Storm Forecast
Valid: Wed 30 Aug 2023 06:00 to Thu 31 Aug 2023 06:00 UTC
Issued: Tue 29 Aug 2023 21:46
Forecaster: GROENEMEIJER

A level 2 was issued across parts of the Baltic States and adjacent regions in Russia and Belarus, for large hail, severe wind gusts, heavy rainfall, and tornadoes.

A level 1 was issued across the level 2 area, additionally including parts of Finland, Poland, Ukraine, Slovakia, Hungary, Romania, Moldova, Bulgaria, and Serbia, mostly for large hail and heavy rainfall.

A level 2 was issued across parts of Tunisia and Algeria, mostly for large hail, but additionally for severe wind gusts and heavy rainfall.

A level 1 was issued across a part of western Croatia for heavy rainfall and tornadoes.

A level 1 was issued across a part of eastern Spain, mainly for large hail.

DISCUSSION

Baltic level 2 area and surroundings...

We expect that elevated convection, probably in the form of one or more mesoscale convective systems will be ongoing across western Lithuania and Estonia by the beginning of the forecast period at 6 UTC. A west-oriented surface warm front should be located approximately at the latitude of central Latvia at 6 UTC and gradually move northward. There is a moderately strong southerly mid-level flow across the Baltic States and adjacent Russia/Belarus with wind speeds of 20-25 m/s at 500 hPa, creating adequate shear for well-organized storms in case they would initiate from the boundary layer. This is expected to happen near and south of the warm front in the course of the afternoon as the unusually warm and humid air-mass has been heated sufficiently to erode the convective inhibition when CAPE has increased to 1500-3000 J/kg. Models do diverge in the amount of convection in the early to mid-afternoon, but there is agreement that the approach of a shortwave trough from the south will help trigger convective initiation by late afternoon. Storm modes will be multicells and supercells with all hazards possible, including large and possibly very large hail, severe wind gusts and local very heavy rainfall. Tornadoes are also possible since low-level hodographs are curved and exhibit sufficient streamwise vorticity to allow for a few tornadoes. The more strongly forced convection at the end of the afternoon into the evening will probably transition into one or two bow echoes tracking north-northeastward from Belarus across the central or eastern parts of the Baltic States, Lithuania, Latvia and Estonia, reaching the Gulf of Finland late at night. The main threat with that system would gradually transition to be severe wind gusts, but very heavy rainfall is another important risk.

Level 1 further south ...

Across the entire remainder of the level 1 area further south, the amount of shear and instability are generally sufficient for well-organized storms, mostly multicells. Given the high amount of moisture, very heavy rainfall is a distinct risk, but large hail mostly in the 2-5 cm range is likely as well, and larger hail may occur locally with any supercell that may form.

Level 2 and level 1 across the Maghreb...

Isolated storms are expected to form across the eastern Atlas mountain range during the afternoon, producing a risk of large hail and, locally, severe winds. During the afternoon and evening, an onshore low-level flow from the east will develop across eastern Tunisia which creates a very unstable situation with MLCAPE in the range of 2500-4000 J/kg. There is strong consensus among NWP models that widespread convection will initiate in the evening. Because of the strong, 20-25 m/s, 0-6 km bulk shear, the storms may include supercells with very large hail and bow echos with severe wind gusts. In addition, very heavy rainfall will be likely, and flash flooding may well occur.

Level 1 across Croatia...

A low-level cyclone is expected to linger around the far northern Adriatic and make landfall just south of the peninsula of Istria. The associated convective storms may locally train over the same areas for a long time, producing rainfall accumulations exceeding 100 mm in a few places. As a result, some local flooding may occur.

Level 1 across eastern Spain

Isolated to scattered storms are expected to develop across the mountains of eastern Spain during the afternoon. Given adequate deep-layer shear for strong multicells and possibly a supercell or two, some 2-5 cm large hail may occur.