Storm Forecast
Valid: Sun 05 Nov 2023 06:00 to Mon 06 Nov 2023 06:00 UTC
Issued: Sat 04 Nov 2023 22:50
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for parts of central Italy, Slovenia, Croatia, Bosnia-Herzegovina, Montenegro and Albania mainly for severe convective wind gusts and excessive convective precipitation, and to a lesser degree for tornadoes and large hail.

A level 1 is issued for E Austria, SW Hungary, Serbia and W Romania mainly for severe convective wind gusts and large hail.

A level is issued for the central Ukraine mainly for large hail, severe convective wind gusts and excessive convective precipitation.

A level 1 is issued for SW France mainly for severe convective wind gusts and to a lesser degree for tornadoes.

SYNOPSIS

A large and extremely intense low-pressure complex continues to cover N, central and W Europe as well as the NE Atlantic Ocean. A mid-level jet runs along its southern flank with an axis near 40N and wind speeds up to 50 m/s across Iberia, Italy and the Balkans, before it turns sharply northward from the Black Sea towards NW Russia.
Near the surface, the dominant cyclone center is located over the North Sea on Sunday morning with an expected minimum pressure near 967 hPa before it gradually fills up. A leading, almost stationary cold front runs from NW Russia to the Black Sea. A second frontal system crosses east-central Europe (already in occluded form), Italy and the Balkans during this forecast period.
Warming and moistening of the maritime polar air over both the Atlantic Ocean and the Mediterranean Sea beneath cold mid levels create marginal CAPE across wide areas, which partly overlaps with the strong to extreme wind field.

DISCUSSION

... central Ukraine into W Russia ...

A frontal wave quickly travels northward along the leading cold front. A plume of very warm air with steep lapse rates of Saharan origin, that allowed unusual CAPE values and a remarkable late-season severe outbreak in W Turkey, E Bulgaria and E Romania on Saturday, is advected far northward towards W Russia in the warm sector. However, as it overspreads an increasingly cool boundary layer that deeply on the continent, it turns more difficult to again create surface-based CAPE on Sunday. The low solar angle and a possible lack of insolation in the warm sector further add to this concern.
Current thinking is that at least a few hundred J/kg of surface-based CAPE can develop in the central Ukraine, aided be sufficient daytime heating and strong lift from a short-wave trough. Under pretty strong 0-6 km shear around 20 m/s, multi- to supercellular storm organization is likely, and a few large hail and severe wind events are possible, next to excessive rain in case of training convection. Storms will likely decouple from the surface and become elevated when they travel N- to NE-ward into Russia, whereby they are expected to lose their severe weather potential despite an even stronger kinematic background (including 0-1 km shear up to 20 m/s). Nonetheless, a careful monitoring of observational data is recommended to better assess the northward extension of surface-based convection in the nowcasting mode.

... central Italy into Balkans, plus E Austria and Hungary ...

Another round of organized storms is expected along the cold front that crosses this region from NW to SE. It will be located along a line from north-central Italy along the Slovenian-Croatian border into Hungary at the beginning of the forecast period and reach Albania, S Serbia and central Romania by evening before it outruns the favorable lift support from a short-wave trough and weakens. Despite strong linear lift from this pronounced trough, the formation of a long and persistent squall line is somewhat doubtful. An angle of more than 45° between the front line (NE-SW) and the background westerly flow rather suggests a break-up into some shorter bowing lines and possibly even discrete storms, which can turn multi- to supercellular. While the kinematics are more than impressive with 0-6 km shear between 25 and 40 m/s and 0-1 km shear between 10 and 20 m/s, the low amount of CAPE (a few hundred J/kg at best) is a limiting factor. The highest severe weather threat is expected along the west-facing coasts of central Italy, Croatia, Bosnia-Herzegovina and Montenegro, where strong orographic lift works on top of the already plentiful synoptic lift. Severe downbursts and excessive rain are the main hazards, while tail-end storms in the coastal belt can also produce large hail and a few tornadoes, especially in case of supercells.
The model pool largely agrees on a strong belt of subsidence immediately behind the cold front. Under quickly clearing skies, daytime heating and enough advection of Adriatic moisture allow the re-buildup of CAPE up to 500 J/kg over the Pannonian plains. Conditional upon the timely arrival of the next short-wave trough(s), a seond, postfrontal round of isolated to scattered, discrete storms is possible in the noon to afternoon hours. Some convection-resolving forecast models simulate this in Serbia, S Hungary or even as far west as E Austria (ICON-D2!). Deep-layer shear around 20 m/s could indeed support a few low-topped multi- or supercells with a still elevated hazard of severe downbursts, marginally large hail or large amounts of small hail in this scenario, which is reflected by a northward extension of the level 1 area.

... France, Belgium, Switzerland, Germany ...

Marginal CAPE and lift from further short-wave troughs allow scattered to widespread shower activity in the maritime polar air. The peak of the activity is expected between late morning and sunset. The majority of the showers will be too shallow to produce lightning. Deep-layer shear relaxes on the cool side of the main frontal zone, but the strong low-level wind field with 850 hPa winds up to 25 m/s and 0-1 km shear well in excess of 10 m/s can allow multicellular organization. A few severe wind gusts and perhaps the brief spin-up of one or two tornadoes are possible, though they will be widely scattered and no obvious hot spots can be identified. A level 1 is added for SW France, where confidence in a few hundred J/Kg CAPE, and therefore deeper convection, is somewhat higher.