Storm Forecast
Valid: Sat 08 Jun 2024 06:00 to Sun 09 Jun 2024 06:00 UTC
Issued: Fri 07 Jun 2024 23:21
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for parts of France, Switzerland, S Germany, Austria, N Italy, the Czech Republic, S Poland, Slovakia, Hungary and Slovenia for large hail, excessive convective precipitation and severe convective wind gusts.

A level 1 and level 2 are issued for parts of Romania, Moldova and the Ukraine for excessive convective precipitation and large hail.

A level 1 and level 2 are issued for N Spain and far SW France mainly for excessive convective precipitation.

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

A level 1 is issued for W-central Turkey mainly for excessive convective precipitation.

A level 1 and level 2 are issued for S and E Turkey for large hail, excessive convective precipitation and severe convective wind gusts.


SYNOPSIS

An extensive mid-level low stretches between Iceland, the British Isles and Scandinavia. A fairly strong 500 hPa jet with wind speeds around 20 m/s rounds its southern flank over central Europe. Two embedded short-wave troughs exit from the Baltic States and Belarus into Russia and move from the British Isles to S Sweden, Denmark and Germany, respectively.
Near the surface, a zonal frontal boundary runs from the Ukraine to France that separates temperate maritime air to the north from warm subtropic air to the south. While it remains rather diffuse, it responds to the passage of the two short-wave troughs by forming two shallow frontal waves. The western one is more pronounced and moves from Germany into Poland.
South of the main frontal zone, two cut-off lows slowly travel eastward over Iberia and over Turkey, respectively. They are separates by a weak mid-level ridge over much of the Mediterranean Basin.

DISCUSSION

... central Europe ...

Aided by plentiful evaporation and converging winds, rich low-level moisture accumulates along the mentioned frontal boundary, often resulting in 2m dewpoints in the 15-18C range. Similar or even slightly better low-level moisture also resides on the southern side of the Alps, further away from the frontal zone. In conjunction with steep lapse rates aloft that detach from the Sahara, the Iberian Plateau and the main Alpine crest and move eastward, CAPE between 500 and 1500 J/kg can be realized across many areas, possibly up to 2000 J/kg in N Italy, Slovenia and adjacent Hungary, where the capping will be stronger, though.
At least scattered thunderstorms are expected to initiate near the frontal boundary as well as over orographic features. Under 0-6 km shear around 20 m/s, 0-3 km shear around 15 m/s and veering wind profiles (the latter especially in the warm sector of the German/Polish frontal wave), an organization into multicells, some supercells, bowing line segments and later probably several large clusters is expected. Some swaths of large hail, severe wind and excessive rain events meeting level 2 criteria are likely to occur. The hail hazard, including a possibility of a few very large hail events, is highest with early, discrete storms, especially towards the south and southeast (e.g., E and S Austria, Slovenia, N Italy, but also E Slovakia), where local initiation at drylines downstream of the Alps or High Tatras, further enhanced CAPE and shear in the Alpine pumping regime and more subtle synoptic forcing are factors favoring undisturbed supercellular structures.
While the model pool does not fully agree on the timing and placement of storms, there is a fairly good agreement that the highest storm coverage will originate over E France, Switzerland and S Germany in the afternoon, aided by a conjunction of synoptic, mesoscale and orographic lift as the the most distinct mid-level vorticity maximum passes and transforms the frontal boundary into a cold front. These storms will increasingly affect Austria, the Italian Alps and possibly Slovenia towards evening.
More scattered storms are possible all along the frontal boundary further upstream and downstream, though less synoptic lift - or even weak synoptic subsidence - along with less prominent mountains will keep storm coverage lower over W France as well as over Hungary and Slovakia (with exception of the High Tatras, see above) during peak heating hours. Another source of uncertainty is how far northward the frontal wave can draw the moist, CAPE-laden warm sector into the Czech Republic or even Poland. The majority of the models increases convection signals there only in the evening and overnight, reducing the chances for surface-based and severe storms to some degree. As a compromise, the level 1 is extended northward into the Czech Republic, but not into Poland. Nonetheless it is worth mentioning that 0-1 km shear strongly increases to 10-15 m/s across the northern half of Poland near the cyclone center, hence surface-based storms forming there would unfold an enhanced hazard of producing severe wind gusts and one or two tornadoes. This scenario appears too conditional for a level 1, but is worth being kept in mind.
While convection will peak in the afternoon and evening, some storms can continue well into the night if enough lift support from mid-level vorticity maxima is present. However, they will gradually turn elevated and weaken then.

... Romania, Moldova, Ukraine ...

Despite weaker lapse rates, plentiful low-level moisture allows CAPE up to 1500 J/kg also in the warm sector of the eastern frontal wave. With otherwise weak synoptic lift support, the diffuse cold front will likely initiate scattered to widespread storms by afternoon. Under 10-15 m/s deep-layer shear, multicells are the expected main convective mode and excessive rain and large hail the expected main hazards. The center of the storm activity will likely be the eastern Carpathians in Romania, from where they will move into Moldova, reflected by a level 2. Convection will turn elevated and weaken after sunset.

... Iberia and Turkey ...

The cyclonic circulation in the range of the two cut-off lows draws sea breezes and maritime moisture quite some distance inland, allowing moderate CAPE up to 1000 J/kg or locally beyond also in interior parts. Also in these regions, scattered to widespread, predominantly daytime-driven storms are expected. At the forward flanks of the cut-off low, steeper lapse rates and enhanced deep-layer shear up to 20 m/s hint at well-organized storms with a predominant hail and wind threat. At their northern and western flanks, moist air under weak shear and plentiful lift shifts the main hazard to excessive rain. The highest (convective) precpitation accumulations are predicted for the Basque country and adjacent parts of N Spain and SW France, warranting an upgrade to a level 2. Otherwise, a lack of clear focal points (and of high-resolution model output) rather suggests broad level 1 areas.

... rest of the Balkans ...

Low to moderate CAPE under quiescent synoptic conditions allow isolated to scattered afternoon storms. Steep lapse rates near the fringe of the Saharan plume and 10-15 m/s deep-layer shear can promote multicells with large hail. However, expected storm coverage is just too low for a level 1 or a high-probability lightning area.