Storm Forecast
Valid: Mon 04 Sep 2023 06:00 to Tue 05 Sep 2023 06:00 UTC
Issued: Sun 03 Sep 2023 22:25
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for parts of Spain for large hail, severe convective wind gusts and excessive convective precipitation.

A level 1 and level 2 are issued for Greece, parts of Bulgaria, NW Turkey and the Aegean Sea mainly for excessive convective precipitation and for non-mesocyclonic tornadoes (waterspouts).

A level 1 and level 2 are issued for SW Turkey mainly for large hail, excessive convective precipitation and to a lesser degree for severe convective wind gusts.

A level 1 is issued for the NE coast of the Black Sea mainly for excessive convective precipitation and for non-mesocyclonic tornadoes (waterspouts).

SYNOPSIS

A blocking high has established over central Europe. It is flanked by a cut-off low W of Portugal, a strong zonal flow over Scandinavia and another cut-off low slipping southward over Greece, creating a classic and persistent "omega" pattern. Rather cool continental air flows southward at the eastern flank of the blocking anticyclone over E Europe, the Balkans and the central Mediterranean region, whereas very hot air is advected from Algeria towards France, BeNeLux and England at its western flank.
Thunderstorm activity is mostly tied to the two cut-off lows.

DISCUSSION

... Spain ...

The strong meridional flow ahead of the western cut-off low advects an elevated mixed layer from the Sahara across the Western Mediterranean region and the E half of Spain, creating CAPE around or above 2000 J/kg over the abundant maritime moisture reservoir and still 500-1500 J/kg over inland areas. Convective initiation will be less widespread than in the past few days and likely confined to a band of synoptic and mesoscale lift, which erodes the capping inversion at the western rim of the CAPE area. It is associated with a mid-level vorticity maximum and an accompanying surface cold front, which slowly shifts NE-ward across Spain. The good model agreement allows drawing a rather compact high-probability lightning area and level 2 area.
Elevated and probably non-severe storms are already possible in the morning near the W rim of the level 1 zone. Surface-based convective initiation will be delayed and struggle for some hours against the strong vertical wind shear, but by afternoon scattered storms are likely to form and should quickly organize into multi- and (right-moving) supercells under strong vertical wind shear, which often exceeds 20 m/s across the 0-6 km layer and 15 m/s across the 0-3 km layer. The primary hazards are large to isolated very large hail and severe downbursts. However, veering wind profiles along with the strong moisture influx from the southeast to east also suggest a possibility of side- or backbuilding. Locally high rainfall accumulations are therefore possible as well, especially in case one or two larger storm clusters should emerge towards evening.
After sunset, convection will gradually decouple from the surface, weaken and move offshore onto the Bay of Biscay.

... Greece, Bulgaria, NW Turkey, Aegean Sea ...

Around the base of the eastern cut-off low, rather cool mid-level air on top of the very warm and moist maritime boundary layer creates CAPE on the order of 500-1500 J/kg, south of Greece locally more than 2000 J/kg. Converging winds allow widespread thunderstorm formation, starting mostly over the Greek mainland during daytime but increasingly moving southward to the islands and over the open sea in the evening and overnight. The main hazards are excessive rain and several non-mesocyclonic tornadoes (waterspouts) at outflow boundaries, nocturnal land breeze fronts and other convergence zones near the center of the surface cyclone which will likely round the Peloponnes peninsula overnight. At its rear flank, increasing NE-erly surface winds advect drier and cooler air and stabilize the airmass over land, while heavy rain with embedded thunderstorms may become an even bigger issue at the windward coasts of the Greek mainland and, to a lesser degree, Bulgaria and NW Turkey.

... SW Turkey ...

At the forward flank of the cut-off low, a similar CAPE reservoir is overspread by SW-erly mid-level flow, creating deep-layer shear around 15 m/s. Scattered, daytime-driven storms can organize into multicells and perhaps some supercells, and grow upscale into large clusters later on. The main hazards are large hail and excessive rain, while the wind hazard is dampened to some degree by weak shear across the lower troposphere. Isolated very large hail is not ruled out with supercells, especially towards the east, where deep-layer shear is strongest (but convective initiation expected to be more isolated), and near the south coast, where CAPE is highest (possibly exceeding 2000 J/kg).

... NE Black Sea coast ...

Scattered to widespread thundery showers will be active throughout the forecast period near another small surface cyclone. Heavy rain and a handful of waterspouts are the main hazards.