Storm Forecast
Valid: Sun 28 Jul 2024 08:00 to Mon 29 Jul 2024 06:00 UTC
Issued: Sun 28 Jul 2024 07:50
Forecaster: TUSCHY

A level 2 was issued across parts of Estonia into Latvia mainly for excessive rain.

A level 1 surrounds that level 2 mainly for heavy to excessive rain (northern part) with otherwise mainly numerous strong to isolated severe gusts (convective) and an isolated tornado risk.

A level 2 was issued for SE Austria into Slovenia and far NE Italy mainly for large hail, severe gusts and heavy rain.

A level 1 surrounds that level 2 for similar hazards but with lower probabilities.

SYNOPSIS

A blocking subtropical ridge persists atop most of the Mediterranean Sea with a building rige from the North Sea towards the Norwegian Sea.

A progressive wave over Denmark/S Sweden (06Z) races E towards the Baltic States. The forecast cyclonic wave breaking in an highly dynamic setup causes a treble-clef-shaped PV anomaly over the Baltic States with forecast impressive tropospheric mass response. An intense depression evolves, which deviates from climatology by 3-4 sigma and features strong clustering in IFS-ENS data in the 990-995 hPa range. This intensification rate is not only driven my the background dynamics but also due to the advection of a very warm/moist airmass towards its center, resulting in a well structured shallow warm-core system. The attendant cold front also impacts Poland and bends back towards the Alps before turning into a wavy boundary over France.


DISCUSSION

... Poland to the Baltic States and surroundings ...

The big player on the European map today will be the rapid cyclogenesis over the highlighted area.

The interaction of the eastbound moving cold front and an evolving/deepening dry slot with already noticeably drying seen over N-CNTRL Germany at 06Z will dictate the thunderstorm risk over E-CNTRL Europe.

The first cold front over S Poland seems to open up into a broad LL confluence zone ahead of the main and rapidly structuring/meridionally aligned cold front, which affects CNTRL Poland around noon and E Poland during the evening hours. This boundary seems to acquire some sort of split front characteristica with meager instability probably the main reason for a more reluctant severe risk.

However with the dynamic nature of this system and rapid potential instability build-up, confidence increases in a growing line of partially electrified convection, which affects CNTRL Poland around noon and E Poland during the afternoon before moving into W Ukraine/Belarus area.
Both ensemble and deterministic data indicate gust potential in the Bft 8-9 range but we cannot rule out a few Bft 10 events with more dominant convective segments along that line with background shear in th 15-20 m/s range for the lowest 3km. A few tonado events are possible as semi-discrete convection persists in a kinematic environment with low/modest LCL heights especially from E Poland into W Belarus and far NW Ukraine.

Some CAA convection is forecast from NE Germany into NW Poland and S Sweden during the day but this activity remains mostly sub-severe, although a few stronger gusts and isolated hail are possible with 15 m/s background shear and even weak UH tracks (f.ex. along the German/Polish border around noon).

The main focus for (extremely) severe evolves over the Baltic States around noon onwards as the depression arrives/deepens.
First there is the cold front, which crosses Lithuania during the afternoon hours with a strong to isolated severe wind gust and isolated tornado threat (similar setup than futher S).

Then the event unfolds with intense LL wind field response and impressive convergence along the deformation zone of this depression, which runs from W Lithuania to W/CNTRL Latvia into Estonia. The highest concern for dangerous flooding conditions arises along the W/NW fringe of the deepening vortex, where influx of unstable air converges into a highly frontogenetic environment. In addition, SSTs of the Gulf of Riga run 5-6 K too high compared to the climatology with readings around 22C. This additional moisture funnels perfectly into the deformation band. QPF of NPW guidance unanomously indicates 24h rainfall amounts in the 90-150 l/qm range from SW Estonia into W/CNTRL Latvia, which is also highlighted by probabilities of 60-70% for more than 80 l/qm in 24h (COSMO-LEPS a tad lower, IFS-EPS and ICON-EU EPS).
We covered this risk as forecast soundings indicate a unstable/very efficient MUCAPE inflow into this depression, so embedded thunderstorms are certainly possible along its periphery. The impact will be exaggerated by intense background flow and surface gusts in the Bft 9-10 range with a few Bft 11 peaks not ruled out.

This event is gradually weakening during the end of this forecast period but it is certainly far from over with additional rain most likely excaberating ongoing flooding issues.


... SE Austria into Slovenia/NE Italy ...

The cold front approaches the Alps and causes scattered CI around noon. The airmass ahead/S of the Alps features MLCAPE values in the 1000-2000 J/kg range with higher values towards NE Italy. DLS oscillates around 15 m/s and southward moving storms may organize into multicells and a few supercells with isolated large hail and strong to severe gusts next to heavy rain. We issued a small level 2 for the area with expected highest severe coverage. During the night, most storms weaken although this could take some time over NE Italy as storms continue to build into an highly unstable airmass.

... Nordic States to Russia ...

A belt of weak shear/modest CAPE provides conducive conditions for slow moving storms with a few heavy rain and isolated hail issues. There is no real focus for a region with higher severe coverage and hence no level upgrade was performed.