Storm Forecast
Valid: Fri 26 Jun 2020 06:00 to Sat 27 Jun 2020 06:00 UTC
Issued: Fri 26 Jun 2020 01:34
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for England, Belgium, France, N Spain, Switzerland, SW Germany and W Austria mainly for excessive convective precipitation, large hail and severe convective wind gusts.

A level 1 and level 2 are issued for E Germany, Poland, E Slovakia and E Hungary mainly for excessive convective precipitation, large hail, severe convective wind gusts and to a lesse degree for tornadoes.

A level 1 and level 2 are issued for the E Czech Republic, E Austria, W Slovakia and W Hungary mainly for large hail and to a lesser degree for severe convective wind gusts and tornadoes.

A level 1 is issued for NW Russia mainly for large hail, severe convective wind gusts and tornadoes.

SYNOPSIS

The blocking pattern softens and starts giving way to a meandering zonal flow. A broad mid-level ridge and an associated surface high cover E Europe, the Baltic region and S Scandinavia. It is opposed by a progressive mid-level trough and an associated surface cyclone that approach the British Isles and France. In-between, two filling cut-off lows with a NE-erly track are placed over the Czech Republic and central Turkey.
Very warm and often moist air masses cover most of Europe (with the exception of Ireland and N Scandinavia) and set the stage for an active day of mostly diurnally driven storms, partly enhanced by dynamics.

DISCUSSION

... central Europe, including N Spain, France, Belgium and England ...

Warm air advection intensifies both ahead of the Atlantic trough and ahead of the filling cut-off low over the Czech Republic. Both warm air advection regimes are supported by steep lapse rates that spread from the Spanish Plateau across France, Belgium and England and from the Alps into the Czech Republic, Slovakia and parts of Poland, respectively. Fuelled by plentiful evaporation and delayed vertical mixing, low-level moisture accumulates and CAPE on the order of 500 to 1500 J/kg is expected to build across wide areas, separated by a wedge of slightly cooler, drier air with more limited instability from the Netherlands across much of Germany into Bohemia (left behind by the Czech cut-off low). The unstable areas largely overlap with slightly enhanced vertical wind shear (around 10 m/s across the lowest 3 km) and veering low-level wind profiles.
At least scattered thunderstorms are expected in numerous regions, though the warm air advection delays convective initiation and adds some uncertainties with respect to its placement and timing. The dominant storm mode will be a mixture of single cells, multicells and loosely organized clusters, which may grow quite large towards evening.
Due to the high coverage, rather slow storm motion and distinct possibilities of training or backbuilding, the primary risk is clearly heavy precipitation. However, especially discrete storms, some of which could temporarily turn supercellular, can also produce large hail, severe downbursts and an isolated tornado.

Areas with an expected particularly high storm coverage, and hence an excelling risk of excessive convective precipitation and flash floods, are:
(1) An arc-shaped convergence zone ahead of the Czech cut-off low from E Germany to W and S Poland, E Slovakia and E Hungary (in particular near its southern end, where a persistent inflow of "uncontaminated" warm air can keep side- to backbuilding storms alive into the night);
(2) The axis of maximized warm air advection ahead of the Atlantic trough in E England, Belgium and NE France; and
(3) A belt from SW to central France, where a long-lived MCS in the evening and night is a likely scenario.

Swaths of more widespread severe wind gusts are most likely in case large storm clusters develop within ~200 km north of the Pyrenees (i.e, S France) and Alps (i.e., N Switzerland and SW Germany), areas that can benefit best from the steep lapse rates that are advected off the elevated terrain. However, this scenario is too questionable to influence the drawing of the level 2 areas.

A particularly intriguing but also volatile environment for organized storms evolves over the eastern parts of the Czech Republic and Austria and the western parts of Slovakia and Hungary, where a southwesterly mid-level wind maximum and a southerly low-level jet east of the Alps and Bohemian Massif create a confined area with 0-3 km shear (storm-relative helicity) between 15-20 m/s (100-300 m^2/s^2) under at least moderate instability. Despite persistent isentropic lift, residual morning clouds and a capping inversion may keep convective initiation rather limited, though.
Confidence in scattered or even widespread storms is highest along the northern margin of the best kinematics over the E Czech Republic, where isentropic lift is maximized near the nose of the low-level jet. Multicells, supercells and later large storm clusters can bring all kinds of severe weather. The primary risks are large (isolated very large) hail in initiating and heavy precipitation in later stages.
Some high-resolution models also simulate isolated convective initiation at the east Alpine rim in Austria. Storms forming here, amidst the best vertical wind profiles, would have a high potential to turn supercellular and bring large to very large hail plus severe downbursts. Rather low cloud bases and 0-1 km shear around 10 m/s also suggest an enhanced tornado risk if storms can survive some time over flat terrain.
Apart from the E Czech Repubic, this discussed area is covered by a level 1 and a low probability lightning area to reflect the high conditional severe weather threat under rather unlikely convective initiation.

... central Finland into NW Russia ...

A frontal wave separating warm and moist air to the south from polar air to the north travels eastward. Low-level moisture accumulates in the warm sector and creates some hundred J/kg CAPE which overlap with strong shear (15-20 m/s across the 0-3 km layer, 10-15 m/s across the 0-1 km layer) thanks to the mid-level jet aloft. The highest CAPE, around 1000 J/kg, is simulated near the warm front in NW Russia, but is somewhat disputed due to the notorious overestimation of low-level moisture by forecast models in this area and in this synoptic environment.
Scattered thunderstorms will likely form in the afternoon and evening and can organize quickly into multicells, supercells and/or bowing line segments. Depending on the storm mode, large hail, severe wind gusts and one or two tornadoes are possible, the latter especially near the warm front, where low-level wind profiles are most supportive.
Limiting factors against a level 2 are a lack of synoptic-scale lift, whose passage more to the NE could limit the overall storm coverage, and the absence of orographic features which would help to nail down sites of convective initiation.

... other thunderstorm areas ...

Isolated to scattered afternoon storms occur in an environment of low to moderate CAPE, weak vertical wind shear and devoid of dynamics. They will mostly be tied to mountains. Their severe weather risk is low.