With a winter weather advisory in the forecast, the city of Janesville has declared a snow emergency.  Police say vehicles should not be parked on city streets beginning at 4:00 p.m. Tuesday until snow and ice control operations are complete.  The city warns predicted freezing rain could create very slippery pavement conditions and advise motorists to take extreme caution.  The city does offer off-street parking in several downtown lots including East Wall Street, South High Street, River/Union Street, City Hall, Hedberg Library, and the North Parker Drive Parking Ramp.