Snow is expected to begin Sunday afternoon and spread north, possibly mixed with some sleet in parts of the area near and south of Interstate 80. As the center of low pressure tracks across southern Illinois into central Indiana, snow will increase during the evening hours. A little before midnight will begin the period when snow will fall the hardest in the Chicago area, perhaps as much as an inch an hour in some areas. Travel conditions will deteriorate rapidly and roads will become slick and hazardous. By early Monday morning rush hour, snow totals will probably range from 3 inches far south to over 6 inches on the north side of the metro area. This will be a heavy wet snow with storm total water equivalent in excess of an inch possible.
Cold air follows
As the storm moves east, snowfall will taper off later Monday morning with light snow or flurries continuing into the evening. Storm totals will probably range from 3 to 5 inches south to over 10 inches north. Behind the storm, with a strong northerly jet stream in place, frigid arctic-source air will flow south with the southern edge of this cold air mass pouring over the Canadian border into the Midwest. High temperatures in the middle 20s will be about the best Chicagoans can look forward to Tuesday through Thursday. A cold front could give a period of snow Tuesday night into Wednesday.
An office debate is raging about the meaning of wind chill temperatures and we are turning to you for clarification. When the temperature is 33 degrees and the wind is creating a wind chill of 23 degrees, will that cause water to freeze?
It will not. Wind chill is perceived air temperature, not actual air temperature. Air removes heat from any object warmer than the air itself, and moving air (wind) removes heat more rapidly. Wind chill describes the rate at which heat is lost from our skin, not the temperature to which skin will be cooled.
Wind chill is the fictitious temperature of still air that would remove heat from our skin as quickly as the existing combination of air temperature and wind is actually removing it. Air at 33 degrees cannot cause water to freeze, regardless of a wind chill temperature well below water’s freezing point of 32 degrees.
Chicagoans are facing the prospect of the second major snowstorm this month. The first one officially dropped 12.9 inches of snow at O’Hare and 9.9 inches of snow at Midway. During that storm the Chicago area along with much of the Midwest was deeply entrenched in arctic air and the only real dilemma that forecasters faced was how much snow would fall. Precipitation type was never an issue-the storm would produce ALL snow-the question was how much? Because of the cold environment the snow was light and flurry and quite easy to shovel. Snow to water ratios were in the nature of 14-16:1 meaning that one inch of liquid precipitation fluffed up to 14 to 16 inches of snow.
The approaching storm expected to reach the Chicago late Sunday is quite different than the one that struck earlier this month. In this storm was air has invaded the southern Midwest where high temperatures Saturday reached the lower 60s along the Ohio River in far southern Illinois. This warmer air will be drawn north into the eastern flank of the storm as it approaches and ultimately result in the rain-snow dividing line moving very close to the Chicago area. Not only do forecasters have to determine how much snow will fall, they will also have to determine where the rain-snow line will set up and whether rain, snow or sleet will fall at any given location and when will the change in precipitation type take place. (a daunting task)
Current thinking brings a mixture of rain, sleet and snow into the far southern portions of the Chicago metro area late Sunday and Sunday night which will dramatically cut snow accumulations there. Further north through the city and into the north and northwest suburban areas the precipitation is expected to remain all snow with totals in the 6 to 12 inch range. Unlike the earlier storm, the snow from this storm will be wet and heavy with snow to water ratios more in the 8-10:1 range This wet, heavy snow will cut down on blowing and drifting when the winds pick up, but the snow will be much harder to shovel and caution is advised.
If the storm continues to develop as expected the Winter Storm Watch will likely be upgraded to a Winter Storm Warning for much of the north and central portions of the Chicago area including the city of Chicago. Farther south, advisories will likely be issued depending upon the final nature and amount of precipitation.
The storm should begin in earnest Sunday night and have an impact on both Monday morning’s and Monday evening’s commute.
The WGN Weather Center will continue to monitor this storm as it evolves and report the latest trends as they become more clearly defined.
Steve Kahn WGN-TV/Chicago Tribune Meteorologist
A Winter Storm Watch continues for much of the Chicago area from Sunday evening through Monday. This Winter Storm Watch may be upgraded to a Warning late tonight or Sunday morning.
Confidence remains fairly low in the exact track of this storm, a track that will determine which Chicagoland communities end up with the most snow.
The latest round of computer model guidance arriving at the WGN Weather Center this afternoon continues to suggest that the heaviest snow should fall over the northern and western suburbs. The snow will likely mix with sleet and rain over southern sections, including northern Indiana. Areas that see all snow can anticipate 8″ or more of heavy, wet snow accumulation by late Monday afternoon.
White: Snow Advisory/Winter Weather Advisory
Yellow: Winter Storm Watch
Red: Winter Storm Warning
The last of the snow showers from a weakening storm system should dissipate or exit the Chicago area by 1PM. A few flurries may linger, but no snow accumulation is expected this afternoon or tonight.
Another, much more potent storm is scheduled to arrive on Sunday evening. For more on that system, check out Tom Skilling’s analysis on the WGN Weather Blog.
In addition, the WGN Weather Center Team will provide a steady stream of storm updates throughout the weekend here on the WGN Severe Weather Blog.