Areas above around 1000 m altitude of Canton Ticino recorded a 24-hour accumulation of fresh snow of 120-150 cm on 25 December 2013.
10 March 2021
25 December 2013
by HansPeter Roesli (Switzerland)
Below the 1000 m level similar amounts of rain (measured in mm) were observed. The snow caused blackouts and closed mountain roads, whereas the rain led to a 50 cm increase in the level of Lago Maggiore in 36 hours. These data are absolute records for the month of December in the region.
The three days straddling 25 December showed one of the classical synoptic situations over Europe — a close-to-stationary deep trough over Western Europe, with an elongated undulating cold front just west of the Alps (See animation, 24 December 18:00 – 26 December 18:00 UTC, ). Ahead of the frontal system warm and humid Mediterranean air was pressed against the alpine south-side. The warm-air advection peaked on 25 December with persistent strong winds during the whole day. Notable in particular, the low-level advection along the Po Valley that allowed for the penetration of warm air (Scirocco) even into the deep-cut valleys of the Ticino area, thus keeping the snow line well above the valley grounds (See black box in full resolution image ), wind arrows in red/green/blue for 06:00/12:00/18:00UTC). Normally, warm-air advection at lower levels is much weaker and precipitation cooling in the stagnant air rapidly lowers the snow line to the valley grounds.
The geopotential height at PV=2 matches well the pattern of the high cloud, in particular the cloud head showing up around 18:00 UTC on 25 December (see full resolution ). Also on 25 December, between 06:00 UTC and 10:00 UTC, lee wave clouds show a very distinct arc-shaped back-end over Austria,opening a foehn gap (See red box in full resolution image ). A bit later on the foehn gap uncovers tiny electric-blue orographic clouds over the mountain range south-west of Salzburg. (See red box in full resolution image at 18:00 UTC on 25 December ).
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