Storm Forecast
Valid: Sun 25 Feb 2018 06:00 to Mon 26 Feb 2018 06:00 UTC
Issued: Sat 24 Feb 2018 21:04
Forecaster: TUSCHY

A level 2 was issued across the N-Ionian Sea and the far NW-Aegean Sea mainly for excessive rain.

A level 1 surrounds both level 2 areas with a large hail, severe wind gust, an excessive rain and low-end tornado risk.

SYNOPSIS

An unusual pattern, driven by a delayed tropospheric reaction of the stratospheric polar vortex split, results in a broad area with positive geopotential height anomalies over far N-Europe and negative anomalies from the Azores to E-Europe. A surge of very cold mid-/upper-tropospheric air is underway towards CNTRL- and W-Europe. The accompanying air mass is dry and cold, so despite sporadic offshore convection over the S-Baltic Sea, no thunderstorm risk exists.

Ahead of the expanding dome of arctic air, a mobile short-wave over the W-CNTRL Mediterranean rotates east towards the Ionian Sea. This wave interacts with a baroclinic zone, which extends from Tunisia towards Sicily and sparks a broad LL vortex, which rapidly shifts to the NE. No warm core anomalie is forecast, but still a robust LL vortex is expected to affect the Ionian Sea during the night.


DISCUSSION

... SE-Sicily and far S-Italy to Greece ...

The air mass south of Sicily is characterized by enhanced low-tropospheric moisture beneath an eastward fanning EML, which create moderate instability in the 500-1000 J/kg MLCAPE range.

The upper-level wave acquires a negative tilt during the forecast and attendant forcing is on the increase during the forecast. The warm sector, initially capped, can expect an upswing of convection between Malta and Greece with strong forcing favoring a growing and organizing cluster of storms. DLS is adequate for organized convection with severe wind gusts the main hazard. Steep mid-level lapse rates and strong DLS both indicate higher probabilities for a few large hail events with maturing storms. Offshore, LL shear is weak, given unidirectional shear in the warm sector, but SRH increases along the coasts of S-Greece with a low-end tornado risk. This activity affects S-Greece after midnight. Heavy rain is certainly possible although the progressive nature of the cluster should limit the risk for training/extreme amounts.

Further north, e.g. over SE-Sicily, the question will be where the surface front resides as the evolving vortex swings to the E/NE. Enhanced buoyancy and LL shear/DLS favor organized and surface based convection just along and to the south of the front with all kind of severe possible. Right now SE-Sicily looks most promising for surface based activity with an attendant severe risk.

Further east, over the N-Ionian and S-Adriatic Sea, conditions for intense upper divergence improve with a structuring coupled jet configuration. 30-40 kt low-tropospheric inflow of moist and unstable marine air from the south beneath the divergent pattern assists in a rapidly growing cluster of showers/thunderstorms with very heavy rainfall forecast. GFS probably suffers from a convective feedback problem (also ramping up the LL vortex and shear too fast), but a large cluster is likely in the level 2 area, bringing excessive rain to the coasts of Albania and NW-Greece during the night. Strong veering profiles also indicate a tornado risk with any more discrete and tail-end cell.

Another area of concern evolves over NE-Greece in the second level 2 area during the night. As the vortex approaches from the SW, a persistent and moistening LL flow is pointed towards NE Greece, where strong LL convergence and orography support a quasi-stationary cluster with intense rain. Thunderstorm probabilities remain questionable for most of the time due to limited CAPE, but the risk increases during the end of the forecast as MLCAPE increases to 400 J/kg. Excessive rain will be the main hazard although a low-end coastal tornado risk is possible with abundant LL speed/directional shear and low LCLs.

Over the W-CNTRL Mediterranean, a low-end waterspout risk exists with offshore storms with graupel and gusty winds also forecast in strongest storms.

... Far SE-Turkey ...

Strong DLS but meager CAPE allow a few better organized storms to move ashore with graupel and gusty winds. We can't exclude an isolated large hail event, but the risk is too slim for a level area.