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by HansPeter Roesli (EUMETSAT)
Jump to imagesBetween the 25 and 28 January 2006, on a rather rare trajectory, an upper low moved from Denmark to the Pyrenees and an associated low-level depression formed in the Western Mediterranean. Over the Alps a southerly flow regime set in during 26 January, advecting moist Mediterranean air over the cold stagnant air that covered the lower levels of the Po Valley (a left-over from the prior cold spell). These are precisely the ingredients required for heavy snow fall along the south side of the Alps and, in particular, over Italian Switzerland (Ticino). The snowfall set in at all height levels on the evening of 26 January, but turned progressively to light rain at low levels in the night of 27-28 January. Over an extended period of time, two to three centimetres of snow per hour were deposited at low levels. On the morning of 28 January, in the Ticino area, the snow depth reached 70 to 90 centimetres, and 60 centimetres were measured at the Milan airport of Malpensa.
The two images from Meteosat-8 show the situation around noon on 27 January, at the height of the snowfall. The Airmass RGB depicts the frontal system associated with the low-pressure system. At that time it extended from the south-eastern coast of Portugal to Northern Italy. Strongly subsiding air north of the frontal cloud band is shown with an orange hue. The Dust RGB shows the major associated cloud systems in dark brown-red colours: one close to Portugal and another one over the northern part of the Adriatic Sea. Zooming in on the Alpine area (see Close-up Look, JPG, 291 KB), just north of tihesecond cloud system, another reddish patch (2) delineates the cloudiness (stau clouds) along the south side of the Alps, giving rise to the snowfall. Along its northern border a tiny pink gap (1) marks a gap in the cloudiness (foehn gap) along parts of the northern Alpine slopes.
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