Storm Forecast
Valid: Sat 18 May 2019 06:00 to Sun 19 May 2019 06:00 UTC
Issued: Fri 17 May 2019 18:38
Forecaster: TUSCHY

A level 2 was issued across large to very large hail and severe wind gusts over N-CNTRL Turkey.

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

A level 1 was issued from NE Poland to NE Bulgaria mainly for large hail, severe wind gusts and locally excessive rain. A low-end tornado threat exists over far W Ukraine and NE Poland.

SYNOPSIS

An extensive gyre of low geopotential heights (negative 2 sigma compared to climatology) covers most of W-Europe. Embedded upper lows circle that feature with a rotating midpoint somewhere over the English Channel. Peripheral mid-level waves affect the W/CNTRL-Mediterranean and CNTRL/E-Europe.
Another upper vortex over W-CNTRL-Turkey lifts leisurely N/NE during the forecast.
An extensive ridge axis downstream of the W-European vortex extends from the W-Black Sea N all the way to the Baltic Sea.

A cold front over Lithuania, SW-Belarus and the Ukraine drifts gradually S/SW. Scattered to widespread thunderstorm development is forecast along/ahead of that synoptic-scale front.


DISCUSSION

... S-Sweden, NE-Poland, SW-Lithuania, SW-Belarus, parts of the Ukraine, Moldova, parts of Romania and Bulgaria...

Multi-day WAA with low-tropospheric moisture advection results in an extensive plume of warm/moist air. Mid-level lapse rates are modest (partially due to convective overturning) but still adequate for widespread 500-1000 J/kg MLCAPE. Air mass is weakly capped with numerous regions probably losing its cap in the course of the early afternoon hours. Otherwise, passing low/mid-level waves cause early CI, which continues all day long within this CAPE plume.

DLS is rather weak over the Ukraine and south, so expect slow moving thunderstorm clusters with isolated large hail, gusty wind and locally excessive rain. The latter risk was responsbile for an upgrade.
Over extreme W-Ukraine, NE-Poland to far S Sweden DLS starts in the 15-20 m/s range and weakens gradually betimes with a NW-ward departing mid-level speed maximum. Hence there is a risk for organized multicells and isolated supercells until the early afternoon before the storm mode turns more messy.

Large hail and severe wind gusts will be the main hazard. Enhanced 0-3 km shear until the early afternoon hours indicates a risk for upscale growing convection with swaths of strong to isolated severe wind gusts over NE-Poland. Despite weak LL shear, temporarily deviating thunderstorms could pose a temporal tornado threat within a low LCL environment. This risk however will be isolated and short-lived in nature. We did no include S-Sweden into the level 1 because of the more elevated nature of thunderstorms.

... N-Turkey ...

With the upper low sitting atop W-/CNTRL-Turkey, the stage is set for another round with severe/organized thunderstorms in the area of interest. Realtime data shows most models struggling with the magnitude of CI (underestimating the amount of convection), so confidence in another round of scattered to widespread thunderstorms is high.

Strong synoptic-scale forcing ahead of the approaching vortex and a diffluent upper flow regime atop, full-time CI is forecast with peak activity until 18 Z. Afterwards the vortex exits N-Turkey to the north with stronger subsidence along its backside affecting the area of interest.
Mid-level lapse rates in excess of 8K/km advect atop a moist BL, so 1000-2000 J/kg MLCAPE are once again forecast. A northward surging mid-level jet pushes DLS into the 20-30 m/s range, so well organized updrafts are anticipated. Supercells with large to very large hail and severe wind gusts are the main concern before the general risk diminishes after sunset. Mainly the wind-driven large hail threat prompted us to upgrade to a small level 2.

...Lightning areas ...

The rest of the lightning areas feature either weak shear or marginal CAPE for an organized thunderstorm threat. Expect gusty winds and graupel the main hazard next to an isolated flash flood risk due to slow moving thunderstorms. The isolated nature and modest moisture content in the troposphere preclude a rainfall-driven upgrade.

The far SW Mediterranean expects better CAPE/shear but all models remain very reluctant with offshore CI due to modest synptic-scale forcing and some capping. A few overnight thunderstorms are forecast but mostly to the north of the strongest CAPE/shear parameters. Hence no upgrade for now.