Storm Ana caused widespread severe weather across Europe, from high winds to heavy snowfall, in the second week of December 2017.
By Andreas Wirth (ZAMG) and Jochen Kerkmann (EUMETSAT)
On 11 December winds of over 160 km/h were reported in western France, near La Rochelle. Spain and Italy were also slammed by the strong winds, with heavy rain and snow affecting many parts of western Europe.
By 12 December the centre of the storm was now heading northeast, bringing further severe weather to northern parts of Europe. Winds of up to 90 km/h were recorded in Poland, and heavy snow was reported in Finland.
Ana's progress can be clearly see on the animation of Meteosat-10 Airmass RGB imagery, 9 December 00:00 UTC–11 December 06:00 UTC (MP4, 1 MB).
A classical rapid cyclogenesis associated with Ana induced a Föhn storm over the Alps on 10 and 11 December 2017. Ana advected moist and warm air from the Mediterranean region into middle Europe. This resulted in rain and snowfall south of the Alps on one hand and a very strong Föhn storm on the other.
Wind speeds up to 200 km/h on mountain peaks were measured in Salzburg and the southern parts of Austria. Because of the relatively warm temperatures, most of the precipitation fell as rain causing local inundations.
Stages of development
Sunday 10 December 2017, 06:00 UTC
The development of the rapid cyclogenesis started on Sunday 10 December at 06:00 UTC and was relatively unimpressive, with the appearance of a wave bulge at the rear side of a west to east bound cold front (Figure 1, above right, click to expand).
While the surface pressure only showed an almost unnoticeable weak trough, the red colour of the wave bulge in the SEVIRI Airmass RGB hits the meteorologist eyes. This colour is uncommon in frontal waves; it is a clear sign for overrunning cold air sinking from higher atmospheric levels — a first indication for a very dynamic development.
This first guess was confirmed by the position of the cloud bulge in respect to the jet; a positive vorticity advection at 300 hPa, was located right above this cloud feature, to the left.
Sunday 10 December 2017, 12:00 UTC
Six hours later on the Airmass RGB the classical signs of a developing Rapid Cyclogenesis can be seen — a comma-shaped cloud feature at the rear of a frontal cloud band which was still overrun by stratospheric air (indicated by the red colour).
The surface pressure started to drop dramatically and cold air advection increased behind the surface cold front. The vertical cross section clearly shows that the surface front was associated with the comma-like cloud feature north of the broad cloud band (see images below).
The broader cloud band in the south was an upper cold front, also known as a 'Split Cold Front' due to the visible separation of the upper front from the surface front in satellite imagery. The vertical cross sections (Figure 3 and 4) indicate that the region of the surface cold front was highly unstable — warm air advection in lower levels superimposed by cold air advection above 600 hPa.
Sunday 10 December 2017, 18:00 UTC
The pressure in the centre of the low Ana dropped to 974 hPa. The cloud head started to transform into an occlusion, cyclonic vorticity increased, and the height of the PV=1.5 surface showed a distinct minimum in the red area of the Airmass RGB (Figure 5).
This was now the peak phase of the rapid cyclogenesis. The position of the jet axis ran along the rear side of the cold front and the dipole of cold and warm air advection was still very strong.
Monday 11 December 2017, 00:00 and 06:00 UTC
The cylogenesis process came to an end, the pressure minimum in the centre of low Ana dropped further to 958 hPa. The Airmass RGB (Figure 6) shows a classical frontal system, a cold conveyor belt occlusion that developed out of the cloud head.
The PV=1.5 minimum lay within the low centre, the jet axis split into two branches, one at the rear side of the cold front and the other ahead of the warm front.
The formation of low Ana goes hand in hand with the typical stages of a rapid cyclogenesis. The very first indication was the cloud head at the rear side of a frontal cloud band.
This wave-like bulge was different from a classical frontal wave insofar that it showed as a red colour in the Airmass RGB.
This is the indication for sinking dry stratospheric air, a process which makes this situation so explosive because it destabilises the lower atmospheric levels.