Valid: Fri 26 Oct 2018 06:00 to Sat 27 Oct 2018 06:00 UTC
Issued: Thu 25 Oct 2018 16:12
A level 2 is issued for Iraq and adjacent Iran mainly for severe convective wind gusts, large hail, flash floods and to a lesser degree tornadoes.
A level 1 is issued for parts of Syria and SE Turkey for severe convective wind gusts, large hail and flash floods.
A mid-level trough that strongly amplified to Turkey on Thursday is leaving our forecast domain to the east. A new mid-level trough amplifies from the British Isles towards Spain on Friday. These shifts cause the mid-level flow to back from NW-erly to SW-erly directions over large parts of the continent.
A strong autumnal jet stream curves around these two mid-level troughs and a shallow mid-level ridge in-between. A distinct vorticity maximum in the left exit region of a wind maximum fosters the formation of a strong surface cyclone over Denmark that will likely deepen to 990 hPa while it travels to Latvia. It ingests unseasonably warm air into its warm sector that covers the central Mediterranean, the Balkans, the Alpine region and spreads into E Europe. In contrast, polar air is in place ahead of its warm front and invades W Europe again behind the long and wavy cold that stretches from Poland to Spain.
Despite the strong dynamics, the day will be mostly quiet convection-wise, apart from the far SE of our forecast domain.
... Syria, SE Turkey, Iraq and adjacent Iran ...
The tip of the eastern mid-level trough and a diffuse surface cold front continue to travel eastward on Friday, exposing a partly unstable and strongly sheared warm air mass to lift.
On Thursday afternoon, surface observations showed highly variable low-level moisture with 2m dewpoints between 6 and 17C over Syria and Iraq, hence the amount of CAPE was questionable. Despite the low station density, there are hints that Mediterranean moisture is advected far inland as soon as the low-level wind veers from S-erly to W-erly directions, allowing some hundred J/kg of CAPE in a transitional belt near the cold front. This assumption is also backed up by scattered to widespread and apparently well-organized storms that formed over Syria.
The situation will likely be similar on Friday, when the most highest threat of organized and severe storms shifts to Iraq. A level 2 is issued where ECMWF, GFS and ICON show robust precipitation signals in an environment of 300-1000 J/kg CAPE under 20 m/s 0-3 km shear. Due to the pronounced wind shift, it is not ruled out that a convective line forms and brings scattered to widespread severe wind gusts, possibly accompanied by violent duststorms in case the outflows outrun their parent clouds. Discrete storms may easily turn supercellular, large hail is likely and one or two tornadoes are not ruled out (0-1 km shear around 10 m/s) in that case. The flash flood risk is especially enhanced in the upslope flow regime in the mountains towards the Iranian border, but in general if a place is exposed to larger storm system or repeated rounds of storms.
SE Turkey and Syria are covered with a level 1, as the forecast models show remaining patches of CAPE after some hours of daytime heating. Both synoptic and orographic lift remain in place and vertical wind shear is still moderately enhanced. Scattered storms with a few severe weather events are still possible.
... W Mediterranean Sea to Morocco ...
Beginning synoptic lift ahead of the western trough starts to contract the moist boundary layer and to cool the mid-levels, resulting in the gradual build-up of some hundred J/kg of CAPE. However, it appears likely that a capping inversion, placed by warm air advection, will still mostly hold on Friday. First embedded thunderstorms are possible in the strong upslope flow in NW Italy overnight, and storm coverage may increase around N Morocco and S Spain towards Saturday morning. Vertical wind shear gradually increases ahead of the W European trough, but limited CAPE and the embedded - and possibly elevated - nature of these storms will keep the severe weather risk low.
... N and NW Europe as well as E Black Sea ...
Scattered, low-topped thundery showers form in the stream of polar air as it moves over comparably warm waters. Strong vertical mixing will keep both CAPE and vertical wind shear on the low side. Severe weather is unlikely.
Marginally severe wind gusts are possible at the S flank of the cyclone in coastal areas of Poland, the Russian exclave of Kaliningrad and Lithuania overnight, but an involvement of deep convection is too unclear to issue a level 1.