Heavy precipitation (snowfalls at higher levels) in north-westerly flow over Switzerland, led to an extreme avalanche risk in the Alps (risk level 5) in January 2018.
By HansPeter Roesli (Switzerland) and Jochen Kerkman (EUMETSAT)
Between 20 and 22 January 2018 a frontal train extended from the Atlantic to the Alps and beyond. Crossing the British Isles, it took a south-easterly track, sending three open warm sectors over the Alps.
This is illustrated by the sequence of Airmass RGB from Meteosat-10, 20 January 2018 00:00 UTC–22 January 23:45 UTC (Figure 2) on which the warm sectors are well marked by extended cirrus shields that travel in rapid succession to the Alps.
Over the Alps very strong warm-air advection occurred on 21 January in the second warm sector, followed by a third, somewhat weaker event, the day after.
Figure 2: Meteosat-10 Airmass animation, 20 January 2018 00:00 UTC–22 January 23:45 UTC
Both events peaked during daytime, so details can be clearly seen in the sequence of black-and-white HRV images, overlaid in semi-transparency by channel IR10.8 (MP4, 7 MB) with warmer temperature in yellow to colder temperature in white.
On both days a classic massive cloud wall (known as Nordstau) got pressed against the northern side of the Alps. Also, on both days the cloud deck not only straddled the Alpine crest, but reached well into the southside, particularly on 21 January.
Rain and sleet were even observed 60 km south of the main Alpine crests in the southern pre-Alps. At Locarno an unofficial rain gauge measured more than 6 mm in nine hours on 21 January. On 22 January another, much shorter and weaker, rain event happened.
The position of the rain gauge is shown as a white point on the VIS0.86/IR11.45 (I2/I5) band combination from VIIRS on S-NPP (Figure 3). The image provides a more detailed (375 m spatial resolution) view of the cloud edge towards the end of the precipitation, when it was essentially wind-driven.
A wind cross-section along the northern side of the Alps (Figures 4 and 5) and the warm advection field at 700 hPa on 21 January at 12:00 UTC (Figure 1, above right, click to expand) show the particular intensity of this event. The most significant areas being the extended warm advection area and the veering wind profiles in the lower troposphere upwind of the Alps, topped by strong (>60 kn) north-westerly winds in the middle troposphere.
On 22 January the meteorological fields (not shown) were similar but less intense, which led to a less interesting event.
Airmass RGB with wind x-section overlaid
22 Jan 12:00 UTC
On the Meteosat-10 Water Vapour image (Figure 6), high level lee waves in cloud free areas over Italy and France, can be clearly seen. Water Vapour images are good tools for detecting lee waves when there are no clouds. See this similar case from 21 January 2005 — Strong North Foehn over the Alps.