A storm system expected to move in to the Midwest Thursday and Friday could drop rain or snow in Wisconsin just as folks head out on roads for the Christmas holiday.
Though it's too early to forecast just when it will hit and what kind of precipitation will fall in southern Wisconsin, the National Weather Service on Monday cautioned people to take the storm into account when making holiday travel plans.
A low pressure system passing to the south of Wisconsin will suck up moisture from the Mississippi River valley area and depending on the path — which is still in flux — folks in southern Wisconsin could see rain, a rain-snow mix or snow. When it will hit on Thursday into Friday is still unknown.
"Right now, there's not a ton of clarity because we're so close to where (the low pressure system is) tracking and because of that the temperatures can vary by a fair amount," said Sean Miller, a National Weather Service meteorologist based in Sullivan.
Thursday is the first day of winter.
It's possible that the Milwaukee area could start out with rain and then transition into snow. Conditions could change drastically over an area of 50 miles or less — that's a big deal when the storm track could vary by more than 150 miles.
The National Weather Service noted that a few degrees difference in temperature at 5,000 feet in the atmosphere will determine what type of precipitation will drop — 32 degrees will mean snow, 34 degrees is likely sleet and 36 degrees or warmer is rain.
The storm system headed this way "tends to bring more precipitation than the weaker Clipper systems we've had more recently," said Miller. "These tend to have more moisture and can be more rambunctious in producing more precipitation."
More accurate forecasts of the winter storm will be available Tuesday and Wednesday as conditions become more clear.
Miller pointed out that no matter where the storm system tracks, it's likely some area of Wisconsin will get hit — something holiday travelers should consider.
"Some portion of the state will see a decent amount of snowfall, several inches. That will be important for folks traveling," Miller said. "Right now, it seems like that might be central Wisconsin but that can easily shift north or south."
Right behind the storm system, a mass of cold air is expected to move in to Wisconsin courtesy of a large and very strong Arctic area of high pressure. Temperatures are forecast to plunge Christmas Eve into Christmas Day, with the mercury dropping to zero or below.
Just how low temperatures will drop depends partly on whether snow falls on Thursday and Friday since a snow pack will cool conditions even more, said Miller.
The last time Milwaukee shivered in subzero temperatures was Dec. 19, 2016, when a low of 7 below zero was recorded. The normal high and low temperatures in Milwaukee this week are 32 and 19.