Storm Forecast
Valid: Fri 19 Aug 2022 10:00 to Sat 20 Aug 2022 06:00 UTC
Issued: Fri 19 Aug 2022 10:26
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for Italy, Switzerland, Austria, SE Germany, the Czech Republic and Poland for for excessive convective precipitation, towards the SE also for severe convective wind gusts and large hail.

A level 1 and level 2 are issued from Bosnia-Herzegovina to Slovakia mainly for severe convective wind gusts.

A level 1 and level 2 are issued for Sweden and Finland for excessive convective precipitation, severe convective wind gusts and large hail.

A level 1 is issued for parts of the Ukraine mainly for excessive convective precipitation.

A level 1 is issued for SW Russia for excessive convective precipitation, severe convective wind gusts and large hail.

A level 1 is issued for the Adriatic Sea and for the E Black Sea for non-supercellular tornadoes (waterspouts).


SYNOPSIS

A deep, steering cyclone is located between Iceland and the Faroe Islands. A mid-level jet curves around it across the British Isles into Scandinavia, overlaid by a low-amplitude trough that gradually connects with a cut-off low in the Alpine region. A long, slow-moving, pronounced yet often diffuse frontal boundary stretches from Finland to Italy and separates very hot air to the east from cool maritime air to the west. Further downstream, warm and rather calm synoptic conditions prevail across E Europe. The large blocking anticyclone slowly recedes into Russia, while a faint cut-off low is parked over the Ukraine and the Black Sea.

DISCUSSION

... Central Europe, Italy, W Balkans ...

A complex scenario unfolds in the range and ahead of the mid-level trough and the cut-off low. Despite less extreme conditions than on Thursday, the overlap of CAPE and shear is still robust enough to allow another round of partly severe storms in some regions.
Well ahead of the frontal zone, a belt of very hot air with an elevated mixed layer (EML) of Saharan origin is advected far northward into east-central Europe. Under intense daytime heating, this EML connects to the surface and drives the temperature to peaks around 40C from Greece to Hungary, but it also mixes out the low-level moisture. This hot and dry airmass is gradually undercut by several surges of cooler but moist westerly winds, as the slowly advancing cold front disintegrates into a mess of outflow boundaries and shallow density currents in response to the intensifying thermal gradient. The leading boundary, which should run from Bosnia-Herzegovina to W Poland at noon, will likely attain the character of a dryline as the moisture accumulates in the wedge of cooler air. Each following surge of westerly winds brings again somewhat cooler but still adequately moist air with 2m dewpoints in the mid- to upper tens.
As strong synoptic and mesoscale lift results in mid-level cooling, CAPE up to 1000 J/kg can materialize across wide inland areas and probably up to 2000 J/kg in coastal areas of Italy and the W Balkan states. Mid-levels winds are weak near the center of the cut-off low. The southeastern and eastern parts of the CAPE reservoir are, in contrast, overspread by a southerly mid-level jet whose axis runs from central Italy to W Poland around noon and from Albania to E Poland around midnight.
Today's convective activity will strongly be modulated by mesoscale details, which results in an unusually large spread of the model pool and reduces the confidence of this forecast. Nonetheless, two hot spots of possibly severe storms, worthy of a level 2, seem to emerge:

(1) Scattered convection will likely form from noon onwards in the range of the leading dryline in Bosnia-Herzegovina, Croatia, Slovenia and Hungary. The window for surface-based convection could be quite narrow in space and time, though: Storms that propagate too far into the hot and dry, deeply mixed air towards the SE will soon dry up, whereas the capping inversion beneath the fringes of the EML may become impermeably strong towards the NW. Those storms that manage to be surface-based can organize fairly well under at least moderate CAPE (up to 1000 J/kg, more in case the Adriatic moisture is advected enough far inland) and deep-layer shear between 15 and 20 m/s. Strong linear lift and intense cold pool formation in the hot and dry air will likely favor bowing line segments with a main hazard of severe wind gusts, perhaps also including a few concentrated swaths with some extreme events. While the mid-level jet will carry these storms northward along the boundary into Hungary and later Slovakia, it is pretty likely that they will also send a strong and organized outflow front eastward, which can still bring severe wind gusts and duststorms in the Pannonian Plains without triggering further storms in the very dry air anymore. In case storms can stay discrete (which appears not too likely, though), large hail becomes another hazard.

(2) The second hot spot will likely be S Germany, where a band of very moist air with a few hundred J/kg CAPE wraps around the cut-off low under very weak vertical wind shear. Scattered to widespread, slow-moving storms are expected in parts of Bavaria with a peak in the afternoon and evening. High precipitation efficiency poses a remarkable risk of excessive rain. In case this convergence zone stays stationary, rainfall peaks well above 100 mm within a few hours are possible and even major flash floods may result! Remnants of the convective are packed into a rainband and advected into SW Germany and Switzerland, still bringing plentiful rain while the contribution of deep convection gradually decreases.

In the entire belt in-between these two areas, the environment shifts from the "German mode" (very moist air, weak vertical wind shear near the cut-off low) to the "Balkans mode" (dry air, moderate to strong vertical wind shear beneath the mid-level jet) from NW to SE, and so does the character of the severe weather hazards: mainly excessive rain to the NW versus mainly severe wind gusts to the SE, plus the odd large hail event in case of multi- to (temporarily) supercellular storms.
Despite decent CAPE, the storm coverage remains somewhat unclear in this entire belt. Several vorticity maxima at mid-levels and numerous convergence zones at low levels should support at least scattered storms throughout the forecast period. The peak of the activity will be in the afternoon over Italy and Austria, whereas a return of mid-level warm air advection (and hence increased lift) can shift it into the evening over the Czech Republic and Poland. Several models predict an MCS that could then cross Bohemia from S to N, which is why the German level 2 is expanded a bit eastward. Otherwise, a large level 1 seems to be the best solution due to the wide range of possible solutions offered by the forecast model pool. After sunset (or even before then in case low-level cold air advection becomes dominant), convection should decouple from the surface, and the wind and hail risks decline.

... Sweden, Finland ...

A surface cyclone forms near the German/Polish border and moves onto the Baltic Sea. Ahead of it, warm air advection increases and the frontal boundary starts pushing northward as a warm front. Despite rather poor lapse rates, low-level moisture accumulation near the warm front and daytime heating allow moderate CAPE up to 1000 J/kg in the warm sector, in Finland probably even more. Much of the CAPE is under rather weak vertical wind shear, but it increases to values up to 15 m/s closer to the front line, much of it concentrated across the 0-3 km or even 0-1 km range.
Scattered storms are expected to form and move NE-ward throughout the forecast period. Closer to the front line, several organized and long-lived storms, especially some bowing line segments with a hazard of severe wind gusts can develop. Discrete storms can turn multi- to supercellular and also pose a risk of large hail, heavy rain and perhaps one or two tornadoes, thanks to low cloud bases and strong low-level shear.
The flat to hilly topography and the broad, diffuse lift in the warm air advection make it difficult to pinpoint any hot spots. Signals for convective initiation are maximized in north-central Finland in the afternoon and in S to central Sweden overnight, when synoptic lift further increases. In the latter case, storms may struggle or fail to stay surface-based, hence the main risk shifts to excessive rain (extending into Saturday).

... Ukraine, SW Russia ...

Scattered to widespread, daytime-driven storms form under moderate CAPE and weak vertical wind shear. The main hazard is excessive rain. Towards the SE (Caucasus mountains and around), steeper lapse rates and slightly enhanced vertical wind shear ahead of the shallow cut-off low can also promote some better organized storms with additional hazards of large hail and severe wind gusts.

... Adriatic Sea and E Black Sea ...

Near and just ahead of these two cut-off lows, mid-level cooling may erode the capping inversion just enough to allow scattered showers and thunderstorms also over the warm water bodies. A few waterspouts are possible at convergence zones, especially nocturnal land breeze fronts.