ECCE Weather Club - Forecasting heavy snowfall on the plains: a tricky forecast. The case of the Caetano snowstorm the 21st and 22nd November 2024.

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Forecasting heavy snowfall on the plains: a tricky forecast. The case of the Caetano snowstorm the 21st and 22nd November 2024.

While forecasting snowfall in the mountains is fairly easy and the expected snow is welcome, on the plains the forecast becomes particularly uncertain and its arrival dreaded because of the systematic chaos it causes on the roads, but also in rail and air transport. In the mountains, as long as the infrastructure and roads are not threatened by exceptional avalanches, a warning of heavy snowfall level 2 with 20 to 40 cm/24h or a level 3 warning with an accumulation of 40 to 60 cm will be seen as profitable by mountain tourism professionals. We may sometimes be criticized for having forecasted the rain-snow limit too low, resulting in rain where snow was forecast. In the mountains on the other hand when accumulations is approaching 100 cm in 1 or 3 days, forecasting becomes trickier and the potential impacts more serious. On the plains, the impact of the snowfall is becoming significant with low accumulations of fresh snow. If a front that gives 2 to 5 mm of rain in 12 hours, is considered to be normal, the same front in winter giving 2 to 5 cm of fresh snow, with locally 10 cm, will often create considerable chaos for all types of transport.

The Analysis of the CAETANO storm, which gave 20 to 40 cm of fresh snow on the northern Swiss plateau and in the central Valais, and only 0 to 3 cm towards the Lake of Geneva basin, give an opportunity to review the factors to be taken into account when forecasting snowfall on low land (type of front, air mass involved, absence or strengthening of the wind, intensification of precipitation and isothermal phenomenon, cold air lake effect, snow lake effect on the Lake Geneva basin, road temperature, case studies, online observation and nowcasting, use of crowdsourcing, weather conditions before and after the snow event, etc.. .)