The maps "new snow" are published daily from 1 November to 30 June and are updated several times a day. There are three different time ranges for the fresh snow maps: 24 hours, 48 hours and 72 hours. The selected time range can transition seamlessly from the past to the future and can be set anywhere between 3 days in the past and 3 days in the future (this feature is currently only available in the Whiterisk app). Time ranges that extend over more than 24 hours are calculated from the sum of the 24-hour fresh snow depths.
The 24-hour review of the current day is compared with measurement data from the current day for the first time at 8.30 am and improved. Up to this point, the fresh snow maps for the current day are based purely on model data. At 10.30 a.m., 4.30 p.m. and 10.30 p.m., the fresh snow analysis is updated again with the measurement data supplied in the meantime.
The new snow maps are based on the snow modelling of the operational snow-hydrological service of the SLF (OSHD). The OSHD model system uses high-resolution meteorological input data from weather forecast models and various reanalysis products from MeteoSwiss. In forecast mode, the new snow model uses data from the COSMO forecast. In addition, the new snow model is operated in analysis mode, whereby an optimal transition between analysis and forecast data is made possible by using precipitation from the nowcasting product INCA (Integrated Nowcasting through Comprehensive Analysis). As part of the operational modelling system, the new snow model is updated each time new forecast and analysis data from COSMO becomes available.
The following model steps lead to differences in the quantity and spatial distribution of the modelled fresh snow and the COSMO precipitation analysis and forecast:
- Downscaling of the meteorological input data from COSMO from 1 km to 250 m resolution. Radiation information is dynamically adjusted using a 25 m resolution terrain model in order to take topographical influences (shading, slope inclination) into account. For wind, topographical information is used to take into account the influence of the local terrain on the wind.
- Division of total precipitation from COSMO into solid and liquid form using air temperature data.
- Altitude-dependent calculation of fresh snow density taking into account air temperature and wind speed. This function was optimized by calibration based on measurements of the density of freshly fallen snow that has settled over 24 hours.
- Assimilation of snow measurement data from around 440 automatic weather stations and observers.
- Physical consideration of various snow processes, such as a) settling of new snow, b) melting of new snow in the event of significant warming or rain, c) melting of new snow when it falls on apery and still warm ground.
The data is modelled for the whole of Switzerland with a resolution of around 250x250 m and then smoothed for display. A time course of the data from individual measuring stations can be viewed on the measured value map.