Storm Forecast
Valid: Sun 29 Oct 2017 06:00 to Mon 30 Oct 2017 06:00 UTC
Issued: Sun 29 Oct 2017 00:12
Forecaster: PISTOTNIK

Threat levels were issued for large parts of central Europe mainly for severe convective wind gusts and to a lesser extent for tornadoes.

SYNOPSIS

Amplified long waves govern the circulation in Europe. A 500 hPa trough over eastern Europe and a deep surface low with its center over Estonia are opposed by a 500 hPa ridge and an extensive surface high over western Europe and the near Atlantic.
In-between, a very strong NW-erly flow curves from Scandinavia across central Europe to the Aegean Sea. Embedded in this mid-level jet streak, a powerful short-wave trough ejects from Denmark (Sun 00 UTC) to the Black Sea (Mon 00 UTC) and reinforces the eastern European long-wave trough. Strong synoptic lift ahead of this short wave and in the jet's left exit also stimulates the formation of a secondary surface cyclone over the Skagerrak Sea Saturday night, which is expected to deepen below 970 hPa while it moves across Poland into Belarus on Sunday. This new cyclone will bring a remarkable autumn storm to central Europe, probably including a round of severe convection along its cold front.

DISCUSSION

... central Europe (cold front passage) ...

A tongue of moist air from the North Sea is advected eastward ahead of the cold front (2m dewpoints around 10°C across N Germany and around 12°C in the Netherlands at Sun 00 UTC, in line with the forecast models). It is overrun by steepening lapse rates, as the approaching short-wave trough lifts and vertically stretches the air. This interaction will likely create neutral to marginally unstable profiles up to 4-5 km height by early Sunday morning, and the formerly stratiform rain shield along the cold front is expected to break up into convective elements in E Germany and W Poland.
Vertical wind shear is more than supportive for storm organization below the mid-level jet streak: 0-3 km bulk shear increases from 10 m/s in central Poland to 30 m/s near the Alps, where 0-3 km storm-relative helicity also peaks above 400 m^2/s^2. The best overlap of all "ingredients" for organized convection is anticipated in the 06 to 14 UTC time frame, as the most active part of the cold front crosses east-central Europe. Linear convection with wind gusts very probably above 25 m/s and possibly, at least locally, above 33 m/s is expected to develop. Fine-meshed models agree on such a scenario.
The cold front will probably be located along a line Wroclaw (Poland) - Prague (Czech Republic) - Nuremberg (Germany) at Sun 06 UTC and will quickly move SE-ward. In the first few hours of this forecast period, the most active line segments with the highest risk of lightning and severe to localized extreme wind gusts will likely follow the Polish / Czech border. By mid-morning, the eastern line segments will gradually weaken and the most active parts should shift across the E and S Czech Republic into E Austria, W Slovakia, and finally Hungary, following the path of the strongest synoptic forcing.
The area to the east of the Alps becomes a particular focus. As the westerly flow increases ahead of the cold front, diurnal heating and westerly downslope winds (which may easily become severe themselves, but are not included into the ESTOFEX threat level scheme) may push the temperatures temporarily close to 20°C in the late morning. The deeper and drier boundary layer further increases the risk of severe downbursts as the convective line arrives. In addition, a pocket of westerly 850 hPa winds up to 40 m/s (!) overspreads this area exactly in the time frame when the cold front is expected to arrive - e.g., around 10 UTC in Vienna and Bratislava, 11 UTC in Budapest and 13 UTC at the Hungarian / Serbian / Croatian border triangle.
All in all, the setup appears menacing enough to issue a confined level 3 area. Prior to the cold front's arrival, the formation of a "dryline" boundary between poorly mixed, more humid air to the east and warmer, drier Foehn subsidence to the west should be monitored! If it appears, it would mark a preferred path for the axis of the most extreme wind gusts.
Convection will gradually weaken in Croatia, Serbia and W Romania in the late afternoon, as both the strong wind field and the synoptic lift start to disintegrate.

... North Sea, Baltic Sea and downstream areas (postfrontal convection) ...

Well-mixed polar air moves southward behind the cold front. CAPE on the order of 100 J/kg may form where the fetch over the North Sea or the Baltic Sea is long enough. Scattered showers are expected. Low-topped multicells will likely be the dominant convective mode.
Another sharp surface trough follows ~6 hours after the cold front and crosses E Germany, SW Poland and the Czech Republic until 15 UTC. The wind field is strong enough to support a second round of widespread wind gusts between 25 and 30 m/s with the passage of this feature, but it is rather unlikely that convection will already grow deep enough to produce lightning by then. Later in the day, the chances for (weakly) electrified convection increase a bit, but at the same time the wind field gradually relaxes, though 0-1 km shear still remains enhanced due to frictional effects. All in all, a level 1 seems to be a proper solution for a conditional risk that deep convection might be involved in some of the expected severe wind gusts.

A side note for all highlighted areas: Though severe straight-line winds are clearly the dominant risk, one or two tornadoes are not ruled out in case particularly strong, discrete updrafts manage to form either within the cold front or in the postfrontal convection. 0-1 km shear is exceptionally strong (>20 m/s) near the cold front, and surface friction still keeps it enhanced (>10 m/s) in the postfrontal air mass.