Extended Forecast
Valid: Tue 10 Aug 2021 06:00 to Wed 11 Aug 2021 06:00 UTC
Issued: Sun 08 Aug 2021 20:52
Forecaster: TUSCHY

A level 2 was issued across N Turkey mainly for excessive rain.

A level 1 surrounds this level 2 with similar hazards but lower probabilities. In addition, isolated hail and a few waterspouts are possible.

A level 1 was issued across parts of Poland into E Czechia and N Slovakia mainly for hail and severe wind gusts.

A level 1 was issued across parts of Austria into NW Hungary mainly for large hail and severe wind gusts.

SYNOPSIS

What a forecast to talk about. General blocking regime keeps going over Europe with two prominent features catching our eyes.

The first one is an upper low, which cuts off from the westerlies and creeps SE over the open NE Atlantic. The upper low offers an impressive negative geopotential height anomaly and so is the anomaly of the evolving LL depression, which features a transient shallow warm seclusion structure (with mixed signals in latest cyclone phase diagrams). This is a -4 to -5 sigma event regarding climatology for MSLP and probably around the climate extreme of the Mclimate (CFSR).
Downstream response assists in spreading out a progressive ridge over far NW Europe with another progressive low-amplitude upper trough passage over Denmark/Germany into Poland.

The second feature of interest is a subtropical anticyclone, which continues to intensify between Italy and Libya/Tunisia. Thickness and geopotential height readings will be impressive with up to 600 gpdm near Sicily and Tunisia. An area-average forecast sounding from GFS for this region reveals intense subsidence with utterly dry air in the 5 to 6 km layer (AGL). The H7 temperature soars into the 13-16 C range with H85 temperature readings over N Tunisia to Sardegna to Sicily oscillating around 30 C (advection of an African air mass along the W fringe of that anticyclone). The air mass could mix up to 500 hPa - extreme values if this verifies. Bone dry air offers certainly no thunderstorm probabilities but very hot BL conditions. A few orographic events over Spain are possible along the fringes of this heat dome. A slow retrograde motion is forecast until 06Z although in this stage it looks more like an expanding subtropical cell instead of any net motion to the W.

Then we have to talk about a quasi-stationary upper trough, which parks over the Black Sea/Turkey into Near East. Despite breaking into numerous pieces, its main centroid seems to stay offshore over the E-/CNTRL Black Sea.
SST anomalies for the 82-11 period reside in the +3 to +5K range (dependant on the type of measurement). Relaxing background shear, abundant latent heat influx from the Sea and a potential PV max entering the scene could result in a tropical-like cyclone scenario. ICON showed this during the past 2 days but now starts to back off a bit in the youngest runs. IFS/GFS remain slow with any development. This certainly has to be monitored closely in the following model runs. Any motion should be to the N with a weak southerly background flow, taking any vortex to the proximity of the Sea of Azov.

Overall NWP guidance is stable in the past few runs - not much of a surprise with such a blocked flow pattern.

DISCUSSION

... N-Turkey to Georgia to the Sea of Azov ...

With the broad and still ill defined vortex over the Black Sea, thunderstorm activity spreads out far away from its centroid. Hence a broad area with excessive rain is forecast in an overall favorable setup with BL moisture in excess of 14-16 g/kg, SBCAPE in the 1-2 kJ/kg range and TPWs between 40-50 mm. DLS is in the 2-5 m/s range, so pulsating convection features very slow forward motions. Any local mesoscale boundary or orographic support could result in quasi-stationary convection with extreme rainfall amounts on a local scale.
We favored IFS/ICON with more realistic BL moisture performance and hence a broad level 1 and a confined level 2 area were issued for excesive rain with a few significant flash flood events well possible. The highest amounts could occur between the Kolkheti Plain and the Sea of Azov (not yet certain enough for a level 2) but also along the W Pontic Mountains.
Initiating cells can also produce an isolated hail event and a low-end waterspout risk is also present with augmented background vorticity and forecast convergence zones next to the coastal areas.

The final magnitude of this event also depends on how fast the vortex spins up atop the E Black Sea.

... N Germany to Poland and E Czechia to N Slovakia ...

A progressive mid-level wave races E during the day and is accompanied by moderate to strong upper level divergence. This wave affects a seasonable moist air mass with 500-1000 J/kg MLCAPE. DLS increases from N to S from 10 to 25 m/s and so does the 0-3 km shear (5 to 15 m/s).
The current idea is that convection evolves over N Germany into N Poland with adequate background shear for a few organized multicells. Strong forcing could increase the degree of organization with some line-ups possible - similar to past events this weekend. A few strong to severe wind gust and hail events are possible.
Shear and CAPE space improves during the afternoon/evening over S/E Poland into E Czechia and N Slovakia, so convection may mature into multicells/transient supercells with an attendant hail and severe wind gust threat.

NWP guidance still indicates substantial differences in the placement and magnitude of the CAPE-shear space, so right now only a level 1 area was issued for the region, where most of the guidance supports organized convection. In fact the overall pattern could support a few long-tracked supercells with swaths of large hail/severe wind gusts so further modifications of this level 1 are certainly possible with a regional upgrade not ruled out.

... SE Austria into NW Hungary ...

Strong westerlies affect the Alps with widespread DLS in the 20 m/s range. Enhanced moisture along the Alpine rim and diurnal driven alpine pumping should support a belt of enhanced moisture within the W part of this level 1, where isolated CI is forecast (little support from the synoptic scale). CAPE-shear space may increase in LAM outputs, which capture this setup better, but overall guidance already points to a few multicells/one or two supercells, crossing this level 1 from W to E. Large hail and severe wind gusts will be the main issue. Storms entering W Hungary should start a weakening trend with increaing capping around sunset and onwards.

... Scandinavia ...

This region is placed beneath the broad upper trough and filled with a moist BL air mass. Widespread 400-800 J/kg MLCAPE is forecast with DLS in the 1-5 m/s range. Quasi-stationary pulsating thunderstorms are forecast on a scattered to widespread scale. TPWs reside in the 20-30 mm range with lower values towards Norway/W-Sweden. Heavy rain will be an issue with merging/clustering cells and/or stationary thunderstorms. General ingredients do not yet justify any upgrade but heavy rain could become an issue on a local scale.

... Eastern Carpathians ...

CAPE shear space could support a few multicells with hail and strong to isolated severe wind gusts but the current setup does not yet jusify an upgrade with weak synoptic-scale forcing beneath ridging. CI could be on the low-end side.