Storm Forecast
Valid: Sat 09 May 2020 06:00 to Sun 10 May 2020 06:00 UTC
Issued: Fri 08 May 2020 23:58
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for Syria, SE Turkey and extreme N Iraq for large hail, severe convective wind gusts, tornadoes and excessive convective precipitation (the latter mostly towards the north).

A level 1 is issued for north-central Iraq for severe convective wind gusts.

A level 1 is issued for N and E Spain, large parts of France, S Germany, the SW Czech Republic and N Austria for large hail and excessive convective precipitation.

SYNOPSIS

While a late outbreak of arctic air starts gathering over the Norwegian Sea and N Scandinavia, most parts of Europe are still under warm air masses and weak pressure gradients. A cut-off low slowly moves eastward from the Ukraine into Russia. Another one enters the Iberian Peninsula from the west. In-between, a broad ridge covers large parts of the continent, and warm air advection continues over much of the Mediterranean Sea, central and E Europe.
Near the SE corner of our domain, a pronounced mid-level trough crosses the Near East and quickly assembles the ingredients for a major severe weather outbreak.

DISCUSSION

... Syria, SE Turkey, N Iraq ...

A surface low forms ahead of the mentioned short-wave trough and is predicted to deepen below 1000 hPa while it travels along the Syrian/Iraq border. Hot desert air with very steep lapse rates (dry isentropic from the surface to around 3 km) is advected northward at the forward flank of the low and overspreads cooler and somewhat moister air. The warm air advection regime results in excellent wind profiles for organized storms with 0-3 km shear up to 25 m/s and storm-relative helicity up to 400 m^2/s^2, respectively. Moisture will betimes be mixed out, resulting in a dryline moving northward in the warm sector. However, ECMWF and ICON indicate that some hundred J/kg CAPE will probably be available between the warm front and the dryline.
Under strong synoptic lift, at least scattered storms will initiate in the late morning to afternoon and will quickly organize into supercells or bowing lines with all kinds of severe weather, especially large hail and severe wind gusts. Depending on the resilience of low-level moisture against mixing and the final CAPE magnitude, some extreme events (hail >5 cm or wind gusts > 32 m/s) are well possible. Despite cloud bases probably higher than 1500m, 0-1 km shear over 10 m/s could also support one or two tornadoes with strong supercells. Later on, upscale growth of convection into one or two large clusters is a possible scenario, quickly moving northeastward and gliding onto cooler air as soon as it crosses the surface warm front. By then, excessive precipitation becomes the main risk, whereas the other risks diminish.
To the south of the dryline in Iraq, convective initiation is quite unlikely but any outflow boundary or the cold front running into the hot and dry desert air can still create strong to severe wind gusts associated with dust storms.

... N and E Spain, France into central Europe ...

A diffuse warm front continues to slowly lift north ahead of the Iberian cut-off low. Steep lapse rates are advected northeastward in the warm sector, and moisture accumulates beneath the frontal inversion. Resulting CAPE will likely be on the order of 500 J/kg in central Europe, but may rise to 1000-1500 J/kg from the Basque country and the French Pyrenees forelands to west-central France (1300 J/kg were already sampled by the Bordeaux Friday 18 UTC sounding).
The wind field, while not particularly strong, will be superimposed by numerous small disturbances at all levels. In general, vertical wind shear will by mostly weak across lower layers, while 0-6 km shear around 15 m/s becomes more likely closer to the warm front, sufficient for moderate storm organization.
While convective initiation is possible across a wide area, the model pool offers an unusually large scatter about the key question where it will indeed happen. In general, several possible foci emerge:
(1) The warm front from north-central France to south-central Germany and Austria;
(2) Mountain ranges (excluding the higher Alps above ca. 2000m due to a residual snow cover); and
(3) The forward flanks of travelling vorticity maxima.
Secondary initiation at outflow boundaries later on is not ruled out but not particularly likely, as the capping over flat terrain will still be rather strong due to ongoing warm air advection.
Latest model runs converge somewhat to two maxima of storm coverage in S Germany, the SW Czech Republic and parts of Austria (where already a few elevated morning storms are possible) and from N and E Spain into W France, respectively. In-between, relatively less convective activity can be assumned from N and E France into Switzerland, where a vorticity minimum passes during peak heating hours and handicaps convective initiation.
Storm mode will likely be a mixture of single cells, multicells and loosely organized clusters, but closer to the warm front a few right-moving supercells are not ruled out. Main risks, which are covered by two level 1 areas, are large hail especially with better organized storms and excessive convective precipitation otherwise. The severe wind risk is kept low by weak low-level shear and not too high cloud bases.
The eastern storms will fade after sunset, whereas some of the western ones can be kept alive by increasing synoptic lift and may spread over larger parts of France, Switzerland, and possibly Belgium, Luxemburg and W Germany overnight. The severe weather risk weakens when they decouple from the surface.

... Portugal into NW and central Spain ...

The approaching cut-off low sends a cold front along with a pronounced trough axis eastward onto the Iberian Peninsula in the afternoon and evening. With marginal CAPE but strong dynamic lift and 10-15 m/s shear across the lowest 3 km, a low-topped convective line with strong to isolated severe wind gusts is possible. Limiting factors against a level 1 is the late daytime before the front could reach deeper and better (but then elevated) instability further inland.

... Spanish east coast, NE Morocco and N Algeria, W Mediterranean Sea ...

A spatially confined but very good overlap of robust surface-based CAPE and enhanced vertical wind shear in the sea breeze regimes will very likely be too strongly capped to be released into storms.
Apart from that, scattered elevated convection from Altocmulus levels can be weakly electrified but does not pose a severe weather risk. In the course of the night, it could increase into an elevated MCS moving offshore from the central Algerian coastline in response to increasing dynamic lift.