Storm Forecast
Valid: Tue 11 Aug 2020 06:00 to Wed 12 Aug 2020 06:00 UTC
Issued: Mon 10 Aug 2020 23:36
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for Spain and SW France for multuiple severe weather risks: mainly excessive convective precipitation towards the N, mainly large hail, severe convective wind gusts and to a lesser degree tornadoes towards the S.

A level 1 is issued for England, Belgium, the rest of France, Switzerland and N Italy mainly for excessive convective precipitation, large hail and severe convective wind gusts.

A level 1 and level 2 are issued for the Netherlands, W and S Germany, Austria, Hungary and parts of the Balkans mainly for excessive convective precipitation and to a lesser degree for large hail.

A level 1 is issued for S Italy mainly for excessive convective precipitation and to a lesser degree for large hail.

A level 1 is issued for the E Black Sea region and W Georgia mainly for excessive convective precipitation and to a lesser degree for large hail.


SYNOPSIS

Weak pressure and temperature gradients cover Europe. A faint mid-level ridge stretches from the central Mediterranean region to Scandinavia, where it keeps a blocking surface anticyclone alive (despite some weakening trend). A trough at low- and mid-levels stretches from Iceland to Portugal. Between these two features, very warm air is transported northward into large parts of W and central Europe. In contrast, cooler and drier air advances southward into E Europe in the wake of a cyclone over NW Russia.

DISCUSSION

... W and central Europe, Iberian Peninsula, Italy, Balkans ...

Warm air advection and a history of synoptic subsidence under the recent blocking anticyclone placed a robust capping inversion into the body of warm air, which allowed rich low-level moisture accumulation and substantial CAPE buildup before too much convective overturning could take place. By Monday afternoon, 2m dewpoints in the upper tens supported CAPE between 400 and 1200 J/kg across wide areas. In sea breeze and Alpine pumping wind regimes, where vertical mixing was further suppressed, 2m dewpoints even reached the lower twenties and CAPE the order of 1000-2000 J/kg (albeit strongly capped), most notably across the W and central Mediterranean Basin, over the French Pyrenees forelands and from N Italy to the Pannonian Plain, but more locally also along the Dutch and German coastlines.
On Tuesday, little changes in the CAPE magnitude are foreseen, while ongoing daytime heating and some sources of synoptic and mesoscale lift further erode the cap. Mostly daytime-driven thunderstorm activity over orographic features, along outflow boundaries and other convergence zones will therefore probably increase a bit. Apart from the far west, weak vertical wind shear keeps storms poorly organized.
Within these general characteristics, the discussed area can be divided into several sub-regions:

(1) Spain and SW France:
Synoptic lift and vertical wind shear increase ahead of the Atlantic trough. Especially the southern Spanish regions of Andalusia, Murcia and Valencia face an impressive overlap of robust CAPE and ~20 m/s deep-layer shear. At least scattered, well-organized thunderstorms are expected and can bring all kinds of severe weather. While the wind risk is enhanced with coastal and inland storms alike, the risk of hail (including a possibility of a few extreme events) and one or two tornadoes is maximized with tail-end storms, which have a high risk of turning supercellular thanks to the abundant maritime CAPE reservoir. Later in the evening and night, and in general towards the north where deep-layer shear is somewhat weaker (10-15 m/s), storms could merge into clusters and the main risk gradually shifts to excessive rain. One or two MCSs are also expected to enter SW France in the evening and night, where the capping inversion could resist for a longer time.
A major source of uncertainty is the behavior of low-level moisture in inland Spain. Under strong insolation and fairly strong southerly flow, it could be mixed out prematurely and/or removed by downslope winds not only over mountainous terrain, but also in the Ebro valley and interior parts of Catalonia. Hence these regions are only placed under a low-probability lightning area and a level 1, mainly for isolated severe wind gusts in case some storms can yet form in this dry environment.

(2) Great Britain, Belgium, rest of France, Switzerland, N Italy:
The capping inversion on top of the moist boundary layer and limited synoptic lift could keep the storm coverage rather low, but the conditional severe weather risk is notably enhanced once storms initiate. The main risk is excessive rain due to plentiful low-level moisture and slow storm motion. However, steep lapse rates in the "Spanish plume" also enhance the risk of large hail and severe downbursts even in the absence of noteworthy vertical wind shear.
Confidence in surface-based storms is limited in England (where the ratio between capping and lift deteriorates), and in general turns very low in coastal areas. Elevated activity on top of the sea breezes or in the warm air advection regime in Great Britain can still bring some heavy rain, but the other risks are significantly lower.

(3) Netherlands, W and S Germany, Austria, Hungary, Slovenia and Croatia:
Slightly cooler and drier air advances westward, and the convergence zone at its leading edge creates a focus for convective initiation. Downwind of the mid-level ridge axis, a slow storm motion from NW to SE (i.e., parallel to the boundary) enhances the risk of flash floods. A belt of 10-15 m/s upper-tropospheric flow might enhance deep-layer shear just enough to allow marginally large hail or plentiful small hail. In general, the wind and hail risk is kept low by the moist airmass with rather skinny CAPE profiles, though.
Two areas with the strongest lift (either synoptic or orographic) and the expected highest storm coverage around the border of Belgium, the Netherlands and Germany and from the Austrian Alps southeastward are upgraded to a level 2. Otherwise, the less clear position of the convergence zone and the possibility of morning convection with ensuing limited daytime heating enhance the uncertainties, and a level 1 seems more appropriate.

(4) Central and S Italy, Balkans:
Despite substantial CAPE, rather dry profiles with an "inverted-V" boundary layer make convective initiation more difficult in the absence of synoptic lift. Isolated to scattered afternoon storms should mainly be confined to mountains. Isolated heavy rain and to a lesser degree marginally large hail are the main risks.

... S Poland, N Ukraine, far-south Belarus into W Russia ...

A cold front moves southward. A zone of enhanced moisture creates some hundred J/kg CAPE (though possibly not as much as models suggest) under weak vertical wind shear ahead of it. Scattered, mostly daytime-driven, poorly organized and rather short-lived storms are expected. Isolated heavy rain is not ruled out, but its coverage should remain below our level 1 criteria.

... E Black Sea region ...

Abundant low-level moisture creates CAPE on the order of 500 to 1500 J/kg. In an otherwise tranquil synoptic environment, a shallow cyclone over the E Black Sea is expected to keep scattered storms going, probably in form of loosely organized clusters. During daytime, activity could shift more towards onshore areas. The main risk is heavy rain. In case discrete storms can hold their ground (which is doubtful), they could benefit from moderately enhanced deep-layer shear and also produce localized large hail.