Storm Forecast
Valid: Sun 14 Apr 2019 06:00 to Mon 15 Apr 2019 06:00 UTC
Issued: Sat 13 Apr 2019 18:29
Forecaster: TUSCHY

A level 1 was issued for parts of Lebanon, Syria and far S Turkey mainly for large hail and severe wind gusts. Isolated excessive rain is also possible.

A level 1 was issued for N-CNTRL Turkey mainly for large hail, severe wind gusts and excessive rain.

SYNOPSIS

An amplified streamline pattern continues over Europe with an extensive positive 2-3 sigma height anomaly over the S Greenland/N Norwegian Sea. A retrograde moving cyclonic vortex exits far W Russia and crosses Belarus during the night. Otherwise an extensive upper trough resides over E-CNTRL and SE Europe with cold mid-levels atop a dry continental BL air mass (yielding near zero onshore CAPE). Numerous embedded cyclonic circulations won't play a role in today's outlook. Also numerous subtle waves circle the main trough. Those wave certainly play a role and dictate today's thunderstorm coverage.
Stout subtropical ridging with deep WAA/high thickness suppresses CI due to capping/meager mid-level lapse rates atop a NE-ward surging subtropical air mass.

Synoptic-scale fronts won't play a role in today's outlook.


DISCUSSION

...Lebanon, Syria and S-Turkey ...

One of the aforementioned subtle waves passes by the area of interest to the north. Upper part of that wave features a diffluent trough configuration while loosing contours at lower levels (curvature nearly levels off at 700 hPa). Steep mid-level lapse rates (sampled nicely at the 12Z Bet Dagan sounding) spread north/northeast ahead of the wave and merge with steeper lapse rates atop the Taurus Mountains/Armenian Highlands.
Passing wave is accompanied by a diffuse/broad LL depression, which crosses Turkey during the day from SW to NE. Attendant onshore flow - orographically channeled - advects a plume of marine Mediterranean air onshore over N Lebanon/NW Syria beneath the steeper lapse rates. Overlap yields 500-1000 J/kg MLCAPE with locally higher values.

The passing wave keeps low/mid-level flow enhanced with 15-30 m/s 0-6 km shear and weaker shear magnitudes below. In an ongoing weak WAA regime, forecast soundings reveal enlarged and curved hodographs, supportive for organized thunderstorms (multicells and isolated supercells). This would cause a large hail threat especially with discrete storms and severe wind gusts due to a well mixed LL air mass. Main activity seems to be orographically driven as ongoing capping may limit coverage to the south. Nevertheless we expanded the level area a bit south away from the mountains to account for potential deviant storm motions. We opted to not upgrade to a confined level 2 along the coastal areas, where best but mostly capped thermodynamic profiles exist. In case of isolated CI a significant hail event is possible.
Clustering storms (increasing heavy rain risk) spread NE with a gradually diminishing severe risk due to the complex orography and lowering CAPE.

...Italy to Greece ...

Active diurnal driven thunderstorms occur beneath the upper trough due to an enhanced vertical temperature gradient, temporal diabatic heating, weak capping and seasonable moist BL air. Peak severity occurs during late noon/afternoon with peak coverage during late afternoon/evening. Forecast soundings show ELs around 350 hPa with elongated thin CAPE profiles, a moist troposphere, lowered WBZ heights and very weak shear. Hence slow moving thunderstorms pose a local heavy rain risk. Main hazard however will be graupel/hail - either lots of small hail or isolated large hail in case of more discrete storms, which take profit of pockets with better SBCAPE (e.g. CNTRL Italy). Thunderstorms may feel enhanced upper ventilation due to a the proximity of 50 m/s jet streaks which could enhance updraft strength on a temporal scale (increasing hail risk). Foreacast soundings also show good LL CAPE support (roughly 150 J/kg) and hence waterspouts and onshore funnels/short lived tornadoes are also possible.
Despite that multifacet risk and localized events equaling level 1 conditions, missing mesoscale or large-scale foci preclude the issuance of broad level 1 areas for now.
During the night the thundestorm activity shifts offshore with similar hazards while onshore activity diminishes rapidly until 21Z.

...W Turkey ...

A similar setup to Italy/Greece affects W Turkey with better DLS (roughly 15 m/s). Strong forcing ahead of the mid-level wave and weak capping cause early and widespread CI. Up to 1000 J/kg SBCAPE combined with the mentioned shear magnitude cause upsscale growing clusters with large hail, a few severe wind gusts and excessive rain reports. The best thermodynamic/kinematic overlap was upgraded along/north of the Pontic Mountains and would not be surprised about swaths of severe wind gusts with inverted-V profiles ahead of the approaching convection. Furhter west, limited diabatic heating and early CI may keep the severe risk too isolated for a broad level 1. During the evening, convection (probably in form of an organized MCS) moves offshore (Black Sea) and weakens.
Lingering cold-core convection beneath the wave's base continues during the night with nothing severe forecast.

... N Portugal and far NW Spain ...

A few WAA driven elevated nocturnal thunderstorms with no severe risk are forecast.