Storm Forecast
Valid: Fri 12 Oct 2018 08:00 to Sat 13 Oct 2018 06:00 UTC
Issued: Fri 12 Oct 2018 08:16
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for Syria mainly for large hail, severe convective wind gusts and to a lesser degree for flash floods.

A level 1 is issued for S Turkey and Cyprus mainly for flash floods and large hail, and for surrounding sea waters for (non-supercellular) tornadoes.

A level 1 and level 2 are issued for S Italy, Malta and Tunisia mainly for flash floods, and for sourrounding sea waters for (non-supercellular) tornadoes.

SYNOPSIS

A pronounced mid-level trough over the N Atlantic and a "high over low" blocking pattern over E Europe govern the large-scale weather situation in Europe.
Ahead of the Atlantic mid-level trough and an associated storm cyclone between Iceland and Ireland, a strong SW-erly flow stretches from the Bay of Biscay to the British Isles, the North Sea and Scandinavia. Unseasonably warm air spreads far north and prolongs almost summer-ilke temperatures in central Europe.
The blocking high is centered over Belarus. It is opposed by a large, diffuse cut-off low over SE Europe. While cooler and drier air spreads southward over the Ionian and Aegean Sea, thunderstorm activity remains vivid over other parts of the central and E Mediterranean region, where plentiful low-level moisture remains in place.

DISCUSSION

... Turkey, Syria, Cyprus and E-most Mediterranean Sea ...

Steepened lapse rates beneath the cut-off low create some hundred J/kg of CAPE over land and possibly more than 1000 J/kg over sea. While the synoptic conditions are quescent in terms of vertical wind shear and lift near the low's center over Turkey, Lebanon and Syria are overspread by synoptic lift (both from warm air advection and a passing short-wave trough) and 0-3 km shear up to 15 m/s at its forward flank.
Global forecast models strongly disagree how far substantial CAPE will spread inland from the Mediterranean Sea into Syria. GFS and ICON predict substantial CAPE on the order of 1000 J/kg, though for different reasons (>8 K/km mid-level lapse rates per GFS versus <7 K/km per ICON, 6-8C 2m dewpoints per GFS versus 12-16C per ICON). ECMWF, in contrast, offers the most conservative solution with neither particularly steep lapse rates nor particularly much low-level moisture, leading to very limited CAPE and hardly any precipitation signals. As a compromise, a level 2 is issued where model agreement in convective initiation is best and a level 1 further inland. Well-organized storms pose a risk of large hail and severe wind gusts. In addition, a few flash floods are possible in case of high storm coverage or upscale growth into an MCS, and in general closer to the coast where low-level moisture is more plentiful.
Scattered afternoon storms will also form in Turkey, especially its S parts. They will mostly stay disorganized, though some instances of marginally large hail or localized flooding are possible. A few storms may also form or propagate offshore in the evening and overnight. Waterspouts may form then, especially along land breeze fronts.

... Tyrrhenian Sea, S Italy (including Sardegna), Tunisia, Malta and surrounding sea waters ...

Low-level moisture is maximized in this region and allows a few hundred (towards the NE) to more than 1000 J/kg of CAPE (towards the SW). Vertical wind shear is weak. A subtle short-wave trough crosses the area from N to S and creates some lift. At the time of writing (Fri 07 UTC), two intense MCSs are active just W of Sicily and halfway between Malta and the Tunisian coast. They will likely persist for some more hours, following a track roughly towards the SW, though it may be somewhat erratic due to propagation along outflow boundaries. Additional storms are expected mostly in the afternoon over Sicily and Calabria, and until beyond the end of this forecast period over sea waters.
Flash floods are once more the main risk, especially in Sicily. In addition, waterspouts are possible at any convergence zone or outflow boundary over the warm sea waters. The hail and wind risk is limited by weak vertical wind shear and poor lapse rates.
Towards the evening and into the night, the lift of the mentioned short-wave trough may also carry some storms into Tunisia. The precipitation signals in forecast models diverge, but flash floods remain a possibility.

... NW of Ireland and Scotland ...

While much of the British Isles will be battered by heavy rain and severe wind gusts of con-convective origin in the deep SW flow, a confined window for low-end CAPE has opened up near the NW coasts, where very strong lift overspreads the cold front in the left exit region of a mid-level jet streak. A few lightning strikes were detected in the past few hours, and the wind field is strong enough to support severe to extreme gusts with any storm that forms. However, since the forecast model pool agrees on a quick dissipation of this volatile setup, the confidence in further storms is too low for a lightning and threat level area.