Storm Forecast
Valid: Fri 18 Oct 2024 06:00 to Sat 19 Oct 2024 06:00 UTC
Issued: Thu 17 Oct 2024 21:36
Forecaster: TUSCHY

Level 2 areas were issued over NE Italy, W-CNTRL Italy, parts of S-Italy, parts of Sicily and the E-Black Sea for excessive rain.

Level 1 areas surround all the level 2 areas for a similar hazard with lower probabilities. In addition a few tornadoes are possible allong the coasts. An isolated large hail risk exists over and E of Tunisia next to a few strong/severe gusts over NW Italy (cold front passage).

A level 1 was issued over NE France into far W Germany mainly for heavy rain.

SYNOPSIS

A blocking anticyclone persists over Scandinavia into NW Russia and drives a cut-off low from the W Mediterranean Sea SE into Tunisia. Another cut-off is placed atop the Black Sea with not much net-motion forecast during this forecast. This static pattern persists until 06Z.

The SE-ward diving cut-off inserts a weak/broad LL depression atop the Gulf of Gabes into the Strait of Sicily with IFS-ENS highlighting a broad member spread regarding a potential spin-up of a LL depression. The main member cloud exists S of Sicily, however past few NWP cycles (GFS/ICON) trended a tad more E with the mass response and low-tropospheric pressure fall.

An extensive frontal boundary runs from CNTRL Europe S into far N Algeria with a wavy/quasi-stationary boundary from Benelux into E France and a slowly eastward moving cold front atop the W/CNTRL Mediterranean. These synoptic fronts act as the main foci for DMC activity.

DISCUSSION

... Tyrrhenian Sea into Sicily/Strait of Sicily and Golf of Gabes north towards W-CNTRL Italy ...

The eastbound moving cold front is forecast to cross the Tyrrhenian Sea from 06Z onwards but slows down substantially during the day and becomes quasi-stationary somewhere atop the CNTRL part of this basin in response to falling pressure over and E of Tunisia. Responding backed low-tropospheric flow creates an extensive low/mid-tropospheric convergence zone, which drifts slowy E and affects Sicily during the day. This boundary extends NE towards Italy (between Rome and Naples).

The background shear starts relaxed with very weak DLS beneath a diffluent upper flow regime but 0-6 km increases steadily during the day as the upper trough approaches from the W. During the night, we see DLS in the range of 15 to more than 20 m/s over the whole area of interest. 0-3 km shear peaks atop and S of Sicily with values in the 10-15 m/s range with local higher peaks along orographically forced flow. This LL flow advects a moist airmass from the S into the vicinity of Sicily, whereas pooled moisture also increases the depth of the marine layer along the cold front atop the Tyrrhenian Sea.
Beside the rich low-tropospheric moisture, the brisk SW-erly flow ahead of the trough pushes a deep plume of moisture into our area of interest, which roots deep into the ITCZ / tropics. Hence, very efficient rainfall rates are forecast with this activity.

During the daytime hours, numerous slow moving thunderstorms and small clusters exist and pose a localized heavy/excessive rainfall threat (most of that activity should stay offshore).

The setup turns more violent during the evening onwards with the structuring low/mid-tropospheric trough and improved upper divergence ahead of the upper trough. A growing quasi-stationary MCS is forecast to erupt along that convergence zone next to Sicily. The question is, if this MCS roots atop Sicily or stays offshore. As NWP guidance shifted a bit E, we now expect this cluster to affect W Sicily and we would not be surprised to see the activity to drift E of Palermo and into the Agrigento/Caltanissetta area thereafter.

The setup certainly has the potential to bring an extreme nocturnal rainfall event to this region with very high hourly and 12-hourly rainfall amounts, especially if this V-shaped MCS roots over Sicily. Flash flooding is likely with significant and life-threatening flash flooding possible. Once again, extreme gradients with the rainfall amounts are forecast with QPF amounts in the 100-400 l/qm range with worst-case scenarios allowing even higher peaks. We expanded the level 2 towards Malta as we cannot rule out convection building that far S into the unstable airmass.

Another area with training convection evolves from the Gulf of Taranto and to the SW. N-ward moving thunderstorms could train along the coastal front/convergence zone and bring numerous localized but extreme rainfall events.

Two separate level 2 areas were issued to highlight the main area of concern. However we cannot rule out a few rainfall events matching a level 2 in the broad/surrounding level 1 area.

Tail-end and coastal thunderstorms also have an isolated tornado risk and large hail is possible over Tunisia and E.

A third level 2 was issued between Naples and Rome, where repeatedly onshore moving thunderstorms pose an heavy to excessive rainfall threat. In case of a stalling MCS event, extreme amounts in excess of 100-250 l/qm are certainly possible.

... N Italy and the Adriatic Sea ...

The cold front crosses N Italy before noon with a band of strong to severe thunderstorms. This activity is progressive so the main hazard will be bursts of heavy rain and an isolated tornado along the NW coast next to a few strong/severe gusts.

This front slows down thereafter and stalls atop the Adriatic Sea with an (orographically) enhanced prefrontal LLJ pushing a moist marine layer N into NE Italy. There, an ongoing upslope flow event with heavy rain until noon turns more into a banded pattern of training convection with localized extreme rainfall amounts. Another level 2 was issued to highlight the main risk for amounts in the 100-200 l/qm range.
A level 1 was expanded far SE along the E coast of the Adriatic Sea into the vicinity of the Lastovski/Mljetski Channel. This area also experiences good background conditions for slow moving and temporarily training convection. Extreme amounts should stay isolated and could occur mainly offshore, so confidence for an upgrade remained too low. Nevertheless, NWP guidances has numerous variable places with rainfall amounts in excess of 100 l/qm on a local scale.

... NE France to Luxembourg into far S Belgium ...

A N-S aligned frontal boundary remains embedded in a moist background with TPWs in the mid thirties and anomalies approaching 200% on a local scale (to the background climatology). In addition, the mentioned plume of tropical mid-level air also spreads N/NW into E France/Germany and adds more mid-level moisture.
A progressive short-wave crosses the highlighted area during the evening and overnight hours atop a meridionally aligned LL pressure channel with embedded weak/diffuse vortices. The IFS-ENS member converged a bit and places a weak LL vortex over NE France and this is also reflected in other model data.

Along the W fringes of that vortex, enhanced convergence assists in a confined area of more stratiform rain with embedded thunderstorm activity. Healthy hourly and 12-h rainfall rates are forecast and amounts in the 50-120 l/qm range are possible (with the higher amounts depending on how well defined the LL vortex becomes). Also the geometry of the low-tropospheric height field determines, how broad this area with heavy rain will be - there are ongoing NWP issues with that topic.
If the more aggressive solutions verifies, we could see some flash flood and localized flood issues in the highlighted area. We used ensemble data and a poor man's ensemble for the placement of this level 1.

... E Black Sea ...

Persistent onshore flow causes ongoing heavy rain with a few embedded thunderstorms. 24 rainfall amounts exceed 100 l/qm in the level 2 area with lower values in the level 1. A few coastal tornadoes are possible.