"Per ATR "Icing conditions" are expected when air temperature is less than 7 degrees Celsius and there is visible moisture in the air, etc, etc."
Well I would say that would approximate very closely to the weather on that morning.
Living in the same region, Ural weather is known for being unpredictable, - sudden changes in temp & humidity with the different opposing airstreams from polar, european, southerly & siberian esp at equinox.