerly flow regime, numerous more or less pronounced short-waves travel north and induce temporarily enhanced synoptic-scale lift.

A sharp temperature and moisture gradient (a quasi-stationary wavy front) remains in place from the Ionian Sea to E-Hungary and Belarus. A broad and rather ill defined LL depression evolves somewhere over W-Romania/E-Hungary and lifts north towards the Baltic States. This vortex assists in a distortion of the aforementioned quasi-stationary front, as a sharpening and also N-ward lifting warm front takes shape over the W-Ukraine. This warm front lifts north towards Belarus. Else, numerous residual outflow boundaries and diabatically/orographically driven mesoscale convergence zones probably exist east of that front, but their final location is impossible to tell 2 days ahead.


DISCUSSION

... Serbia to W-Romania to Belarus and the Baltic States ...

Ingredients show a favorable environment for well organized thunderstorms. Forecast trajectories of the lower troposphere indicate a long lasting connection to a reservoir of rich BL moisture over far SW Russia. Combined with ongoing evapotranspiration, LL mixing ratios in excess of 10-13 g/kg seem plausible with higher peaks in regions with maximized confluent flow (e.g. the warm front itself). This moisture advects beneath very steep mid-level lapse rates of locally more than 8 K/km, which offers widespread 1.5 to 3 kJ/kg MLCAPE. Even higher values are possible on an isolated scale.

Deep layer shear oscillates a bit with maximized values during the passage of short-waves (e.g. 700 hPa winds up to 25 m/s). DLS of 20-30 m/s will cover the very unstable air mass which supports long-lived supercells. A bit more uncertain is the degree of LL shear, which heavily depends on the strength of the northbound moving LL depression. Right now, strongest backed low/mid-tropospheric flow becomes established over Belarus, which is also reflected by SRH-1 values in excess of 250 m^2/s^2.

To summarize, Serbia up to the W-Ukraine will see very favorable conditions for initially discrete and long-lived supercells with damaging hail in excess of 5 cm, severe to locally hurricane-force wind gusts, torrential rain and a few tornado events. An isolated significant tornado event is possible. Betimes, upscale growth into one or two severe MCS events with swaths of damaging wind gusts is possible, either fostered by intense cold-pools or a passing short-wave.

In general, the tornado risk increases towards Belarus and Lithuania with elongated and curved hodographs. The final path of the aforementioned depression both dictates the potential for a significant tornado risk but also a risk for excessive rain due to slow moving thunderstorms within the center of the vortex over parts of CNTRL/E-Poland.

Finally the level 2 was expanded far west over the S-Balkan States, where less MLCAPE but even stronger DLS in excess of 30 m/s overlap. All kind of severe is forecast with long-lived supercells and a few extreme events are possible.

Parts of Hungary were excluded from the level 2 for now due to uncertainties how fast the air mass recovery occurs after a potential nocturnal cluster from the previous day. However, not much air mass modification is needed for organized DMC and an upgrade may be needed in the following outlook.

... CNTRL Italy ...

A level 1 for slow moving thunderstorms with an excessive rainfall and isolated tornado risk was issued for CNTRL Italy beneath the weakening upper trough.