Extended Forecast
Valid: Thu 24 Jun 2021 06:00 to Fri 25 Jun 2021 06:00 UTC
Issued: Tue 22 Jun 2021 10:45
Forecaster: TUSCHY

A level 2 was issued across SE Germany, N Austria to the Czech Republic into Poland and NW Slovakia mainly for large to very large hail, severe to damaging wind gusts and heavy rain.

A level 1 surrounds that level 2 for similar hazards but with less coverage. An isolated tornado event is possible.

A level 1 was issued for E France to CNTRL Germany mainly for heavy rain.

A level 1 was issued for NW Italy mainly for hail and severe wind gusts.

A level 1 was issued for parts of SE Europe mainly for excessive rain, hail and a few severe downburst events.

A level 1 was issued for NE Europe mainly for hail, severe wind gusts and heavy rain. An isolated tornado risk is possible.


SYNOPSIS

Damped but still ongoing Rossby-wave dynamic from the W causes a (still) standing wave over W/CNTRL Europe, before it starts to progress E beyond our forecast period. Positive geopotential height anomalies persist over Scandinavia into W-Russia and S of Greenland with a broad trough in-between over W-Europe. This trough features numerous smaller scale upper-level vortices with one lifting from SW France to S Germany while weakening/opening up into a wave.
Another trough is placed over far SE Europe with no progression/motion until 06Z.

The main attention (synoptic-wise) is directed to this weakening vortex over CNTRL Europe. NWP guidance still has a not so good handling, where to place the main lobe of energy within that wave with a bumpy run-to-run consistency. The last run of GFS now indicates a merging of energy with a more focused wave crossing Switzerland during the day (better curvature with a more focused mid-level jet to its SE). This however is the first run compared to the past 4 runs of GFS, so more consistency is needed to buy this. ICON comes up with a similar trend but has this wave a bit slower than GFS and with still more cyclonic vorticity along its base. With no substantial jet entering this wave from the W, no further amplification is forecast (which could slow down this lifting wave further). IFS is more on track with GFS and indicates a more progressive wave compared to the past 3 runs.

To summarize, NWP guidance of this wave is still not the best although a trend to a more progessive wave is noted. Timing of this wave is OK and the track handling is very good.

At the surface a wavy frontal boundary runs from France to Poland to Finland with a very unstable/warm air mass to its south.
The overall pattern supports another round of organized convection for parts of Europe.

DISCUSSION

With ongoing uncertainties regarding the upper-level dynamic and with uncertainties about the mesoscale dynamic of ongoing (from the previous night) and developing convection, only a rough deliniation of threat areas will be performed. Especially along its fringes, lightning/level area changes may be needed in later outlooks.

Switzerland and NW Italy to S-Germany/E France to NE-Austria into the Czech Republic and Poland ...
The area from Switzerland into SW Germany is placed beneath the upper trough/wave with ongoing convection and new CI during the day. Concerns exist in limited diabatic heating/lots of ongoing showers/thunderstorms so although a line of convection could evolve over Switzerland, its severe risk should stay low with rather weak 0-3km shear. Depending on temporal diabatic heating (MLCAPE in the 400-800 J/kg range) and better timing of the wave, a level 1 upgrade may be needed later-on.
DLS and CAPE increase towards NW Italy (25 m/s with 1500 J/kg MLCAPE) where bowing line segments could bring severe wind gusts next to a large hail threat. During CI, multicells/a few splitting supercells are possible, too. Airmass to the SE remains very dry with ongoing strong cross-mountain range flow, so a sharp gradient in CI is forecast.

A similar kinematic setup but with somewhat less CAPE and better low/mid-level veering should support a few supercells during the afternoon/evening over SE Germany, which pose a large hail/severe wind gust threat. Upscale growth into a forward propagating MCS towards the W/NW Czech Republic is possible during the evening.

Of most concern is the area from NE Austria into the E Czech Republic/SW Poland, where shear increases also in the 0-3 km layer during the passage of an anomalous strong 700 hPa jet. Orientation of the thickness gradient and CAPE plume favor a motion into the far E/NE Czech Republic with scattered to widespread damaging winds and large hail. Very large hail is possible with IFS/GFS guidance due to lots of mid-level CAPE and strong SR flow at low to mid-levels. With the trend of a more focused/progressive mid-level wave and currently good run-to-run consistency regarding CI, a level 2 was added for this area. Uncertain remains the question how far into Poland this progressive MCS will move with ongoing favorable shear/CAPE overlap well into the night. Modifications may be needed including further (potential substantial) expansion to the E/NE into CNTRL Poland.
Despite increasing CIN over E Austria into NW Slovakia, the environmental conditions are supportive for isolated discrete and long-lived supercells if CI verifies (although current ensemble data has very low probabilities for CI). A grazing UVV maximum and potential outflow boundary interaction with the orography indicate a non-zero risk right now. Hence we expanded the level 2 a bit into this area but modifications may be needed. All kind of (extremely) severe is possible including an isolated tornado risk (despite rather large LCLs) if CI occurs.

Enhanced moisture flux convergence along the N fringe of that wave may support scattered slow moving and clustering thunderstorms with an heavy rainfall risk over CNTRL Germany to E France. Very moist vertical profiles wth DLS of 5 m/s and moderate (skinny) CAPE support that idea.

E/SE Europe ... has a very unstable but partially capped air mass and little QG forcing to work with. The main concern for CI currently remains along the orography but also with a short-wave (crossing Belarus) and lowering thickness towards the Ukraine.
Any developing updraft has 1.5 to 3 kJ/kg MLCAPE to work with so large hail/damaging downbursts become the main hazard. 3/6 km shear oscillates around 10 m/s over Belarus so upscale growth into numerous bowing line segments is possible which could produce confined swaths of severe wind gusts. This risk should subside after sunset with strengthening CIN.
Another multi-model focus for CI exists from E /CNTRL Romania/Moldova to the S Ukraine next to the upper low. Slow moving intense updrafts may bring copious amounts of hail, excessive rain and locally damaging downdrafts.
A broad level 1 was issued where currently confidence in CI is adequate but further modifications will be needed including a potential upgrade over Belarus / NW Ukraine due to many severe reports. Also a broad level 1 with low lightning probabilities were added to highlight chances for isolated CI. If this trend verifies, the lightning probabilities may see substantial adjustments in this area with very high CAPE supporting intense updrafts.

Baltic States to NW Russia ... Models differ with the CAPE/shear space with ICON showing a raher focused overlap, whereas others come in more diffuse. However, the overall pattern supports the passage of numerous short waves along/over a frontal boundary and 15 m/s shear in the lowest 3 and 6km. Combined with 1 to 1.5 kJ/kg MLCAPE, organized multicells/transient supercells tend to merge rapidly into bowing lines with an attendant severe wind gust/hail risk - similar to the initiating convection. Convection along the boundary also poses a tornado risk with enlarged LL hodographs due to backed flow. With ongoing NWP discrepancies only a broad level 1 area was added.