Storm Forecast
Valid: Fri 28 Aug 2020 06:00 to Sat 29 Aug 2020 06:00 UTC
Issued: Fri 28 Aug 2020 00:23
Forecaster: PISTOTNIK

Level 2 areas are issued for the Swiss Ticino, the Italian Alps and for E Spain for large hail, excessive convective rain, severe convective wind gusts and tornadoes.

A level 1 is issued from S France to Austria and the SW Czech Republic mainly for excessive convective rain, large hail and to a lesser degree for severe convective wind gusts.

A level 1 is issued for a part of W Russia mainly for excessive convective rain (mainly towards the NW) and large hail (mainly towards the SE).

A level 1 is issued for the Sea of Azov for non-supercellular tornadoes (waterspouts).

SYNOPSIS

An unseasonably strong and southward displaced zonal flow with an axis around 45-50N remains in place. An embedded mid-level trough moves from Belarus and the Ukraine into W Russia. Another one amplifies from the British Isles towards BeNeLux, France and Spain. Surface cyclones have formed at their forward flanks, the stronger one is the western one with a core pressure near 995 hPa just off the Dutch coast.
Increasing warm air advection into central Europe ahead of the western frough supports a shallow ridge and tilts the flow to a more SW-erly direction. The ingredients for a major severe weather outbreak start to come together in this flow regime.

DISCUSSION

... greater Alpine region to Czech Republic and W Poland ...

Intense warm air advection overspreads central Europe east of an almost stationary cold front from E Germany to S France. Low-level moisture increases due to surface evaporation, converging winds and suppressed vertical mixing, but limited daytime heating of the boundary layer beneath strong warm air advection aloft delays the CAPE buildup. CAPE will likely be limited to a few hundred J/kg near the cold front from W Poland to S France, where the SW-erly to W-erly flow lacks an upstream source of steep lapse rates. Further southeast, lapse rates steepen both due to advection of an elevated mixed layer from NW Africa and heating of elevated terrain over the Alps, and the CAPE magnitude rises: by late afternoon or evening, 500-1000 J/kg should become available in Austria, 1000-2000 J/kg in N Italy, and possibly up to 4000 J/kg near the Adriatic coast, where very moist sea breezes with dewpoints around 22C cut beneath these steep lapse rates. With the increasing mid-level flow and the ongoing warm air advection, this CAPE reservoir is overspread by outstanding vertical wind shear (0-6 km bulk shear often between 20 and 25 m/s) and veering wind profiles (0-3 km storm-relative helicity often between 100 and 400 m^2/s^2).
Scattered convection may already form before noon near the cold front and will increasingly spread into S Germany and Austria in the afternoon, the Czech Republic in the evening and Poland overnight, following strong synoptic lift from warm air advection and travelling vorticity lobes. The majority of storms is expected to stay elevated, and heavy rain is the main risk in that case. If storms can manage to root down to the surface, they can organize into multi- or even supercells and pose additional risks of large hail and isolated severe wind gusts.
The highest chances for a few surface-based storms exist along the north Alpine rim in Austria, where the dryline (the convergence zone between South Foehn and thermally driven upvalley flow) provides a preferred axis of initiation. High-resolution models uniformly simulate some storms there but disagree on their placement and tracks, hence no upgrade to a level 2 is performed despite a higher conditional severe weather risk.

The concern for beginning severe to extreme convective weather is higher in the robust CAPE-and-shear overlap on the southern side of the Alps, but it is challenging to assess where and when the capping inversion will break. Generally speaking, the probability of convective initiation rises steadily throughout the forecast period in the upslope flow regime (both thermally and dynamically driven), most notably in the Swiss Ticino and the Lombardian Alps in Italy, where repeated and backbuilding rounds of storms may already dump up to 100 mm rain by Saturday morning (followed by even more afterwards). While the excessive rain risk is maximized over mountainous terrain, tail-end storms over the foothills can easily turn supercellular with additional risks of large (or locally very large) hail, severe downbursts and tornadoes.
Due to a strong cap and fading synoptic lift support, it is unclear how far east the storms will manage to propagate overnight. However, it needs to be emphasized that the environment of extreme CAPE and significantly enhanced 0-1 km shear in NE Italy would support supercells with even a strong tornado if the capping inversion breaks. A particularly careful monitoring of the Po Valley for convective initiation is therefore recommended.

... E Spain ...

Sea breezes have advected abundant low-level moisture (2m dewpoints of 22-25C) some distance inland on Thursday. The combination with steep lapse rates aloft creates CAPE on the order of 2000 J/kg per Thu 12 UTC Barcelona and Palma soundings, overspread by increasing vertical wind shear and synoptic lift as the mid-level trough approaches. By Friday night, 0-6 km bulk shear will probably exceed 20 m/s.
First storms will be confined to the mountains and will stay rather isolated until the afternoon, but then coverage will increase and secondary storms over lower areas become more likely. A very good organization into strong multi- and supercells is predicted, and large to very large hail and severe downbursts must be expected. Especially on the southern side of the Pyrenees, constantly backbuilding convection can enable very high rainfall accumulations and some dangerous flash floods, too.
In the evening and overnight, synoptic lift ahead of the trough and mesoscale lift from outflow boundaries erodes the cap. Convection can then also propagate to coastal and offshore areas, especially if it manages to grow upscale into an MCS. Despite some drier and cooler air gradually moving south from France, the CAPE reservoir may be big enough to allow a storm system to graze or even hit the Balearic Island until the end of this forecast period. Next to the other mentioned risks, tornadoes become possible when storms move over the maritime boundary layer with its abundant moisture and enhanced low-level shear.

... NW Europe ...

Fairly warm sea surfaces create some hundred J/kg CAPE in the polar air just ahead and beneath the main trough. Scattered thundery showers will be active thoughout the forecast period. Deep vertical mixing and resulting weak vertical wind shear keep the severe weather risk low.
A few heavy rain events are not ruled out over Denmark and S Sweden, where half-circular hodographs (albeit under low speed values) ahead of the occluding frontal system can promote side- and backward propagation. Banded convection with a possibility of higher rainfall intensities also wraps around the low-pressure center, but should stay offshore.
At the N and W flank of the cyclone center, a mesoscale area of strong to marginally severe wind gusts will develop and could affect coasts of E England and late in the forecast period also the Netherlands. They should be of mostly non-convective origin and are therefore not covered by a risk level area.

... W Russia ...

Forecast models predict some hundred to locally up to 1000 J/kg CAPE in the warm sector of the eastern cyclone, though there is considerable disagreement with respect to the placement of this blob of unstable air. Scattered, mostly daytime-driven and rather poorly organized thunderstorms are expected. A level 1 is issued for the southeastern part of the unstable airmass, where the CAPE signals seem to be most consistent and where 10-15 m/s deep-layer shear and steeper lapse rates also allow some multicellular organization. Heavy rain (mainly towards the NW) and marginally large hail (mainly towards the SE) are the main risks.
A temporary tornado risk evolves until ~09 UTC over the Sea of Azov when it is crossed by the tail of the cold front. Converging surface winds under very moist, unstable air should allow some hours of thunderstorm activity.