Nearly-record snowfall in the Western Alps in December 2020

Nearly-record snowfall in the western Alps in December 2020

4 December 2020 00:00 UTC-10 December 09:00 UTC

Nearly-record snowfall in the Western Alps in December 2020
Nearly-record snowfall in the Western Alps in December 2020

After a rather dry November, the meteorological winter of 2020 started with more than 2 m snow in the Alps and an extreme amount of rain.

Last Updated

16, December 2020

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

Winter 2020 started quite abruptly, with up to 3 m 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).

Meteoalarm warnings
Figure 1: Meteoalarm warnings for 6 and 8 December 2020

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.

Figure 2: Meteosat-11 Airmass RGB loop from 4 December 00:00 UTC to 10 December 09:00 UTC

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 500 hPa was strengthening ('digging' southward) with the trough axis moving to the east (Figure 3).

Figure 3: Meteosat-11 Airmass RGB image overlaid with 500 hPa height, loop from 4 December 06:00 UTC to 7 December 06:00 UTC. (Credit: EUMeTrain)

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).

Total Column Water content forecast for 5 Dec 00 UTC, 06 Dec 06 and 12 UTC
Figure 4: Total Column Water content forecast for 5 Dec 00:00 UTC, 6 Dec 06:00 and 12:00 UTC, showing the atmospheric river bringing moisture from Mediterranean and Adriatic Sea towards the Alps. (Credit: ECMWF)

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).

Figure 5: MIMIC TPW product 3-hourly loop from 4 December 06:00 UTC to 9 December 15:00 UTC. (Credit: University of Wisconsin-CIMMS)

Due to upper-level flow being perpendicular to the Alpine mountain range, in the 500 hPa 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 700 hPa 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.

Meteosat-11 IR 10.8
Figure 6: Meteosat-11 IR 10.8 image overlaid with 500 hPa height in cyan colour and omega (vertical velocity) field at 700 hPa in red (rising) and dark blue (sinking), 5 December 00:00 UTC. The axis of the small short-wave trough is marked by yellow dashed line.

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 30 mm/h and continually occurring in the same area (Figure 7). Regions of East Tyrol and Carinthia were particularly badly hit, locally receiving 370 mm of precipitation. Heavy rainfall caused many mudslides and raised avalanche risks, in some areas to highest, level 5.

Figure 7: NWC SAF Convective Rainfall Rate based on Cloud Physical Properties (CRR-Ph product) hourly accumulation loop from 5 December 14:00 UTC to 6 December 09:00 UTC

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.

Meteosat-11 image comparison

Meteosat-11 HRV Fog RGB, 13 December 12:00 UTC compare1
compare2
 

Figure 8: Meteosat-11 HRV Fog RGB on 26 November 2020, 12 UTC (left) and 13 December 2020, 12 UTC (right). Snow cover is depicted in bright cyan hues.

The Mediterranean cyclone was strong enough to raise dust in northern Africa, which is quite unusual for the month of December (Figure 9).

Meteosat-11 DUST RGB image on 09 December 2020, 09 UTC
Figure 9: Meteosat-11 Dust RGB, 9 December 09:00 UTC

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