Storm Forecast
Valid: Fri 21 Aug 2020 06:00 to Sat 22 Aug 2020 06:00 UTC
Issued: Thu 20 Aug 2020 23:32
Forecaster: PISTOTNIK

A level 1 is issued for S Norway mainly for excessive convective precipitation.

A level 1 is issued for a belt form S Sweden to N Spain mainly for excessive convective precipitation and to a lesser degree for large hail, severe convective wind gusts and tornadoes.

A level 1 is issued for the central Ukraine for excessive convective precipitation (mainly to the NW), large hail and severe convective wind gusts (mainly to the SE).

A level 1 is issued from the E Black Sea to the Russian Caucasus region, Georgia and Azerbaijan for excessive convective precipitation (mainly to the NW), large hail and severe convective wind gusts (mainly to the SE).

SYNOPSIS

An unseasonably strong cyclone (which on Thursday even set new August records for the lowest sea-level pressure and the highest sustained winds in Ireland according to Met Eireann) is placed just north of the British Isles. Warm air advection with a strong SW-erly flow at its forward side supports a mid-level ridge and a weak surface anticyclone from Algeria all the way to Finland and NW Russia.
Over E Europe, temperate and unsettled conditions prevail under slightly lower 500 hPa geopotential and surface pressure.

DISCUSSION

... S Scandinavia to Germany, France and N Spain ...

The warm front of the British storm cyclone lifts northward into Scandinavia, whereas its cold front is almost flow-parallel and makes only very slow progress across NW Germany, central France and Spain. The deep SW-erly flow creates strong vertical wind shear across all levels (0-1 km: >10 m/s, 0-3 km: >15 m/s, 0-6 km: >20 m/s). The presence of CAPE is more difficult to assess. Forecast models indicate a few hundred J/kg CAPE only in a narrow belt just ahead of the cold front, associated with a surge of forerunning cooler but moist NW-erly winds at low levels. Near the warm front, a similar CAPE magnitude is predicted over a larger area. In these areas, obviously a capping inversion can bottle up some low-level moisture. In contrast, no CAPE is simulated outside of these zones - neither in the warm sector nor behind the cold front - due to the unusually strong background wind field and effective mixing of low-level moisture. To sum it up, CAPE is on the meagre side and substantial synoptic lift is needed to allow its release against a capping inversion.
Current thinking is that the rain shield of the warm front will include isolated to scattered, embedded thunderstorms from S Norway to Denmark early in this forecast period, which continue to move NE-ward across Scandinavia. Along the cold front, synoptic subsidence prevails during daytime, and convection - if any - will be confined to a narrow belt. Confidence in surface-based convection is very low until late afternoon or evening, and heavy rainfall should be the only risk as long as storms stay elavated.
From late afternoon onwards, the area of interest is finally overspread by increasing synoptic lift ahead of travelling mid-level vorticity maxima. Convective activity is expected to experience an upswing around 18 UTC, and chances increase that a few storms can either initiate from the surface or root down to it. The confidence is highest in N Spain and S France, where diurnal heating is strongest and orographic features provide preferred sites of initiation. Further northward, surface-based convection is less likely but still possible. In general, storms pose additional risks of large hail and severe downbursts as soon as they manage to become surface-based, and especially towards the north (N Germany, Denmark, S Sweden) an isolated tornado is not ruled out thanks to excellent wind profiles with strong low-level shear and veering.
Ongoing synoptic lift will likely keep scattered storms alive until Saturday morning, but the time window for surface-based storms gradually closes again and the remaining risk should shift back to isolated heavy rain in the course of the night.

... British Isles ...

At the southern flank of the cyclone and close to its center, forecast models predict marginal CAPE under strong vertical wind shear and storm-relativ helicity especially near the surface. Showers may bring isolated downbursts or tornadoes, especially when they move onshore and encounter frictional convergence and further enhanced vertical wind shear. In addition, severe downbursts are possible as well, though it will be difficult to disentangle the convective contribution, since scattered severe wind gusts are likely even outside of convection. Due to the limited cloud top heights and the very low probabilities of lightning, no threat levels are issued.

... E Europe ...

Two short-wave troughs swing SE-ward and lift unstable air with some hundred, perhaps locally up to 1000 J/kg CAPE across Belarus and the Ukraine and from the E Black Sea into Georgia, Azerbaijan and the Russian Caucasus region, respectively. Vertical wind shear is low across low- to mid-levels, before it starts to increase to 10-15 m/s across the 0-6 km layer (and even more to anvil levels). Scattered thunderstorms are expected, the majority of them in the afternoon and evening with the support of daytime heating.
In both areas, forecast models show a pretty similar synoptic situation with a dryline-like boundary which separates warmer and drier air to the SE (S and E Ukraine and inland parts of Turkey, Georgia and Azerbaijan, respectively) from cooler and moister air to the NW (Belarus, N and W Ukraine and E Black Sea, coastal areas of Turkey and Georgia and the Russian Caucasus forelands), respectively.
Large hail and severe downbursts are the main risks in the deeply mixed air on the warm side of these boundaries and especially with early, discrete storms. Towards the NW and in general in the course of the evening and night, the primary risk shifts to heavy rain. Convection is expected to weaken overnight, except for the E Black Sea, where it can continue or even intensify until Saturday morning.