Extended Forecast
Valid: Sat 19 Oct 2024 06:00 to Sun 20 Oct 2024 06:00 UTC
Issued: Thu 17 Oct 2024 22:37
Forecaster: TUSCHY

A level 2 was issued for parts of S Italy and the E Black Sea for excessive rain.

The level 1 surrounds both level 2 areas with heavy rain the main risk. Coastal areas could see an isolated tornado risk.

SYNOPSIS

There is not much change compared to the severe weather outlook day 1 (SWODY 1) with ongoing blocking over much of Europe.

The cut-off over Tunisia/Sicily and Sardegna has no room to escape and hence only changes in its geometry of the 500 hPa height field are forecast. The same for the cut-off over the E-Black Sea, which remains also in place.

An extensive low pressuve channel evolves atop the CNTRL Mediterranean with IFS-ENS member highlighting numerous separate LL vortices (e.g. one shifting E/NE just S of Sicily and another one atop the Tyrrhenian Sea). A wavy front is embedded in this low pressure channel with a focused prefrontal moisture advection towards S-Italy.

As a side-note: This event could evolve into a multi-day heavy rain event for parts of S Italy with a potential focus for catastrophic flooding over parts of Calabria, where multi-day rainfall amounts could exceed 300-600 l/qm on a broad scale! Please monitor this setup closely.

DISCUSSION

... Italy ...

Not much detail will be discussed with this SWODY 2 as still some discrepancies exist regarding where to place any mesoscale boundaries or even the main synoptic front. Its final position determines the area of maximized rainfall amounts.

A divergent upper streamline pattern atop a plume of 1000-2000 J/k MUCAPE is present all day long. A brisk S-erly flow advects rich BL and mid-level moisture towards S Italy and efficient rainfall rates are likely. There might be a lull in activity during the afternoon, but otherwise the risk of slow moving /back-building clusters with extreme rainfall amounts is present (this might be another nocturnal peak of most extreme rainfall amounts similar to the SWODY 1). Local peaks in the 200-400 l/qm range are possible with extreme regional gradients of the final rainfall amounts.

Another area with heavy to extreme rainfall amounts could evolve in the SE Tyrrhenian Sea and surrounding areas, especially in case of a developing confined LL vortex.

Coastal areas also see an isolated tornado threat.

A broad level 2 was issued for the current area of highest concern.

The level 2 may need to be expanded into the Palermo area of NW Sicily in case of a stronger and more to the S displaced vortex atop the Tyrrhenian Sea, which could induce another round of onshore moving thunderstorm activity. Confidence in this event remains too low for now to go with an upgrade.

Another focus for heavy to excessive rain evolves over NE Italy (e.g. Emilia-Romagna to SW Veneto). A prolonged period of moist/unstable onshore flow into the orography assists in scattered to widespread thunderstorm activity with 24h rainfall amounts in the 40-80 l/qm range and even higher peaks along the orography. A broad level 1 was issued due to ongoing model uncertainties regarding the final path of the moisture advection, which could shift the final QPF peaks more N/S during following model updates. We also miss a more pronounded convergence zone with more of an orographic rain event for now. An upgrade could be needed along the orography, but also along the coast in case of a better defined convergence zone. This level 1 also includes parts of the E coast of the Adriatic Sea.

... E Black Sea ...

Ongoing onshore flow causes heavy rain into the same area, which already experienced high rainfall amounts during the past few days. Mainly due to the long-lasting event (not necessarily due to the rainfall amounts), we once again issued a confined level 2 for very heavy rain. The far E Black Sea area has to be monitored for signs of a more vigorous LL vortex spin-up beneath/peripheral of the upper cut-off. This could increase the final rainfall risk.