Storm Forecast
Valid: Fri 15 May 2020 06:00 to Sat 16 May 2020 06:00 UTC
Issued: Fri 15 May 2020 00:40
Forecaster: PISTOTNIK

A level 2 is issued for central Romania mainly for large hail and to a lesser degree for severe convective wind gusts.

A level 1 is issued for a belt from NE Hungary and SE Slovakia across the Ukraine to SW Russia for excessive convective precipitation (mainly towards the north) and for large hail and severe convective wind gusts (mainly towards the south).

A level 1 is issued for N Italy and Slovenia mainly for excessive convective precipitation and to a lesser degree for large hail.

A level 1 is issued for Spain mainly for excessive convective precipitation.

SYNOPSIS

Low 500 hPa geopotential stretches from Iceland to NW Russia and the polar jet runs south of it. An unusually strong subtropic jet with speed maxima in excess of 40 m/s at 500 hPa curves around a cut-off low over Morocoo and Iberia before it extends NE-ward to Italy, the Balkans and the Black Sea region, where it gradually merges with the fringes of the polar jet.
At lower levels, the majority of Europe is still under cold air, left behind from the late arctic outbreak at the beginning of this week. In contrast, a strong SW-erly flow with pronounced warm air advection has overspread SE Europe and will even increase on Friday, when an elevated mixed layer (EML) from the Sahara arrives (already sampled by the Thu 12 UTC soundings from Decimomannu, Trapani, Rome, Brindisi and Zadar).
The extremely pronounced and almost stationary frontal zone separating these two air masses runs from the W Mediterranean Sea along the S side of the Alps to Romania and the Ukraine. Some moisture accumulates and CAPE forms on the warm side of the frontal boundary, overlapping with very strong vertical wind shear.

DISCUSSION

... Slovakia, Hungary, much of Romania, Moldova, Ukraine eastward into Russia ...

Thick clouds, rain and occasionally embedded thunderstorms will prevail along the frontal zone from Austria and Slovakia eastward across the Ukraine and SW Russia. Thursday's 12 UTC soundings in vicinity of the frontal zone (Budapest, Poprad, Shepetivka) and the widespread stratus/ stratocumulus decks frazzling southward indicate a pronounced frontal inversion with a very gentle inclinination towards the south, hence a broad area will only experience elevated and limited CAPE. However, going further southward, mostly clear skies eventually allow the formation of surface-based CAPE in response to daytime heating. The return of warm air and the accumulation of moisture near the front line have just started on Thursday, but 2m dewpoints by Thursday evening show a maximum around 16C in W Romania and are largely in line with model forecasts. Hence the buildup of CAPE on the order of 1000 to 1500 J/kg over W and central Romania appears realistic on Friday, decreasing towards the west, north and east. This CAPE overlaps with outstanding vertical wind shear and storm-relative helicity with values up to 25 m/s and 500 m^2/s^2 across the lowest 3 km, respectively.
Despite the highly dynamic setup, forecast models (ECMWF, GFS, ICON) largely agree on the formation of two MCSs near the frontal zone in the afternoon to evening, one over SW Russia just east of the Ukraine and another one in the bordering region of Slovakia, Hungary and the Ukraine. Confidence in the western one is higher, as it is backed up by the lift of a pronounced short-wave trough, whereas synoptic support for the eastern one is hardly recognizable. Nonetheless, both MCSs, once they form, could persist for many hours well into the night and travel far distances - the eastern one could leave our forecast domain and the western one could reach the Asow Sea if it survives until Saturday morning.
Key question is whether the initially elevated storms will manage to start propagating towards the warmer air and become surface-based, which would strongly increase their severe weather risk. Excessive rain is the main risk in the northern parts (where storms are elevated), while the risk of severe wind gusts and large hail prevails towards the south - the more the stronger the propagation towards the warmer air is. Due to large uncertainties about the strom tracks, a broad level 1 corridor is issued, which also extends into the low propability lightning zone.
Models disagree about the initiation of discrete storms in the area of the highest CAPE in Romania. However, with pronounced synoptic lift of the short-wave trough and the help of orographic features, the confidence in scattered afternoon to evening storms over the Carpathian and Apuseni mountains is high enough to upgrade this area to a level 2. Any persistent updraft that forms can easily turn supercellular and produce large to very large hail plus severe downbursts. Storms will likely decay once they detach too far from the mountains, where the air will likely be strongly capped.

... N Italy, W Slovenia, far-south Austria ...

After a very rainy and thundery night (which could still imply an ongoing risk of flash floods at the beginning of this forecast period) in the range of the mentioned short-wave trough, temporary synoptic subsidence causes some clearing on Friday morning. By afternoon, the formation of a few hundred J/kg CAPE and the arrival of the next vorticity maxima will create a new round of scattered convection. Early storms could still benefit from enhanced shear and produce marginally large hail. Afterwards, the mid-level jet will quickly depart to the east, and heavy rain remains the main risk, especially as storms will have a tendency to backbuild towards the warmer air. Activity will weaken around sunset while it moves eastward into Slovenia and possibly N Croatia.

... Balkans (including Bulgaria and S Romania) ...

Low-level moisture under the strong warm air advection regime, and hence CAPE, was highly variable on Thursday. 2m dewpoints were as high as 20C e.g. in the bordering region of Turkey, Greece and Bulgaria, but dropped below 0C (!) where downslope winds mixed down the warm air from aloft, e.g. in Bosnia-Herzegovia, but surprisingly also along parts of the Croatian and Montenegrin coast. Considering the dry history, not much moisture recovery is expected overnight, and mixing processes will be even quicker and more efficient on Friday under 850 hPa winds around 20 m/s and the presence of the very dry EML aloft. Hence hot and dry downslope winds from the mountains will likely proceed NE-ward from the late morning onwards, driving temperatures into the low- to mid-30ies or in Bulgaria even into the upper 30s. By evening, the moist air over the forelands with CAPE on the order of 2000 J/kg and excellent wind profiles will likely be pushed back all the way to the Black Sea without producing any single storm. Nonetheless, the progressing dryline(s) should be monitored for possible convective initiation, as any storm in such an environment could turn very severe.

... Iberia ...

In the range of the cut-off low, daytime heating creates some hundred J/kg CAPE under mostly weak vertical wind shear. Scattered to widespread, mostly daytime-driven storms form mainly over the mountains and along sea breeze fronts. They pose a main risk of excessive rain with a few flash floods. In addition, large amounts of small hail as well as one or two non-mesocyclonic tornadoes along convergence zones are possible.

... N and NE Europe ...

Low-topped and weakly electrified daytime convection in well-mixed polar air does not pose a noteworthy severe weather risk.