Extended Forecast
Valid: Fri 12 Aug 2022 06:00 to Sat 13 Aug 2022 06:00 UTC
Issued: Wed 10 Aug 2022 16:38
Forecaster: TUSCHY
A level 2 was issued across Corse and surrounding areas mainly for excessive rain, large to very large hail and severe to damaging wind gusts. A tornado event is possible.
A level 1 surrounds that level 2 for similar hazards but with lower probabilities.
A level 1 was issued for S Italy mainly for heavy rain and a few waterspout reports.
SYNOPSIS
The protagonist will be the cold-core cut-off over the E-Alps during the start, which moves towards the Adriatic Sea until 06Z. Numerous waves with lower amplitude circle that cut-off and insert copious areas with enhanced thunderstorm probabilities.
This cut-off is well forecast in the numerical guidance with a rather good handling of track and intensity that far out. Fine-tuning is certainly needed later-on with respect to placement of all those waves.
DISCUSSION
... S-Italy ...
An ongoing cluster from the previous night affects this area with flash flood producing rain and an upgrade to a rain-driven level 2 may become necessary later-on. 1000 - 2000 J/kg MUCAPE and DLS less than 10 m/s favor slow moving and clustering convection. Residual outflow boundaries may serve as foci for temporal training with an enhanced flash flood threat. In addition, mature cells may pose a temporal hail risk next to a waterspout risk. This activity weakens around sunset / shifts to the Ionian Sea respectively.
... S Balkan States ...
As the background flow ahead of the cut-off turns to the SW during the day, a moist marine air mass advects well inland. Mixed with some diurnal heating, 500-1000 J/kg MUCAPE evolves in a weakly capped environment. Scattered thunderstorms with isolated hail and heavy rain are forecast. This activity continues during the night with some nocturnal flash flood issues. A level 1 may be needed in later updates when regional hot spots with augmented severe become more obvious.
... Ukraine ...
A moist BL beneath neutral mid-level lapse rates combined with diurnal heating assist in 1000-2000 J/kg MUCAPE. DLS remains weak (5 m/s range), so clustering convection poses mainly an isolated hail and heavy rain threat. A level 1 may be needed in later updates when regional hot spots with augmented severe become more obvious.
...Ligurian Sea to Corse during the overnight hours ...
Probably the most volatile setup of the day evolves during the overnight hours as the main trough axis of the cut-off approaches from the N and overspreads 27-28 SSTs (2-3 K positive anomaly) of the Ligurian Sea. A constantly tightening geopotential height gradient between the cut-off and the subtropical ridge over SW Europe results in an impressive mid to upper-level jet, which overspreads the area of interest. SBCAPE in the 2 kJ/kg range with 20-25 m/s DLS and intensifying 0-3 km shear cause a favorable setup for organized convection. Scattered CI is forecast beyond midnight over the Ligurian Sea and upscale growth into a large and forward propagating cluster is expected. Excessive rain, large hail and severe wind gusts are possible with extreme events possible (very large hail and damaging wind gusts up to hurricane force). In addition, embedded supercells may pose a tornado threat due to enlarged hodographs in an high SRH-environment (more than 200 m^2/s^2 in the lowest 3 km AGL) with low LCLs.
We expanded the level 1 far south to account for faster cold pool driven activity than currently anticipated and we also added a level 2 for the area with the highest severe risk for now. This level 2 may be enlarged in later outlooks when fine-tuning becomes possible (to the W and S).
The level 1 was also expanded far west as uncertainties remain how far the activity builds into an extremely unstable but increasingly capped marine air mass W of Corse (MUCAPE in the 3000-4000 J/kg range with similar shear values). We also need to monitor the W fringe of the cluster in case of near parallel alignement towards the background flow betimes which would point to a concerning flash flood risk for Corse and N Sardegna during the end of this forecast period.
