Change to winter tyres
From 1st November to 15th April the following year there is a weather-dependent winter equipment rule. Winter tyres must be fitted on all wheels for snow-covered roads, slush or ice.
If you drive in such conditions without winter tyres you risk getting a 60 Euro fine. If any other vehicles are put at risk this can theoretically rise to 5,000 Euros.
Further details are available in German here.