Mesoscale Discussion
Valid: Tue 30 May 2017 11:00 to Tue 30 May 2017 15:00 UTC
Issued: Tue 30 May 2017 11:35
Forecaster: PISTOTNIK

This Mesoscale Discussion is issued to highlight latest developments in severe thunderstorm activity across central Europe.

... W Poland ...

Two MCSs have crossed many parts of Germany overnight and in the morning. The southern one has recently entered W Poland (the voivodeships of Lubusz and Lower Silesia) and the northern one is about to enter NW Poland. The former gap in-between has been closed by new intense storm developments around Berlin. Hence probably a single, long bowing thunderstorm line will cross W Poland in the next couple of hours.
Widespread severe wind gusts are expected along this line and may even include a few extreme events (>32 m/s). They are supported by enhanced vertical wind shear towards the north and particularly hot air (i.e., strong evaporative cooling) towards the south. Large hail is possible in case more discrete updrafts are embedded in the line or develop ahead of it.
The air turns constantly cooler and drier towards the NE across Poland, hence the fully developed line is expected to propagate towards the higher CAPE reservoir (around 1000 J/kg should be available) and move more to the SE instead of the E as time proceeds.

... central Germany ...

A wind shift line marks the cold front which crosses Germany from W to E. Despite lower temperatures than yesterday, CAPE between 500 and 1500 J/Kg should still be available and 0-3 km shear increases towards 15 m/s. Storms have fired up near the W rim of the highlighted area. They will likely intensify, move eastward and organize into multi- and supercells. Large hail (especially with supercells) and severe wind gusts (especially in case another line should emerge later on) are the main risks.

... Czech Republic ...

A pronounced outflow boundary from the Polish storm system will enter the Czech Republic from the north and may cross much of it by early evening. Latest 11z surface observations show hot but mostly dry conditions (temperatures around 30°C, dewpoints mostly between 11 and 15°C), though overnight showers and thunderstorms have left pockets of enhanced low-level moisture with dewpoints up to 18°C.
Fields of towering cumuls clouds are visible on satellite imagery and first isolated storms have already formed. They are still suffering from strong entrainment, but activity will likely increase in the next few hours. With plentiful CAPE but rather weak vertical wind shear, pulse storms may bring 2-4 cm sized hail, localized flash floods or severe downbursts.
Depending on the strength of the mentioned outflow boundary and its capability to "pool" and enhance the low-level moisture, activity may turn widespread and poses an additional risk of flash floods. However, this is mostly beyong 15z, and later observations need to be avaited to confirm or reject this scenario.