Storm Forecast
Valid: Sat 25 Jul 2020 06:00 to Sun 26 Jul 2020 06:00 UTC
Issued: Fri 24 Jul 2020 18:15
Forecaster: PISTOTNIK

A level 2 is issued over Serbia, SW Romania and W Bulgaria mainly for large hail and excessive convective precipitation.

A level 1 is issued for the W Ukraine, NW Romania, Hungary, Croatia and Bosnia-Herzegovina mainly for excessive convective precipitation.

A level 1 is issued for the north-central Adriatic Sea for non-mesocyclonic tornadoes (waterspouts).

A level 1 is issued for S Italy mainly for large hail.

A level 1 is issued for NE Turkey, Georgia, Armenia and the Russian Caucasus region mainly for large hail and severe convective wind gusts.

SYNOPSIS

A broad, zonal mid-level jet covers most of Europe with an axis between 50 and 55N. Two large cyclones are placed over central Scandinavia and NW of the British Isles, respectively, and short-wave troughs move eastward at their southern flanks. On the anticyclonic side of the main frontal zone, a weak secondary cyclone stretches from Hungary to the central Adriatic Sea and dissolves again in the course of this forecast period.
The zonal weather pattern implies cooler-than-usual, unsettled conditions towards the north and warmer-than-usual, stable conditions towards the south. However, a pronounced frontal boundary between these contrasting airmasses is missing.

DISCUSSION

... central Mediterranean region to Balkans, Hungary and W Ukraine ...

Left over by abundant convective activity Friday evening and night, large areas of rain and some embedded thunderstorms will likely still be active at the beginning of this forecast period from Hungary and W Romania to the north-central Adriatic Sea. This activity poses an ongoing risk of high rainfall accumulations. In addition, a few waterspouts are possible over the Adriatic Sea.
As the shallow cyclone gradually fills up, convection will soon die off over coastal and offshore areas, whereas new rounds of scattered to widespread storms are expected over land. They will primarily initiate over mountains and propagate towards flatter areas later on.
The CAPE magnitude rises from ~500 J/kg in the W Ukraine to ~1000 J/kg over the central Balkans and up to 1500 J/kg over the southern half of Italy and adjacent sea areas (where it will be strongly capped, though). Vertical wind shear and synoptic lift decrease in comparison to Friday, but 10-15 m/s deep-layer shear and subtle synoptic lift are still available. Both the amount of lift and vertical wind shear can be enhanced on smaller scales by the complex terrain and resulting modifications of the low-level wind field (most notably by upvalley and upslope circulations).
The main risk is excessive rain towards the north, where a high coverage of disorganized, clustered and partly embedded storms in moist, weakly capped air is expected. Towards the south, storms can organize better due to steeper lapse rates, more discrete initiation (i.e., less "competition") and slightler stronger vertical wind shear, and the main risk shifts to large hail. In case of supercells, one or two instances of very large hail are not ruled out.
The severe weather risk appears highest over Serbia and adjacent areas of W Bulgaria and SW Romania, where (1) both orographic features and outflow boundaries from overnight's large storm cluster may act as initiation sources, and (2) low-level flow modifications supportive for better storm organization are most likely NE of the mountains. This area is upgraded to a level 2.

... NE Europe and NW Europe ...

While decent moisture is available in the warm sectors of both cyclones, the Atlantic origin of the involved airmasses keeps the lapse rates on the poor side. Marginal CAPE with low cloud-top heights only allow low-topped and weakly electrified convection. The two affected areas stretch from Finland to NW Russia and Belarus and cross the British Isles, repectively. While enhanced vertical wind shear and strong synoptic lift ahead of the short-wave troughs might promote some strong wind gusts, the low depth of convection keeps the risk of severe events too low for a level 1.

... NE Turkey, Georgia, Armenia, Russian Caucasus region ...

Thanks to a long "fetch" over the Turkish plateau, the mid-level flow advects very steep lapse rates on top of rich moisture in lowlands and coastal areas. This process allows an overlap of robust CAPE (500-1500 J/kg) and enhanced 0-6 km shear (15-20 m/s). Afternoon storms that manage to propagate from the mountains into the lowlands can organize into multi- and supercells and pose a main risk of large to very large hail. In the drier air over higher, more mountainous terrain, storms will be high-based and less organized, and the main risk shifts to localized (possibly dry) downbursts.
Limiting factor is the expected low storm coverage due to a lack of synoptic lift support and very dry environmental air entraining into convective updrafts.