Storm Forecast
Valid: Wed 19 Jul 2023 06:00 to Thu 20 Jul 2023 06:00 UTC
Issued: Tue 18 Jul 2023 22:28
Forecaster: GROENEMEIJER

A level 3 was issued across parts of Italy, Austria, Slovenia, Hungary, Croatia, Bosnia and Serbia, firstly for very large hail and damaging winds, and secondly for very heavy precipitation.

A level 2 was issued for the same hazards across an area surrounding the level 3.

A level 2 was issued for eastern Slovakia, central Romania, and eastern Hungary for large hail, very heavy rainfall, and to a lesser extent for tornadoes.

A level 1 was issued across an area surrounding these level 2 areas, for heavy precipitation and large hail.

A level 1 was issued across eastern Spain for a risk of severe downburst winds, and, very locally, large hail.

SYNOPSIS

A strong zonal jet streak is located across central Europe at the northern fringe of a pronounced elevated mixed layer. The combination of the associated steep lapse rates and ample low-level humidity across Po Valley and the northwestern Balkans lead to very high MLCAPE in a zone from Serbia through the northwestern Balkans, with values of 2000-3000 J/kg, into the Po Valley, where values in the 3000-5000 J/kg range are predicted. A high-end severe weather situation with very large hail and widespread severe winds is expected to develop as a subtle upper-level vorticity maximum moves into the region, setting the stage for convective initiation of this volatile environment.

DISCUSSION

Alpine region, Pannonian Plain, and the northwestern, western and central Balkans...

NWP models suggest that on Wednesday morning scattered rapidly-moving elevated storms may be ongoing across the large level 2 area. Forecasting their locations and intensity is very difficult even 6 to 8 hours ahead. Although these storms pose some risk of severe winds and large hail, their elevated nature limits this risk, and stronger storms are expected to develop later in the day.

During the afternoon, as the aforementioned vorticity maximum approaches the Alps from the west, per all NWP guidance, widespread initiation is expected across the southern and central Alps. The storms will do so in a very unstable and strongly sheared environment, so that rapid evolution into supercells, followed by a transition to one or more bow echoes is expected. Additional storms could form across the Dinaric Alps, and possibly across the Po Valley as parcels are lifted ahead of outflow boundaries from the Alps.

The very high instability and strong storm-relative low-level wind in this environment suggest that some storms will likely produce very large hail in the 5-10 cm range. The transition in a bow echo is expected during the evening as the storms move across the northwestern Balkans, and possibly Hungary. Widespread severe winds are expected as a result, including some extremely severe winds of more than 32 m/s. Overnight, the system should very gradually diminish in intensity as an environment with lower CAPE is encountered across southern Serbia, Romania, and Bulgaria.

Slovakia, eastern Hungary, West Ukraine, Northwest and Central Romania...

Storm initiation is expected as a subtle frontal wave with some warm advection provides additional lift across this section of the Carpathian and Abuseni mountains. With moderate CAPE and fairly strong wind shear in place, some of the storms that develop in response will likely become severe. They include supercells with a risk of large hail and very heavy rainfall. In addition, the warm advection creates amounts of low-level storm-relative helicity that may be adequate for one or two tornadoes to develop with the highest risk located across Central Romania.

East Spain...

A few storms may form across the mountains of eastern Spain in response to the diurnal strengthening of the inland and upslope flow. Given the very deep and dry boundary layer, strong evaporational cooling suggests a risk of downbursts with any stronger storm. Given high CAPE and 20 m/s of deep-layer shear, some large hail is possible as well.