Storm Forecast

Storm Forecast
Valid: Fri 22 May 2020 06:00 to Sat 23 May 2020 06:00 UTC
Issued: Fri 22 May 2020 00:00
Forecaster: PISTOTNIK

A level 1 and level 2 are issued from SW Turkey across N Turkey, Georgia, Azerbaijan into SW Russia for large hail, severe convective wind gusts, excessive convective rainfall and to a lesser degree for tornadoes.

A level 1 is issued from central Turkey across E Turkey into Armenia mainly for severe convective wind gusts and to a lesser degree for large hail.


The steering mid-level features in the European sector are two lows near the Belorussian / Ukrainian / Russian border and NW of Scotland, respectively. Between them, a narrow but amplified ridge stretches from Iberia to Scandinavia. The eastern low is largely filled with cool air that spreads east- and southward over the Black Sea, Turkey and the E Mediterranean region. The western low is connected with an unseasonably strong surface cyclone that affects the British Isles with strong to severe, non-convective wind gusts.
A meandering zonal flow entwines both mid-level lows to the south, stronger on their forward sides.


... Turkey, Armenia, Georgia, Azerbaijan, SW Russia ...

Once more, the ingredients for organized and severe thunderstorms come together in the strong SW-erly flow ahead of the Ukrainian mid-level low. Daytime heating creates very steep lapse rates over the Turkish plateau and the Caucasus mountains. While low-level moisture is quickly mixed out in the mountainous areas (resulting in 2m dewpoints dropping often below 0C on Thursday and, probably, again on Friday), the heated plume overspreads somewhat cooler air with accumulated moisture when it is advected over flatlands and coastal areas. This process can create CAPE on the order of 500 to 1500 J/kg (possibly more locally along the Turkish south coast), which overlaps with moderate to strong vertical wind shear, often approaching or exceeding 15 m/s across the lowest 3 km. Near the Black Sea coastlines, however, cooler sea waters dampen the CAPE magnitude and create a capping inversion that is almost impermeable for surface-based convection (see the Thu 12 UTC Samsun sounding).
On Friday, a pronounced cold front starts pushing the cooler air from the Black Sea more vehemently inland. At the same time, a mid-level trough crosses Turkey from west to east. With plenty of synoptic, frontal and orographic lift mechanisms in place, at least scattered thunderstorms are likely to form and to organize well, including a possibility of supercells and upscale growth into several large clusters.
The highest coverage is expected near the front line, but front-parallel storm motion and destructive interference could result in somewhat messy storm modes. Besides, convection will decouple from the surface as soon as it is undercut by the cold front. In this regime, the main risk will gradually shift to excessive rain.
The opposite regime is the hot and dry prefrontal airmass well inland. Deep vertical mixing limits CAPE to a few hundred J/kg and also reduces vertical wind shear, though it will still be sufficient for storm organization. Convective initiation is expected to be more isolated and mostly tied to mountains, but any storm that forms can produce severe wind gusts (one or two extreme events are not ruled out). In addition, more persistent updrafts can support large hail.
The key question is whether a belt emerges in-between whose environment can combine the advantages of both regimes, i.e. the moisture is thick enough to resist vertical mixing and result in robust CAPE, while at the same time the low-level air is heated enough to break the capping inversion. In that case, discrete supercells could easily produce all kinds of severe weather, most notably large to very large hail. The areas with the highest risk of such a scenario are upgraded to a level 2: The Caucasus forelands, The plains of E Georgia and Azerbaijan, and coastal areas of SW Turkey. Large parts of Turkey are, however, left below a level 1, since the advancing moist air from the Black Sea is so much cooler that daytime heating may not suffice for surface-based convection anymore, leaving only elevated CAPE with much less efficient vertical wind shear.
While storms in Turkey will gradually weaken and decay in the evening and overnight, the later arrival of the mid-level trough could shift the maximum activity towards evening in Georgia, Azerbaijan and SW Russia. In particular ECMWF and ICON uniformly show signs of an MCS that shall work its way eastward over the Russian Caucasus forelands all night long. If this happens, the severe weather risk would still be enhanced.

... Germany and surroundings ...

An overlap of some CAPE with moderate vertical shear and veering profiles is also present in the warm sector of the British cyclone. The model pool indicates that CAPE will be lower on Friday than it was on Thursday across France (e.g., 1600 J/kg according to the 12 UTC Bordeaux sounding), mainly due to a combination of beginning cold air advection at low-levels and still synoptic subsidence at mid-levels ahead of the ridge axis. However, model consistency is remarkably low. The GFS and ICON model families provide a more aggressive scenario with CAPE between 500 and 1000 J/kg and to rounds of scattered to widespread storms near the warm front and along the cold front, which would affect large parts of Germany. In contrast, the ECMWF and UKMO model families show CAPE not higher than 100 or 200 J/kg and consequently a much lower storm coverage.
After an inspection of latest observational data, this forecast leans to the more conservative scenario. With an expected capping inversion and limited (if any) synoptic lift, it is questionable whether daytime heating will be strong enough to achieve surface-based initiation, especially near the warm front. Therefore, only limited and weaky electrified, often embedded and/or elevated convective activity is foreseen for a prolonged time until the late afternoon.
Rather late, in the 16 to 22 UTC time frame, the cold front and a small-scale vorticity maximum temporarily enhance the storm risk across central Germany. If this convection can root down to the surface, an organization into multicells or perhaps a few transient supercells is possible. While all kinds of severe weather might occur in that case (most notably severe wind gusts and one or two tornadoes, given rather cloud bases and 0-1 km shear of 10-15 m/s), the scenario is just a bit too conditional for a level 1.
A second maximum of elevated and weakly electrified convective activity is possible towards Saturday morning from France into SW Germany. Severe weather is not expected.

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