Winter thunderstorms over the southern side of the Alps

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On 9 January 2018 numerous thunderstorms brought heavy rain, snow, hail and frequent lightning to the south western parts of the Alps.

Date & Time
09 January 2018 09:00–14:00 UTC
Meteosat-10, Suomi-NPP
High Resolution Visible, Infrared Channel

By HansPeter Roesli (Switzerland)

On 9 January strong southerly winds (130–160 km/h over the Alps) advected a Mediterranean airmass beyond the Alps. Being lifted over the southern side of the Alps, it became unstable and released numerous thunderstorms over northern Piedmont and southern Switzerland. Under these skies it felt like late spring rather than the middle of winter.

Figure 1
Figure 1: Lightning strokes over Switzerland, 8 Jan 12:00 UTC to 9 January 12:00 UTC. Source: Lighting Maps

According to a blog by MeteoSwiss (in Italian), the temperature in the valleys was particularly mild (between 7–11 °C), so it was raining, instead of snowing, well beyond 1500 m altitude. However, the snow pack did increase considerably above 2000 m — accumulations of fresh snow in excess of 1 m were measured in less than 24 hours close to the Alpine crests.

The thunderstorm activity was exceptional for January. While isolated electric discharges have sometimes occurred the past during similar winter weather, discharges of such a magnitude have never been registered before, since objective monitoring began in 1981.

The level of intensity of lightning strokes over Switzerland can be clearly seen on the map of lightning strokes from the Lightning Maps website (Figure 1).

Added to this were local coffee-bean sized hail, high winds at low levels, and cloud bursts of nearly 40 mm/h intensity.

The image sequence between 09:00 and 14:00 UTC, of the Meteosat-10 HRV band overlaid by the semi-transparent band IR10.8 (Figure 2), shows the situation around the peak of the activity, which took place in full daylight.

Among many other convective clouds, a line of thunderstorms stood out at 11:00 UTC, just left of the red arrow on Figure 3.

Figure 2: Met-10, 09 January, 09:00–14:00 UTC
Animated gif, HRV with IR.108 overlaid
Figure 3: Met-10, 09 January, 11:00 UTC
HRV with IR.108 overlaid

VIIRS on Suomi-NPP flew over the area at this time. The satellite's high resolution bands showed evidence of a line of four distinct cumulonimbus clouds, rather than of a fuzzy mass of high cloud as seen on the Meteosat-10 imagery.

Figure 4 shows the temperature of the cloud tops in band I5 (IR11.45), while Figure 5, a combination of band I2 (VIS0.86) overlaid with the semi-transparent I5 (IR11.45), goes from white (cold) to yellow (warm), highlighting the cloud structures even more.

Figure 4: Suomi-NPP, 09 January, 11:01 UTC
VIIRS infrared (IR11.45)
Figure 5: Suomi-NPP, 09 January, 11:01 UTC
VIIRS Visible (VIS0.86) with semi-transparent infrared (IR11.45) overlaid
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