Storm Forecast
Valid: Sat 10 Aug 2019 06:00 to Sun 11 Aug 2019 06:00 UTC
Issued: Fri 09 Aug 2019 15:08
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for parts of N Italy, Switzerland, SE Germany, the Czech Republic, Austria, Slovakia and SE Poland mainly for excessive convective precipitation and large hail and to a lesser degree for severe convective wind gusts.

A level 1 is issued for parts of Denmark, S Sweden, NE Germany and NW Poland mainly for severe convective wind gusts and to a lesser degree for tornadoes.

A level 1 is issued for an area in W Russia mainly for excessive convective precipitation, large hail and to a lesser degree for severe convective wind gusts.

SYNOPSIS

The zonal flow remains unseasonably strong. A long-wave trough exits our forecast domain to the east. Another long-wave trough with a positively tilted axis approaches W Europe. In-between, a shallow ridge translates from central to E Europe. The mid-level jet axis runs from Iberia to Germany to the Ukraine.
At the surface, a deep and mature cyclone moves from England to the North Sea. Its cold front crosses S Scandinavia and Poland before its tail becomes inactive and almost stationary across the Czech Republic, S Germany, France, Spain and Portugal. Very hot air covers the entire Mediterraenean region and is advected into the Alpine region and the Balkans ahead of the cold front. Maritime polar air is present on the cold side of the frontal zone.

DISCUSSION

... Alpine region, Czech Republic, Slovakia, S Poland into far-W Ukraine ...

Despite rather modest lapse rates (mostly below 7 K/km), rich low-level moisture ahead of the cold front allows the development of some hundred J/kg CAPE under ~20 m/s deep-layer shear. Some rain areas with a few elevated leftover storms from the previous night will likely move E- to NE-ward across Switzerland, S Germany and Bohemia on Saturday morning, while outflows and thermally direct cross-circulations make several NW wind surges run some distance ahead into the prefrontal warm air in Moravia, Austria and later also Slovakia. In the course of the day, chances increase that either storms will re-activate at the leading edge of the rain shield or form anew at one of the preceding boundaries, but model signals are widely diverging: Whereas global models predict a large cluster of convective precipitation to move from N Austria into Slovakia, SE Poland and the W Ukraine in the evening and overnight, finer limited-area models place strong activity over the Austrian Alps in the afternoon and evening (discussed below in more detail), but little to none anywhere E to NE of it. In general, a lack of synoptic lift and a cap from preceding warm air advection may indeed keep storm coverage on the low side, especially over flat terrain. Nonetheless, a conditional level 1 is extended far E-ward, as any storm that forms especially in the 12 to 20 UTC time frame may become surface-based and well-organized with a risk of large hail, excessive precipitation and severe wind gusts. Before and after this time frame, convection will mostly stay elevated, and excessive rain in case of training activity along the flow-parallel frontal boundary is the main risk.
The outflows from the morning storms will be blocked by the Austrian Alps and diverted around their E and SE foothills. Thick moisture (with predicted dewpoints of 18-21C at low levels and 14-16C at 850 hPa) and insolation should easily allow 1000-1500 J/kg CAPE, and seeing the strong, persistent convergence during peak heating hours, there is little to be said against at least scattered storm initiation in the afternoon and evening, and even the most "aggressive" precipitation output by AROME and WRF could still be too conservative. Under 20 m/s deep-layer shear and very favorable wind profiles in the deflected wind regime over the E and SE Alpine foothills (0-3 km storm-relative helicity likely between 200 and 400 m^2/s^2), a quick organization into multi- and supercells is expected, and one or two large clusters may emerge in the course of the evening. The wind profiles and the orography favor side- and backbuilding, hence the wind risk is kept rather low, while there is a distinct possibility of large hail and flash floods. Tail-end storms that descend into the forelands might even produce very large hail (despite the limiting factor in form of modest lapse rates) and an isolated tornado. However, convection will probably succumb to the strong cap within 50-100 km after detaching from the mountains.
Even more CAPE is in place in N Italy, probably around 2000 J/kg towards Piedmonte, but a strong cap and a lack of synoptic lift will keep convective initiation on the isolated side and restricted to the mountains. However, any storm that forms can equally become well-orgaized and severe, and the level 1 is extended to the Italian Alps and the Swiss Ticino for that reason.

... E Spain ...

CAPE up to 1500 J/kg and strong 0-6 km shear in excess of 20 m/s overlap in the sea breeze and upslope flow regime along the Spanish East coast. Synoptic lift is absent, hence convective initiation is unlikely and will stay isolated at best. In case persistent updrafts form, large hail and severe downbursts would become likely.

... British Isles to Baltic region ...

Scattered to widespread showers will occur in the stream of deeply mixed, maritime polar air behind the cold front. After some hours of daytime heating, the strongest showers can grow deep enough to produce some thunder and to organize into multicells under 0-3 km shear between 10 and 15 m/s, especially over S Sweden and N Poland, where daytime heating and lift by a vorticity maximum may allow CAPE up to 500 J/kg. Since this area partly overlaps with an 850 hPa wind maximum around 25 m/s further south (from England to Denmark and NE Germany), marginally severe wind gusts of partly convective origin are possible. In addition, an isolated tornado is not ruled out, especially at coastal convergence zones. These risks diminsh in the evening, when the wind field relaxes and the boundary layer stabilizes.
(Side note: expected non-convective severe wind gusts under the 850 hPa wind maximum from England to N Germany are not included into the ESTOFEX risk level scheme.)

... far-E Ukraine and adjacent Russia ...

Daytime heating and lift by a passing vorticity maximum will create some hundred J/kg CAPE. Most of it will be under weak vertical wind shear, but especially early storms may grow into slightly stronger mid-level winds, resulting in 0-3 km shear up to 15 m/s. Scattered, mostly daytime-driven and multicellular storms are expected. A few large hail and excessive rain events plus isolated downbursts are possible.