Extended Forecast
Valid: Fri 28 May 2021 06:00 to Sat 29 May 2021 06:00 UTC
Issued: Wed 26 May 2021 22:49
Forecaster: TUSCHY

A level 2 was issued across far NE Algeria mainly for very large hail, severe wind gusts and locally excessive rain.

A level 1 was issued for parts of Turkey mainly for large hail, severe wind gusts and heavy rain.

A level 1 was issued for parts of N Greece and far S Bulgaria mainly for severe wind gusts and large hail.

A level 1 was issued for parts of CNTRL Bulgaria into far S Romania mainly for excessive rain.

A level 1 was issued for parts of the Ukraine into W Russia mainly for large hail, severe wind gusts and an isolated tornado event.

A level 1 was issued for Archangelsk Oblast mainly for hail, severe wind gusts and a low-end tornado risk.

SYNOPSIS

The disturbed streamline pattern with AO+ conditions continues and results in various slow moving troughs with variable amplitude. The main change is a building ridge over SW/W Europe towards a positive geopotential height anomaly just east of Greenland. Any eastbound progression of Atlantic troughs remains suppressed.
An extensive longwave trough still covers most of NE/E Europe with another low latitude trough placed just W of Portugal. Both waves feature only slow net motion to the E and S respectively. Anomalous winds at 250 hPa cover far N Morocco/Algeria and parts of SE/E Europe into W Russia.

The main focus for CI in proximity to synoptic-scale fronts will be a wavy cold/warm front, which runs from the Balkan States to Romania, the Ukraine into W-Russia. Elsewhere today's thunderstorm probabilities remain detached from any synoptic-scale front.


DISCUSSION

... E Tell Atlas mountain range into Aures Mountains ...

This area resides N of a retrograde moving subtropical ridge, which covers most of Algeria until the end of the forecast period. A low-amplitude wave circles that ridge while crossing the SW Mediterranean, inserting lowering thickness and modest synoptic-scale forcing to this area.

Main concern is the BL moisture content. An offshore flow event over S France during the previous day advects dry continental air far S and probably all the way to the coast of NE Algeria/N Tunisia (although modified in the surface layer). Models however try to push a pool of deep LL moisture (accumulated during the previous 36-48h between the Middle Atlas and the Quarseenis Massif) to the east on Friday which seems reasonable, given a structuring cyclonic vortex over/just S of the Balearic Islands. This wind shift also pushes a plume of well mixed mid-level air E, but enough temporal overlap exists for robust CAPE build-up in the area of interest.

Point source forecast soundings (ICON) indicate 2000-3000 J/kg MLCAPE and 15 m/s / 20 m/s shear in the 3km / 6km layer. Despite the well known capping issues in this area, a gradual cooling trend below 700 hPa is forecast and hence CIN decrease rapidly from W to E. With this in mind, most of the shear can be used effectively so supercells are possible with this setup. IFS however still indicates a stronger vortex over the Balearic Islands with more robust WAA downstream, hence stronger capping, less CAPE and somewhat weaker DLS with little QPF.

As GFS/ICON agree in many parts regarding the spatio-temporal ingredient overlap and with GEFS growing as high as 40% for some measurable QPF (e.g. CI), we added a confined level 2 already for parts of the mountains. Initiating storms will bring large to very large hail and severe wind gusts. Excessive rain could become a concern in case this activity moves off the mountains towards the coastal areas, where numerous convergence zones and abundant LL moisture exist. Due to cap concerns the level 1 was broadened to the N. Modifications to both level areas may be needed later-on as models (hopefully) converge.

... CNTRL Urkaine to W Russia ...

Moisture pooling along the synoptic scale front assists in BL mixing ratios well in excess of 10 g/kg. Mid-level lapse rates remain weak beneath the warm anticylone. Nevertheless, 800-1500 J/kg MLCAPE are forecast with cap eroding early in the day (probably well before noon in some places). Despite a gradual lowering of the thickness values, no substantial synoptic-scale forcing mechanism is forecast to affect that region. Hence, orography, the frontal boundary and weakening cap all steer CI during this forecast period. DLS in the 10-15 m/s range is adequate for multicells and a transient supercell risk with initiating storms posing a hail and severe wind gust threat. Betimes, clustering/upscale growth increases the rainfall threat, although a missing confined LL wind maximum probably keeps this rainfall risk clear for most areas. In case thunderstorms initiate also in the proximity to the front, locally enhanced LL shear with 100-150 J/kg LLCAPE may also support an isolated tornado threat.

... Romania to N Greece ...

Large uncertainties exist for this area with an ongoing/probably decaying cluster of thunderstorms from the previous night shifting E/SE and resulting in a substantial overturning of the airmass including weakening mid-level lapse rates but also question the amount of diabatic heating during the day.
Although models indicate a good amount of rain with this cluster (30-70 l/qm/12h until 18z) ove E-CNTRL Romania, there are currently no hints for any excessice rainfall probabilities. Postfrontal clearing (W-Romania) with recovering lapse rates and residual BL moisture may spark a few more storms during the afternoon hours but weak shear/CAPE preclude any severe for now.

Over N/CNTRL Bulgaria, MLCAPE up to 1000 J/kg and 5 m/s DLS favor slow moving thunderstorms with an excessive rainfall threat next to isolated hail.

Further south, over far S Bulgaria to N Greece, DLS increase to 20 m/s with straight hodographs and a drying mid-level air mass (next to well mixed low levels). Hence the severe risk shifts to severe wind gusts and large hail with multicells/isolated supercells. We combined all those risks in a broad level 1 for now.

... Turkey ...

A lead wave ahead of the main trough axis crosses Turkey from W to E on a more southerly track (along the S coast of Turkey) before the main trough approaches W Turkey from the NW until 06Z. Most parts see a combination of roughly 1000 J/kg MLCAPE and less than 10 m/s DLS with a well mixed and deep subcloud layer. Copious amounts of small hail / isolated large hail and severe downbursts are forecast, next to locally heavy rain.

Confluent SW-erly flow regime funnels rich moisture into the Gulf of Iskenderun with 2m dewpoints in the upper tens. Combined with moderately steep mid-level lapse rates, 1500 J/kg elevated CAPE are forecast as DLS gradually increases from 5 to 15 m/s during the day. Capping will be an issue but the passing mid-level wave and persistent onshore/upslope flow should support a few severe thunderstorms with large hail and severe wind gusts. Hence the level 1 was expanded into this region.

Another level 1 was issued for far NE Turkey mainly for large hail and severe wind gusts (1000-1500 J/kg MLCAPE and 10-15 m/s DLS). This risk vanishes around sunset.

... Archangelsk Oblast ...

Brisk westerlies within a plume of 300-500 J/kg MLCAPE create a nice shear/CAPE setup for a few multicells. With 0-3 km shear in the 15 m/s and 0-6 km shear in the 20 m/s range, hail and strong to severe wind gusts are forecast. Weak capping and early CI probably hampers the overall severe risk but any more discrete storm before noon also poses an isolatd tornado threat with curved 0-3 km hodographs and LCLs arund 500-600 m. This risk diminishes thereafter with separating ingredients.

... N Germany, Poland and far S Sweden ...

Placed beneath the upper low, regionally enhanced BL moisture dictates that thunderstorm risk. 200-400 J/kg SBCAPE and weak shear cause pulsating and probably sub-severe convection.
DLS increases to 15-20 m/s over S Sweden and N-CNTRL Germany but there is the concern of low topped convection, which cannot take profit of all the shear. With missing ensemble guidance regarding notable rainfall amounts, we only expect gusty winds and graupel on a local scale. Hence only lightning areas were added.

... Spain ...

Slow moving pulsating convection brings a risk of hail/gusty winds and locally heavy rain with a decrease during the evening hours onwards.