Extended Forecast
Valid: Fri 21 May 2021 06:00 to Sat 22 May 2021 06:00 UTC
Issued: Wed 19 May 2021 14:52
Forecaster: TUSCHY

A level 1 was issued across the Netherlands into far NW Germany mainly for an isolated tornado/hail and strong to severe wind gust threat.

A level 1 was issued for NE Germany into Poland mainly for isolated large hail and severe wind gusts. An isolated tornado event is possible.

SYNOPSIS

An anomlaous height field yields an unusually far south displaced and strong jet, which is placed over the S Bay of Biscay to the Alpine region before bending south towards Greece. Focusing on the height field, we see a minimum over the S North Sea in response to a filling upper low, which is part of a large-scale gyre over W/N Europe. This area is surrounded by positive geopotential height anomalies, which extend from the NE Atlantic to N Africa to S/E Russia/Siberia. The only interruption is an upper trough with negative anomalies, which crosses Greece during the forecast and shifts leisurely E towards W-Turkey.

At the surface the depression over UK/later S North Sea pushes an occlusion E over CNTRL Europe although it weakens and could transform into a detached cold/warm front wave, which frames E/SE Germany. The occlusion bends back towards the Iberian Peninsula in form of a wavy / slow moving frontal boundary.
Another depression over Finland features a classic warm / cold front and occlusion pattern with a narrowing warm sector. Elsewhere, regionally enhanced BL forcing by numerous convergence zones will be the main foci for enhanced convective activity.

DISCUSSION

... W/CNTRL Europe ...

Ahead of the English depression, a broad fetch of a maritime airmass advects E and beneath cold mid-levels, offering once again low/moderate CAPE for a vast area. No connections to subtropical regions exist so best LL moisture is forecast along the frontal boundaries from the Bay of Biscay to Belgium/the Netherlands/N-Germany into Poland.

Destructive interference of synoptic-scale subsidence and weak SBCAPE keeps thunderstorm probabilities for N France/Belgium and the Netherlands on the isolated to scattered side with some improvement during the evening hours over the Netherlands. This time frame (late afternoon/evening into the first part of the night hours) is currently the main focus for organized thunderstorms. Diffuse forcing (no clear post frontal convergence signal) and good diabatic heating assist in numerous thunderstorms which stay discrete for some time. With constantly tightening isobars due to the approaching depression, LL wind field response is strong with SRH3 increasing to 150-300 m^2/s^2 (slightly less in the 1km layer). Forecast hodographs reveal an increase of LL curvature especially for deviating storms. Combined with LCLs in the 600-800 m range, a few organized shallow mesocyclones could bring a few severe wind gusts, hail (in case of a few taller updrafts) and an isolated tornado. Limiting ingredient currently is elongated/thin CAPE profiles with 150-300 J/kg MUCAPE. However this area needs to be monitored closely for better CAPE build-up and was already highlighted with a level 1.

Further NE, over N Germany into Poland, MUCAPE increases a bit with peak values of 400-600 J/kg for NE Germany into Poland (ICON more aggressive than IFS). Mixed signals exists in how active a short wave will cross this area during the day and hence some models show substantial cloud cover and precipitation which would limit CAPE build-up. Nevertheless, at least temporal clearing is forecast and 15 m/s DLS would support a few organized thunderstorms with severe wind gusts and hail. Dependant on the strength of the wave and temporal backing of the LL wind field, an isolated tornado threat could exists with 150-200 m^2/s^2 SRH1. Right now we added a level 1 for the whole area with best shear/CAPE overlap.

A more potent wave ejects out of the base of the upper trough and crosses CNTRL Germany during the evening/night hours. Impressive synoptic-scale lift interacts with somewhat better LL moisture from SE France and low-end MUCAPE build-up is forecast. Dependant on the final magnitude of moisture advection any near surface based thunderstorm could bring strong to isolated severe wind gusts with 15-20 m/s shear in the lowest km AGL. Uncertainty in CAPE release prelcudes a level 1 for now.

... NE Europe ...

The thinning warm sector features LL mixing ratios in excess of 10 g/kg beneath weak mid-level lapse rates, offering 500-1000 J/kg weakly capped MLCAPE. Forecast hodographs show a mixture of rather short/straight and anticyclonically curved and elongated hodographs, so splitting storms with also partially dominant left moving cells is forecast. The degree of severe risk is probably hampered by rapid clustering, but at least during initiation/storm merging a risk of heavy rain, hail and strong wind gusts exists. This risk is too diffuse right now to add a focused level 1 area but a potential upgrade is possible.

... Elsewehere ...

Better moisture advection is forecast from the Caspian Sea to the Southern Federal District with peak valuse along the Krasnodar Krai. During the overnight hours, a tongue of enhanced MUCAPE shifts over Rostov Oblast to Volgograd Oblast in an overall anticyclonically curved streamline pattern. IFS/ICON differ in how robust this CAPE plume will be but in case of IFS a few elevated thunderstorms with large hail are possible during the night. Airmass remains capped up to 700 hPa, so not much effective shear exists and no level 1 was issued right now.

Very unstable but capped air mass resides at Krasnodar Krai with no CI forecast right now.

Far NW Turkey expects weak onshore flow with enhanced BL moisture but rapidly weakening mid-level lapse rates. Initiating storms could take profit of a well mixed BL air mass and they pose an isolated severe wind gust/hail risk. Low-end CAPE build-up precludes a level 1.