Storm Forecast
Valid: Thu 17 Oct 2024 06:00 to Fri 18 Oct 2024 06:00 UTC
Issued: Wed 16 Oct 2024 17:11
Forecaster: TUSCHY

A level 2 was issued across W Georgia into extreme NE Turkey mainly for excessive rain.

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

Two level 2 areas were issued for NW Italy and S-CNTRL France for excessive rain.

A broad level 1 surrounds both level 2 areas for heavy to locally excessive rain. In addition, a few hail and gust events with an isolated tornado are possible from NE Spain into far S France. In addition an isolated tornado risk exists along the coastal areas in the level 1 area.

A level 1 was issued for E Turkey into far N Syria/Iraq and Armenia mainly for heavy rain.

SYNOPSIS

The evolving pattern points towards an high-impact event during the following days for the western and central Mediterranean.
This pattern is composed of a gradually eastward moving trough over W-Europe, which weakens and cuts off over the Gulf de Lion and the Corso-Ligurian Basin. This occurs in response to rising hights to its E (atop the Adriatic Sea), where blocking/ridging persist, while connecting a blocking Azores high with an intensifying deep anticyclone over Scandinavia / NW Russia. We therefore see another period with more or less quasi-stationary wave pattern over much of Europe.

The cut-off is accompanied by a broad and meridionally aligned low pressure channel, which extrends from the Gulf of Lion N into CNTRL France. NWP guidance remains optimistic in developing a closed depression with crowded IFS-ENS members along this channel, supporting a temporarily deepening vortex somewhere over S into CNTRL France. This vortex probably fills during the late overnight hours while lifting NE towards E France. It drives numerous fronts E atop the NW Mediterranean including a cold front and potential numerous prefrontal convergence zones. Onshore, a slowly westward pushing warm front affects most of S into CNTRL France during this forecast.

Another cut-off evolves just N of the Black Sea and drifts atop the warm waters. This feature assists in another heavy rainfall event for this area.


DISCUSSION

... Parts of Italy ...

A rather straightforward setup evolves from the Gulf of Lion into the Ligurian Sea/Gulf of Genoa and probably all the way S towards the vicinity of Rome and into the Strait of Sicily.

An eastbound moving cold front crosses the Gulf of Lion during the late afternoon hours before approaching Corsica during the late overnight hours from the W.

The prefrontal air mass features weakly capped 500-1000 J/kg MUCAPE with regionally higher values. DLS is mostly in the 15 m/s to locally 20 m/s range but of main interest is the 15 m/s 0-3 km shear, caused by backed BL flow into the lowering pressure over S-CNTRL France (and enhanced along the orography). This flow will bring rich moisture into any existing/evolving cluster.

One area of augmented concern for a quasi-stationary MCS event with excessive and flash flood triggering rain exists over the N-Gulf of Genoa from 06Z onwards. This risk spreads S into the N Tyrrhenian Sea during the day. Mass response to the approachiong trough/cut-off cause a deep N-S aligned band with anomalous high moisture content from the BL into the mid troposphere (TPWs up to 40 mm with anomalies in the 150% range to the background climatology). This moisture flows N into the Gulf of Genoa and adds rich moisture influx into ongoing and evolving clusters. We expect the main event to occur around/E of Genoa but its final magnitude depends on how widespread convection erupts to the S (Corsica and to the E). Nevertheless, expect 12-18h rainfall amounts in the 100-300 l/qm range with flash flooding possible.

During the afternoon hours, falling pressure/heights and a passing lead short-wave should assist in scattered to widespread CI over Sardinia and Corse and to the E. NWP guidance still diverges in where to place the main activity, but the general setup also assists in numerous back-building clusters, partially offshore or along the coast, with excessive rain the main risk but more on a local scale. This convection also builds S into the Strait of Sicily with slow moving/clustering DMC activity.

Right now we focused on the Genoa area with a rainfall-driven level 2. Elsewhere a broad level 1 was issued to reflect ongoing uncertainties regarding placement of training convection. Level 2 events on a local scale are certainly possible however.

Tail-end convection and thunderstorms along the coast/existing convergence zones also pose a tornado threat.

Warm conveyor belt (WCB) moisture also advects N into the S Alps, where WCB rain with embedded thunderstorms brings heavy to excessive precipitation with 12h amounts in the 50-100 l/qm range including local higher peaks. Flash flooding is possible but the broad extent of the rainfall peaks also raises the concern for general flooding issues. A broad level 1 was issued, as CAPE remains on the marginal side.

... Most of S-CNTRL France and NE Spain ...

We start with a band of gradually weakening convection along the S/E fringes of the Massif Central which could survive until the cold front approaches from the W during the afternoon. Heavy to excessive rain on a local scale is already possible with this activity.

The focus however then shifts to NE Spain/S-France from noon onwards, as increasing background forcing plus the eastward surging cold front interact with the onshore advecting unstable airmass. Scattered to widespread convection erupts over far NE Spain into S-CNTRL France, probably in form of an extensive MCS, which shifts E/NE. Of concern is the S/E-Massif Central area, where prefrontal convection could stay stapled along the orography for some time, before being overtaken by the MCS/cold front activity. 12-h rainfall amounts in the 50-150 l/qm range are possible, especially along the S-CNTRL fringes of the Massif Central with 24h amounts in the 100-200 l/qm range. We will most likely see high rainfall amount gradients.

This purely convective driven event merges with another event further W, which caused as rather broad level 2 area issuance.
This event further W also starts during the morning/forenoon hours with a band of slow moving and clustering convection, which is N-S aligned and has impressive PWs of 30-35 mm to work with. As the meridionally aligned low pressure channel evolves/intensifies and offers a temporarily structuring LL depression over S/CNTRL France, this area will see a transformation of a convective event more into a stratiform event with isolated embedded convection. This more stratiform event will still be able to produce healthy hourly and 12h rainfall amounts for areas, which were already affected by the previous thunderstorm activity. Flash flooding and flooding in general becomes an issue.

Beside the rainfall threat, initiating storms over NE Spain into the Toulouse area also bring a temporal hail and gust threat next to an isolated tornado risk.

... E-Black Sea ..

Another cut-off impacts our forecast and moves atop the warm Black Sea with positive SST anomalies in the 2-4 K range. Lowering MSLP causes persistent onshore flow of a moist/unstable airmass into the mountainous area of W Georgia and far NE Turkey. Heavy to excessive rain with flash flooding is forecast.

... E Turkey into far N Syria/Iraq and Armenia ...

A deep moisture plume along an extensive warm conveyor belt interacts with the rough orography and assists in heavy rain with embedded thunderstorms. Flash flooding is likely in this area with 24h rainfall amounts in the 40-80 l/qm forecast with local peaks up to 100 l/qm. We thought about a level 2 upgrade due to the enhanced flood risk along this complex orgroaphy but the extent of thunderstorm activity seems too limited for a full level 2 upgrade right now. Nevertheless, this is an high-impact event for this area, especially for the region N of Lake Van.