Storm Forecast
Valid: Fri 10 Jul 2020 06:00 to Sat 11 Jul 2020 06:00 UTC
Issued: Fri 10 Jul 2020 00:04
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for Poland and Belarus for severe convective wind gusts, tornadoes and large hail.

A level 1 and level 2 are issued for SE France, Switzerland, SE Germany, the Czech Republic, Austria and N Italy mainly for excessive convective precipitation, large hail and to a lesser degree for severe convective wind gusts.

Level 1 areas issued for the Pyrenees and for E Turkey for excessive convective precipitation and large hail.

SYNOPSIS

A high-index pattern is in place. Low 500 hPa geopotential and sea-level pressure cover the Norwegian Sea, the British Isles and Scandinavia. A fairly strong mid-level jet runs along its southern flank with an axis initially from the Channel to Belarus. In the course of this forecast period, it increasingly tilts from a W-erly to a SW-erly direction as a mid-level trough moves from the British Isles towards France, BeNeLux and Germany.
Ahead of this trough, a developing surface cyclone is predicted to move from the Netherlands (Fri 06 UTC) to Estonia (Sat 06 UTC). Latest forecast model runs narrowed down its track, but not yet the degree of its deepening (by Sat 06 UTC: 1000 hPa according to ECMWF versus 1006 hPa according to GFS). Very warm air - with a plume of steep lapse rates originating from Spain and reinforced over the Alps - is advected into central Europe in its warm sector, a substantial CAPE-and-shear overlap builds and an outbreak of severe thunderstorms is expected.
Calm and warm to very warm midsummer conditions are in place further south and southeast. A weak surface anticyclone is centered from Romania to the Ukraine.

DISCUSSION

... Poland to Belarus ...

CAPE buildup may be a bit sluggish at first, but should reach 300-1000 J/kg until the late afternoon under warm air advection, synoptic lift, converging low-level winds and some hours of insolation. Higher CAPE is possible towards the south, depending on how far north the plume of steeper lapse rates can spread. In any case, this CAPE will overlap with strong vertical wind shear, e.g. often 15-20 m/s across the 0-3 km layer and 10-15 m/s across the lowest kilometer. Especially near the warm front, a strongly sheared and helical flow is already available very close to the surface.
Under plentiful synoptic and mesoscale lift, scattered storms will likely initiate by afternoon along the frontal boundary and slightly south of it. A quick organization into supercells with a risk of large hail, tornadoes and severe downbursts or bowing line segments with a main risk of more concentrated swaths of severe wind gusts is expected. The closer to the frontal boundary, which runs roughly along a line Poznan - slightly north of Warsaw - Bialystok - Minsk, the more likely the discrete, supercellular mode appears, as lower cloud bases should keep storms inflow-dominated for a prolonged time. Especially in this mentioned belt, the tornado risk is significantly enhanced, and even a strong and long-lived tornado cannot be ruled out! Forming or existing storms should be closely monitored if they are tracking into areas of more strongly backed (i.e., southerly to easterly) surface winds, which might quickly develop in this highly dynamic and volatile setup.
Despite limiting factors, namely in form of rather low equilibrium level heights and possible cloud fields in the warm sector, the threat appears high enough to warrant a level 2.
Convection will continue to move eastward into Belarus and Russia in the evening, while it gradually becomes elevated and weakens.

... E France to Austria and Czech Republic ...

The southern level 2 area is characterized by more CAPE (500-1500 J/kg) under steeper lapse rates, but weaker kinematics and dynamics on the anticyclonic side of the jet stream. However, while vertical wind shear is weak in the lower tropopshere, deep-layer (0-6 km) shear increases from 10 m/s on the southern side of the Alps to 20 m/s in S Germany and the Czech Republic, sufficient for some storm organization.
Scattered to widespread convection is expected in the afternoon first over the Massif Central and the Alps, in particular over Austria. Convection-resolving models simulate numerous storms propagating to the right and left, some of them transient supercells. East and south of the Alps, the air mass is strongly capped, hence the right-movers will decay as soon as they leave mountainous terrain. In contrast, the left-movers running into the northern Alpine forelands encounter a gradually weakening cap due to subtle synoptic lift and the approaching cold front. It might need two or three attempts of outflow boundaries running out of the Alps, but in the evening the cap should be broken and even a large storm cluster may propagate from the Alps into SE Germany into N Austria.
The main risk is large hail in initiating stages (isolated very large hail not ruled out in case of supercells), but it soon shifts to excessive precipitation due to the high storm coverage and a tendency for backbuilding or training. By evening, several or even numerous flash flood events are a distinct scenario, most notably along the north Alpine rim. In addition, a few cold-pool driven strong to severe wind gusts are not ruled out.
Convection will gradually become elevated and decay overnight, but can still locally flare up when the cold front reaches and crosses the Alps. In particular, first storms may also spread into N Italy towards Saturday morning.

... NE Spain, Italy, E Turkey, Georgia, Armenia, Azerbaijan, Caucasus mountains ...

Scattered afternoon storms in an environment of moderate CAPE and weak vertical wind shear pose a risk of isolated flash floods and marginally large hail. Convection will be strictly tied to orographic features due to a strong cap and a lack of synoptic lift.