Storm Forecast
Valid: Fri 19 Jul 2019 06:00 to Sat 20 Jul 2019 06:00 UTC
Issued: Thu 18 Jul 2019 20:59
Forecaster: PISTOTNIK

A level 1 is issued for Ireland and England mainly for excessive convective precipitation and tornadoes.

Level 1 areas are issued for various parts of central Europe, SE Europe, the Georgian and Russian Caucasus region and NW Russia mainly for excessive convective precipitation and to a lesser degree for large hail.

A level 1 is issued for the E Black Sea mainly for non-supercellular tornadoes (waterspouts).

SYNOPSIS

A broad, weak to moderate westerly flow stretches across Europe. Two mature cyclones are located between Iceland and Scotland and over NW Russia, respectively. The tails of their largely occluded frontal systems fan out into a main frontal zone that runs between 50 and 55N and sharpens somewhat to the west, where a frontal wave crosses Ireland and Scotland.
A subtropic high pressure systeme with very warm air covers the S half of Europe.

DISCUSSION

... Ireland and England ...

Warm and very moist air is advected into Ireland and England in the warm sector of the frontal wave. Despite poor lapse rates, daytime heating will allow around 500 J/kg of CAPE over Ireland, while this buildup will be slower and weaker over England. Moderate to strong vertical wind shear (0-3 km: ~15 m/s, 0-1 km: ~10 m/s) overspreads the unstable air. Vertical wind profiles are strongly veering near the warm front with 0-3 km storm-relative helicity on the order of 200 m^2/s^2, before they straighten to some degree in the warm sector.
The rain shield of the warm front, which might already include a few embedded thunderstorms, will largely clear Ireland by 12 UTC and England by 15 UTC. Scattered thunderstorms will then initiate in the warm sector in the late morning to afternoon, aided by daytime heating, sea breeze fronts and strong synoptic lift ahead of a short-wave trough, and will organize into multicells and perhaps a few supercells. Possibly training storms and high precipitation efficiency in the very moist air could support some heavy rain events with a possibility of flooding. In addition, one or two tornadoes could spin up, especially if clearing skies and insolation follow soon after the warm front passage, while wind profiles are still most supportive. The hail and wind risk is limited by poor lapse rates and very moist air, though marginally large hail or a hail cover are not ruled out in case of a supercell.
Storms will gradually decay when they move offshore onto the North Sea. The cold front may push new rounds of (partly embedded and weakly electrified) convection SE-ward into S England and the Channel region overnight, but they will struggle to stay surface-based. The severe weather risk decreases overnight, though plentiful rain could still accumulate.

... Spain ...

Forecast models agree on patchy CAPE on the order of some hundred J/kg in the sea breeze and upvalley flow regimes that push deeply inland from the N and E coast and up the Ebro valley. These thermal circulations are opposed to the mid-level flow and therefore also feature enhanced 0-3 km shear around 15 m/s.
In the absence of synoptic lift, convective initiation is the critical issue. Any updraft will encounter strong entrainment of very dry air. The most likely scenario is that only a few struggling and short-lived evening storms will form. However, in case the capping inversion on top of the sea breezes and the entrainment can be overcome for a longer time, large hail and (dry) downbursts are possible.

... other continental thunderstorm areas ...

Quiescent synoptic conditions will allow some hundred J/kg of CAPE over wide sreas. Isolated to scattered, mostly daytime-driven storms are expected, especially over orographic features and along the occluding frontal system of the Russian cyclone.
Vertical wind shear is weak across low levels before it regionally rises to moderate values across the 0-6 km layer (e.g. around 10 m/s over Germany, the Czech Republic and Austria, parts of Romania, Turkey and Georgia and surroundings, and up to 15 m/s in the Russian warm sector). Most storms will therefore be single- or multicellular and pose a main risk of heavy rain. A few moderately large hail events are possible with the strongest pulse storms, especially in the regions with slightly enhanced shear mentioned above and over the Balkans, where the CAPE magnitude is highest. These regions wioth a combined heavy rain and hail risk are upgraded to a level 1.
The severe weather risk appears relatively highest in the warm sector of the Russian cyclone, but some doubts exist whether the 800-1500 J/kg CAPE suggested by ECMWF, ICON and GFS can indeed materialize. A possibility of too early, widespread initiation and a lack of foci for a higher concentration of severe weather events therefore preclude a level 2.

... E Black Sea ...

A shallow warm-core depression will likely allow some offshore convection, especially early in the forecast period. Low cloud bases and converging winds could allow a few non-supercellular tornadoes (waterspouts).