Nearly-record snowfall in the western Alps

By Natasa Strelec Mahovic and Jochen Kerkmann (EUMETSAT) and Ivan Smiljanic (CGI)

Winter 2020 started quite abruptly, with up to 3m snow in some parts of the Austrian and Italian Alps. It mostly fell during the weekend of 5-6 December, but there were also some heavy falls in the following days, on 8 and 9 December.

The Austrian Met Service initially issued a red snow warning for the south-western parts of Austria over the weekend, followed by another one for Tuesday and Wednesday (Figure 1).

The snow came in two bouts. The first episode, occurring during the weekend, was stronger, influencing the western part of the Alps. The second one, on Tuesday and Wednesday, brought snow to central and eastern Alps, but not as abundant as the first one.

The Airmass RGB loop in Figure 2 shows the movement of the frontal system across the Alps, and later the formation of the new cyclone in the Mediterranean that brought new precipitation from 8-10 December.

The storm on 5-6 December was caused by a cold front connected to a cyclone with a centre over north west France. While the surface, as well as upper-level, low pressure centres remained stationary over north west France and the Channel, the trough at 500hPa was strengthening ('digging' southward) with the trough axis moving to the east (Figure 3).

A strong south-westerly flow brought humid Mediterranean air to the Alpine slopes. With the turning of the trough axis, this atmospheric river flow turned from south-westerly to southerly, spanning from North Africa all across the Mediterranean, and later to south-easterly, fetching additional humidity over the Adriatic Sea on the 6 December, as seen in the ECMWF forecast Total Column Water content (Figure 4).

The loop of MIMIC-TPW product images, made from retrievals using AMSU-B and MSU data from NOAA-18, NOAA-19, Metop-A and Metop-B satellites and ATMS of Suomi-NPP, reveals that the primary source of humidity was in the Atlantic. Entering upper-level trough circulation, it was additionally intensified, picking up the humidity above the Mediterranean. The Total Precipitable Water content was highest over southern Italy and the Adriatic Sea on 6 December (Figure 5).

Due to upper-level flow being perpendicular to the Alpine mountain range, in the 500hPa height field, a small shortwave trough formed on the southern edge of the western Alps (Figure 6). Shortwave troughs force small areas of horizontal divergence and enhance rising motion. In the field of vertical velocity at 700hPa in Figure 6, upward motion can clearly be seen on the southern side of the Alps (red isolines). Note the sinking motion on the lee side causes clouds to dissolve.

Precipitation often gets concentrated near a shortwave trough, and moves along with it. In this case, the prolonged period of heavy snowfall (rain in lower terrain), was caused by the constant flow of very moist air towards the slopes and the shortwave trough, together with maximum of rising motion, remaining quasi-stationary on the southern slopes of the western Alps.

The loop of the NWC SAF Convective Rainfall Rate (CRR - Ph) hourly accumulations clearly shows how the precipitation on the evening and night from 5-6 December was concentrated over northern Italy and western parts of Austria, with hourly amounts of precipitation estimated to be larger than 30mm/h and continually occurring in the same area (Figure 7). Regions of East Tyrol and Carinthia were particularly badly hit, locally receiving 370mm of precipitation. Heavy rainfall caused many mudslides and raised avalanche risks, in some areas to highest, level 5.

Only after 10 December did the skies clear enough to allow for a comparison of the snow extent over most of the Alps before and after the consecutive snow events. This comparison is shown in Figure 8. Snow appears in cyan hues on the HRV Fog RGB (RGB NIR1.6, HRV, HRV). Due to the high resolution of the visible image, the topography of the Alps with snow-free valleys is clearly highlighted. It is interesting to note, by the end of November 2020, there was much less snow over Alps than in previous years around that time.