Storm Forecast
Valid: Mon 29 May 2017 07:00 to Tue 30 May 2017 06:00 UTC
Issued: Mon 29 May 2017 07:00
Forecaster: PISTOTNIK

A level 1 and level 2 were issued for Belarus and NW Russia mainly for tornadoes, severe wind gusts and large hail.

A level 1 was issued for central Poland, the NW Czech Republic and parts of Germany for large hail, excessive precipitation and severe wind gusts.

A level 1 and level 2 were issued for parts of England, BeNeLux, parts of France and N Spain mainly for large hail, severe wind gusts and to a lesse extent for excessive precipitation.

A level 1 was issued for W Turkey mainly for excessive precipitation.

SYNOPSIS

A subtropical ridge stretches from Algeria towards central Europe. It is flanked by two weak long-wave troughs over eastern Europe (with a cut-off low over Greece and Turkey) and from Ireland to Spain, yielding an ill-defined, stationary omega pattern. To its north, a strong zonal flow is in place from N Scandinavia into NW Russia, where a surface cyclone intensifies at the southern rim of a body of arctic air.
A long, wavy frontal boundary from NW Russia across Belarus, Poland, Germany and France into Spain is the main focus for possible thunderstorm development. Further south, warm to hot and mostly dry conditions prevail.

DISCUSSION

... Belarus into NW Russia ...

The frontal boundary becomes ingested into the circulation of the Russian cyclone. Aided by insolation, warm air advection and delayed vertical mixing, a strong rise of 2m temperatures and dewpoints is predicted in the warm sector across Belarus and adjacent Russia. Though the sparse surface observations make it difficult to validate the model forecasts, the WRF solution with values around 23/15°C by afternoon appears realistic. It would result in CAPE up to 1000 J/kg over a large area, maximized ahead of the cold front.
A small but intense upper-level trough travels from the Baltic States eastward into the area of interest. The westerly flow increases ahead of that feature and on the southern flank of the surface cyclone. As a result, 0-3 km shear around 15 m/s and even 0-1 km shear in excess of 10 m/s will likely overspread the robust CAPE reservoir of the warm sector.
Storms are expected to initiate by noon, aided by sufficient lift near the frontal boundary and ahead of the upper-level trough. Due to the trailing nature of the cold front, a convective line appears rather unlikely and convection could stay discrete for some hours. It will likely organize into supercells with a risk of large hail, severe wind gusts and tornadoes. If strong diurnal heating extends all the way northward to the occlusion point, where vertical wind shear is strongest, even one or two strong and long-lived tornadoes are not ruled out!
Storms will cluster later on and move eastward. Their severe weather risk will gradually decrease overnight. Stronger capping will likely preclude initiation further south over the Ukraine, though CAPE is present and vertical wind shear would still suffice for organized convection.

... Poland, Czech Republic, Germany ...

The westward continuation of the frontal boundary lies under quiescent synoptic conditions or even large-scale subsidence near the upper-level ridge. A cool maritime boundary layer with sea fog will be advected inland from the Baltic Sea, which will quickly cut any thunderstorm options on its northern side.
Similar to Sunday, a belt of rich low-level moisture will be trapped and maintained by the convergent flow near the frontal boundary. Its arc will stretch from central Poland across north-central Germany into BeNeLux and N France on Monday. 2m dewpoints around 18°C beneath steep lapse rates aloft (remnants of a "Spanish plume") will likely support CAPE on the order of 1000 to 2000 J/kg (see Sunday 12z Trappes and Idar-Oberstein soundings). Vertical wind shear is weak.
With a lack lift mechanisms, breaking the strong cap may be a difficult venture. High-resolution models uniformly show isolated to scattered convective initiation, but do not agree on the stretch of the frontal boundary where it shall happen. Pulse storms that form in this high CAPE / low shear environment can bring isolated large hail, severe downbursts and flash floods. Convection may continue into the night with an erratic, cold-pool-driven motion if one or two larger clusters form.

... England across BeNeLux and France into N Spain ...

The tail of the frontal boundary fans out over western Europe. Vertical wind shear increases again ahead of the next long-wave trough (rising from 10 to 20 m/s across the lowest 3 km from S to N). The plume of the richest low-level moisture is advected into N France and Belgium, where 2m dewpoints slightly above 20°C may even support CAPE up to 3000 J/kg. Otherwise, CAPE is expected mostly on the order of 500 to 1000 J/kg and its overlap with the increasing shear regime appears quite solid.
Scattered storms will initiate when diurnal heating manages to erode and break the cap, or aided by sea breeze fronts or outflow boundaries. They will organize into multi- and supercells with a primary risk of large hail, severe downbursts and excessive precipitation. Low-level shear is not particularly enhanced, but a tornado is not ruled out in case of favorable interaction of outflow boundaries with the sea breeze front or with each other.
Upscale growth into an MCS may occur and excessive precipitation may become another hazard at this later stage in the evening. The timing and placement of such a cluster are still uncertain, but the level 2 area with the most plentiful low-level moisture appears to be at the highest risk. It could continue into the night and may travel into the Netherlands and westernmost Germany with an ongoing, slowly decreasing severe weather threat.

... SE Europe ...

Scattered, mostly daytime-driven showers and thunderstorms will form in the well-mixed airmass beneath the upper-level low over Turkey, the S Balkans and Greece (including some Aegean islands). High storm coverage may result in one or two flash floods in W Turkey. Otherwise, severe weather is unlikely under limited CAPE and weak vertical wind shear.