Storm Forecast
Valid: Fri 21 Sep 2018 06:00 to Sat 22 Sep 2018 06:00 UTC
Issued: Fri 21 Sep 2018 01:09
Forecaster: PISTOTNIK

A level 2 is issued for NE Germany and NW Poland for severe convective wind gusts.

A level 2 is issued for S Denmark and N-ernmost Germany mainly for severe convective wind gusts and to a lesser extent for tornadoes.

A level 1 is issued from England to SW Poland and the Baltic Sea for severe convective wind gusts.

A level 1 is issued for the W Czech Republic for severe convective wind gusts and large hail.

A level 1 is issued for Sicily for flash floods.

SYNOPSIS

A strong frontal zone from the N Atlantic into the N half of Europe sets the stage for a first series of autumnal storm cyclones. A short-wave trough travels eastward from the British Isles towards Sweden and Poland in the current forecast period and supports an explosive deepening of a surface cyclone over the North Sea. Its cold front finally terminates the longest summer on record in central Europe.
Quiescent, warm late summer conditions still remain in place from Iberia to the entire Mediterranean region, E Europe and Russia.

DISCUSSION

... central Europe ...

An interesting convective scenario is expected with the passage of both the cold front and the main trough axis.

Strong daytime heating will push prefrontal maximum temperatures in the upper 20s, maybe locally even to 30C ahead of the cold front in the SE half of Germany, Poland and the Czech Republic. This differential heating will accelerate the cold front around noon, and a narrow convective rainband will develop and race eastward with a sudden drop of temperature, wind shift and strong to severe wind gusts. Since this band is placed well on the anticyclonic side of the mid-level jet, it is uncertain whether sufficient synoptic lift will make it grow deep enough to produce lightning and, simultaneously, to enhance its severe wind potential due to deeper and stronger downdrafts. Convection-resolving models yet agree on pretty aggressive scenarios with deep convection and widespread severe wind gusts in the 25-30 m/s range in NE Germany and NW Poland in the 12 to 18 UTC time frame, and a level 2 is issued for this area. Further south, a level 1 suffices due to less synoptic lift and lower convective cloud top heights.
Vertical wind shear would be more than adequate for the development of an organized and long-lived convective line with 0-3 km bulk values around 20 m/s in NE Germany and NW Poland and 15 m/s towards the Czech Republic. A close monitoring of the cold front behavior is therefore recommended. Lightning will be a strong indicator that widespread severe wind gusts must be expected! The risk decreases over land after sunset (the cold front will be across central Poland by then) and shifts to offshore areas of the Baltic Sea.

The tail of the cold front in S Germany will be spared be the short-wave trough, which leaves only modest synoptic lift from prefrontal warm air advection. However, this might be compensated by a bit more CAPE, up to 500 J/kg at peak daytime heating. Isolated to scattered, discrete thunderstorms initiation is predicted by convection-resolving models around 15 UTC, when vertical wind shear starts to increase strongly under the approaching jet stream. If indeed storms form (probably starting from Altocumulus clouds) and root down to the surface, they will likely turn supercellular and pose a risk of severe downbursts and 2-3 cm sized hail while they move into the W Czech Republic. The time frame for surface-based convection is limited to a few hours, but a (low-end) level 1 is extended into Bohemia to account for that scenario.

After a few hours of subsidence induced by strong cold air advection, a new round of convection is expected well behind the cold front. It will probably start over England and the North Sea around noon and move onshore in the Netherlands, N Germany, Denmark and S Sweden in the evening, possibly in form of one or several comma-like features. Vertical wind shear decreases behind the cold front, but the northern parts of the deepest unstable air partly overlap well with the most intense wind field at the rear flank of the storm cyclone. Widespread wind gusts in excess of 33 m/s are likely overnight in coastal areas of Germany, in Denmark, S Sweden and later on the S Baltic Sea! Please remember that ESTOFEX only addresses the risk of convective, but not non-convective hazards. The northern half of the exposed area is therefore not included into the threat levels. Its southern half, in contrast, is covered with a level 2, as there is a limited but still noteworthy chance that deep, electrified convection will form with any of these commas and further contribute to severe to extreme wind gusts at the surface. In addition, one or two tornadoes are not ruled out in the onshore part of the level (i.e, near the border Denmark/Germany), where surface friction keeps low-level shear enhanced to 10-15 m/s.

... Latvia, Estonia, S Finland into NW Russia ...

A plume of enhanced low-level moisture is picked up by the fringe of the frontal zone and advected northeastward. Several central European soundings from Thursday showed CAPE up to 600 J/kg (e.g., Warszawa) in this plume. A similar amount of CAPE is expected again on Friday under mostly weak vertical wind shear, increasing only towards the north.
Vertical profiles of this CAPE will be long and "skinny" according to observed and forecasted soundings. Dry mid-level air and a lack of synoptic lift are other factors detrimental for convective initiation, hence this unseasonable CAPE reservoir will likely remain untouched. Since no other forecast model than GFS shows precipitation signals, no lightning areas are issued.

... central Mediterranean ...

A subtropic warm-core cyclone has formed beneath a decaying cut-off low over the Tyrrhenian Sea, feeding on some hundred, locally up to 1500 J/kg of CAPE (see Thursday's Pratica di Mare sounding) under little to no capping and almost zero vertical wind shear. It is predicted to slowly fill on Friday, but scattered thunderstorms will likely go on and circle around the cyclone's center throughout the forecast period. Most of the convection will stay offshore, apart from the E side of Corsica in the first half and N Tunisia in the second half of the forecast period, hence the risk of heavy rain is too limited for a level 1.

In the afternoon, scattered orographic storms will also form over S Italy and parts of the W Balkans, most notably Bosnia-Herzegovina. Plenty of low-level moisture is fed into upslope circulations and these almost stationary storms, therefore one or two flash floods are possible. A level 1 is issued for Sicily, where this risk appears highest.