Difficult: The roadway is completely covered with precipitation. Accumulation is to the point that the roadway markings are completely obscured making it difficult to differentiate between the roadway and its surroundings. These conditions make travel very difficult. This is a hazardous environment. Some weather conditions can be severe enough that roadway treatments, such as salt or sand, will not work. Emergency vehicles and snow removal equipment may be called off of the roadway due to the conditions. In the event of an emergency, it may be impossible to get emergency vehicles to a scene.
My mother thinks I'm making this up to ruin her Christmas Eve. There is a special place in Hell for Norman Rockwell.
OTOH, I'm sure the kids would rather see Aunt Heather show up on Christmas Day than not at all. Aunt Heather certainly prefers to drive tomorrow instead of spending the night in a motel room in Dyersville waiting for Friday to roll around to get her car towed/fixed (Aunt Heather has already totalled enough cars on U.S. 20 for one lifetime).
So, what's there to do in Milwaukee on Christmas Eve Day in the snow?