Storm Forecast
Valid: Sun 24 May 2020 06:00 to Mon 25 May 2020 06:00 UTC
Issued: Sat 23 May 2020 23:16
Forecaster: PISTOTNIK
A level 1 and level 2 are issued for SE Turkey for large hail, severe convective wind gusts, excessive convective precipitation and to a lesser degree for tornadoes.
SYNOPSIS
A long-wave trough extends from Scandinavia to central Europe and amplifies towards the Balkans. It also loosely connects with two cut-off lows over W Russia and Syria. This configuration leaves much of Europe under rather low 500 hPa geopotential and cool air masses.
A mid-level ridge hesitantly builds from Iberia towards the British Isles and the North Sea. A surface anticyclone forms over the Bay of Biscay and France.
DISCUSSION
... SE Turkey and around ...
Beneath the cut-off low, a surface cyclone travels slowly eastward near the Syrian-Turkish border. At its forward flank, steep lapse rates from the Syrian and Iraqi deserts overspread somewhat cooler but moister air. The resulting area of some hundred, locally perhaps up to 1000 J/kg CAPE overlaps with strong vertical wind shear (0-3 km: 15-20 m/s) and storm-relative helicity (0-3 km: up to 500 m^2/s^2). The warm air advection and a vorticity maximum aloft provide strong synoptic lift and support at least scattered convective initiation especially near the warm front and/or over orographic features, later also along outflow boundaries.
A pronounced lower-tropospheric temperature gradient is in place across the frontal zone (15K from north to south across E Turkey at 850 hPa). Hence instability in the northern parts will likely be elevated, and storms drifting northward will weaken and become embedded in the cloud and rain shield. Towards the south, surface-based convective initiation becomes more likely in the course of the day. The CAPE-and-shear overlap allows well-organized storms with all kinds of severe weather, including supercells with large to very large hail. Later on, upscale growth into MCSs with an increasing wind and flooding risk is possible. 0-1 km shear around 10 m/s and strongly veering profiles might also support one or two superceullular tornadoes, though rather high cloud bases are a limiting factor. A level 2 is issued for the areas at the highest severe weather risk. Convection will gradually weaken in the evening and overnight, but may not fully decay due to still ongoing synoptic and frontal lift.
Even further south in Syria and Iraq, low-level moisture will soon be mixed out by daytime heating. The cold front travelling eastward, or outflow boundaries running into the hot and dry air, may create strong to locally severe wind gusts with dust storms.
... Portugal, Spain, Italy, Montenegro, Albania, North Macedonia, Greece ...
The W and central Mediterranean region lies in moderately warm air under quiescent synoptic conditions. Forecast models indicate some patches of a few hundred J/kg CAPE, mostly where moist thermal upslope flows undercut steep lapse rates formed by diurnal heating over mountains. Weak vertical wind shear across low levels and rising deep-layer shear (15-20 m/s) would create a typical environment for multicells with a local hail risk. However, under lacking synoptic lift support, convective initiation will be isolated at best.
Ahead of a vorticity maximum, weak elevated convection may also form overnight over north-central Spain.
... S Scandinavia, central and E Europe ...
Scattered to numerous daytime-driven showers form across wide areas in the well-mixed, weakly sheared polar air. Especially ahead of travelling vorticity maxima and associated surface convergence zones, they can grow deep enough for limited lightning activity. Severe weather is not expected.
