Storm Forecast
Valid: Tue 24 Sep 2024 06:00 to Wed 25 Sep 2024 06:00 UTC
Issued: Mon 23 Sep 2024 15:06
Forecaster: TUSCHY

A level 2 was issued across the E Adriatic Sea mainly for excessive rain.

A level 1 surrounds that level 2 for similar hazards with lower probabilities, including a tornado risk.
This level 1 extends into NE Italy/SE Austria for some hail and gust reports. A low-end tornado risk exists.

A level 2 was issued for the E-Black Sea mainly for excessive rain.

A level 1 surrounds that level 2 for similar hazards with lower probabilities including a tornado risk.

A level 1 was issued for parts of the W-Mediterranean mainly for severe gusts, hail, excessive rain and a tornado risk.

A level 1 was issued for parts of NW Italy mainly for heavy rain and an isolated tornado event.


SYNOPSIS

A longwave trough is situated over the Norwegian Sea down to the W-Mediterranean and gets framed by a building ridge to its E (e.g. W-Russia) and an impressive positive geopotential height anomaly towards Greenland. In addition, rather healthy looking subtropical anticyclones down in N Africa remain strong and keep an augmented N/S height gradient in place.

Despite being more or less quasi-stationary, lots of "noise" / waves with lower amplitude circle that trough and add a more dynamic environment to parts of the forecast area. A lifting wave over CNTRL Europe loses its amplitude but more short-waves circle even this weakening wave.
Finally the main trough also interacts with a potent upper low (roughly 50N/30W), which re-intensifies the main trough and pushes negative height anomalies just W of the Bay of Biscay into the -2 sigma range regarding background climatology. Mass field response also adds a potent mid/upper jet, which approaches W-Europe during the end of the forecast.

During the past 2 days or so, numerous transient spin-ups of weak depressions occurred over the Black Sea but pulsating convection now caused a rather well defined low-level closed circulation (LLCC) atop the E-Black Sea, which is in line with NWP guidance. More focused/persistent convection occurred S of the center, probably more a signature of enhanced coastal convergence. A ship just W of the circulation reports a MSLP of 1007 hPa and winds of 19kt/290 degree. Local floaters support NWP guidance with SSTs in the lower to mid twenties and these floaters also support the thin mixed layer with peak SST values and a deep/sharp vertical thermocline, so upwelling is certainly an issue for this forecast period.
Nevertheless, a compact vortex is present and another round of enhanced convection is possible with the next diurnal cycle. Phase diagrams support a shallow warm core feature and an OSI SAF bullseye snapshot supports winds in the 20-30 kt range. A cocoon of moist/unstable air is present in latest data with no substantial dry intrusion forecast next to absent frontal structures, so there is a risk of this vortex to approach the E coast as an organized subtropical cyclone (e.g. far W Georgia into Abkhazia and the mountainous area of the Caucasus). Rapid weakening due to the complex orography is likely during the overnight hours (into the 25th September).

DISCUSSION

... E-Black Sea ...

Excessive rain is likely in the level 2 area as the subtropical depression/storm approaches from the W. Landfall is forecast during the evening into the overnight hours. Persistent onshore flow of organized DMC bands into the complex orography spark very efficient rainfall rates with 24h amounts in the 100-300 l/qm range. Dependant on how organized this system will be, outer feeder bands may expand the severe risk far away from the vortex' center. Along the coast, severe gusts are possible but also depend on how fast this cyclone intensifies. No major upper support is forecast and ensemble data only supports modest intensification until landfall, so the gust risk remains uncertain for now.

... W into CNTRL Mediterranean ...

Numerous passing waves keep DLS in the 15-20 m/s ranges with pockets of enhanced 0-3 km shear in an unstable environment.

One regional focus for enhaced DMC activity is a wave, which exits the Adriatic Sea during the start of this forecast and shifts E. Ongoing convection especially along the E coast (e.g. next to the coastal convergence zones but also just onshore) keeps going well into the day and training convection may persist until the evening hours in the level 2 area.

The level 2 area has the best background support for training convection. Prolonged inflow of moist/unstable air (1000-1500 J/kg MUCAPE) through the Strait of Ortanto into the E-South Adriatic Basin roots into a deep tropical moisture plume, which was analyzed in latest layered PW data (1335 Z) over Tunisia (emerging from a deep moisture plume of the ITCZ). 24h rainfall amounts in the 100-200 l/qm range are forecast and a coastal tornado risk is certainly present, as curved hodographs overlap with adequate CAPE.
After an evening lull in activity behind the passing wave, another uptick of DMC activity along the coast is forecast, including more training convection.

Another fetch with enhanced DMC activity was highlighted by a level 1 over the W-Mediterranean. A favorable CAPE/shear space exists and numerous organized thunderstorms including supercells are forecast with all kind of hazards. As background forcing remains diffuse with numerous eastward shifting waves (includig NWP guidance issues regarding timing/placement of those waves) there remains a higher than normal uncertainty regarding the final coverage of stoms. Any more dominant convergence zone could become a focus for clustered convection with latest UK10 run showing 300 l/qm just SW of Corsica. Overall all hazards are possible including excessive rain, a few tornadoes and strong to severe gusts. Uncertainty in convective coverage kept this in a broad level 1.

Another level 1 was added for nocturnal convection along the NW coast of Italy as better forcing on the synoptic-scale approaches from the W. Excessive rain on the local scale and a coastal spout/tornado event are possible.

... NE Italy into SE Austria and surrounding areas ...

Brisk SW-erly flow advects a moist/unstable airmass into the area of interest with MUCAPE in the 400-800 J/kg range. 20 m/s DLS and long/straight hodographs support organized / splitting convection with a few supercells possible. Mature cells moving into far E Slovenia and W Hungary could bring hail/strong to isolated severe gusts and some healthy rainfall rates (progressive nature should keep heavy rain risk rather low however). Low LCLs and some LLCAPE also add an isolated tornado risk to this activity. This is also valid for NE Italy.

... NE Germany ...

An unstabe airmass resides ahead of the occlusion with 15 m/s DLS present, so we won't rule out a few stronger cells along the E fringe of the more stratiform rain area. Strongest cells may bring some graupel/hail and gusts but the overall severe risk seems too marginal for any upgrade. Of note: ICON D2 support a few longer lived mesos with this convection due to more 0-3 km SRH including low LCLs for an area N of Berlin. This trend needs to be monitored in following model runs with peak activity before/around noon.