Storm Forecast
Valid: Fri 27 Jul 2018 06:00 to Sat 28 Jul 2018 06:00 UTC
Issued: Thu 26 Jul 2018 23:32
Forecaster: PISTOTNIK

A level 1 and level 2 are issued from central France into E England mainly for large hail, severe convective wind gusts and to a lesser extent tornadoes.

A level 1 and level 2 are issued for Belgium and parts of the Netherlands mainly for severe convective wind gusts.

A level 1 is issued for N parts of the Black Sea for waterspouts.

All other level 1 areas, and a level 2 for parts of Moldova and the Ukraine, are issued mainly for flash floods and to a lesser extent for large hail and severe convective wind gusts.

SYNOPSIS

Another upper-level trough amplifies west of the British Isles towards N Spain. Otherwise, pressure gradients are weak and very warm air covers most of the continent. A large but diffuse upper-level low is placed between Austria, the Ukraine and Greece. A blocking anticyclone
regenerates over N Scandinavia and deflects the polar jet to the far north. The subtropical jet stretches from Morocco to the Near East.

DISCUSSION

... from France into England ...

As the Atlantic upper-level trough slowly moves eastward, the S-erly flow increases over W Europe. A "Spanish plume" with steep lapse rates is advected across France into E England and overspreads a moist boundary layer. CAPE between 500 and 1000 J/kg will likely build over E England, overlapping with strong vertical wind shear (~15 m/s across the lowest 3 km). Even more CAPE is expected in N and central France, though under somewhat weaker shear (~10 m/s across the lowest 3 km).
A few non-severe and partly embedded thunderstorms may already be active in the morning along the approaching cold front over central England. Supported by synoptic lift from warm air advection and various travelling vorticity maxima, additional storm become likely in the afternoon at the cold front, at prefrontal convergence lines or sea breeze fronts in E England and N France. Convective initiation will be less frequent and will often await the evening or even night further south over central France towards the Massif Central.
Storms will organize into multi- and supercells before a growth into several clusters is expected in the evening. The severe weather risks shift from large hail in initiating stages to severe wind gusts and flash floods later on. Finally, one or two tornadoes are not ruled out in the sea breeze regimes where low-level shear is maximized, if these sea breezes penetrate far enough inland to become exposed to sufficient daytime heating.
Convection will gradually become elevated and weaken after sunset, and in general when it moves onto the North Sea.

... Belgium, Netherlands ...

The environment further ahead of the cold front will be hot and dry, yielding very large spreads between expected maximum temperatures up to 35C and dewpoints in the 10 to 15C range. Despite limited CAPE and weak vertical wind shear, strong evaporative cooling in downdrafts may create severe wind gusts under any storm that forms.
A similar environment was already in place on Thursday (cf the 12z soundings from Essen and Idar-Oberstein), when multiple severe to extreme downbursts occurred in Belgium and the Netherlands. Latest model runs emphasized the possibility of a similar scenario on Friday, hence the level 1 and level 2 are extended eastward.

... other thunderstorm areas ...

The warm air mass in a synoptically quiescent setup provides an environment of low to moderate CAPE and weak vertical wind shear over large parts of the continent. Scattered storms will initiate mainly over orographic features in the afternoon, but may propagate rather erratically along outflow boundaries later on.
The majority of these storms will stay disorganized. Vertical wind shear may be just enough for multicell formation with a risk of marginally large hail especially where thermal upslope flows are opposed to the mid-level winds, most notably in the Russian Caucasus forelands, in NW Turkey and along the S Alpine rim in N Italy. Otherwise, a few flash floods and isolated severe downbursts are the main risks.

Of special concern are large parts of the Ukraine and Moldova, where steep lapse rates from the SE (originally created over the Caucasus and the Persian highlands) are advected on top of a very moist boundary layer. Forecast models show CAPE maxima on the order of 1000 to 2500 J/kg, though there is neither much agreement on their placement nor on the location, timing and frequency of convective initiation. Further east where the moist boundary layer is mixed out, hot and dry conditions with deep subcloud layers favor the hail and wind risk, while the risk of excessive rain decreases. The entirety of these hazards seems to be maximized over parts of Moldova and the Ukraine, and these areas are upgraded to a level 2.
Scattered convection (not necessarily electrified) is also possible along nocturnal land breeze fronts over parts of the Black Sea on Friday morning and then again late at night. The cyclonic background wind field may aid in the spin-up of a number of waterspouts.