Midwest braces for first frost/freeze of the season

Frost and Freeze Warnings are up for parts of Minnesota and Western Wisconsin through Wednesday morning as a Canadian high pressure system calms and cools the atmosphere. Residents of Northern Minnesota are most likely to wake up to frosty car windshields and wilted flowers. However, frosty conditions may be seen in rural areas as far south as Northern Iowa and Northern Illinois.

Some forecast low temperatures for Wednesday morning:
Brainerd, Minnesota...31°
St. Cloud, Minnesota...33°
Rhinelander, Wisconsin...34°
Wisconsin Dells, Wisconsin...37°
Waterloo, Iowa...37°
Rockford, Illinois...39°

So, how does frost form when the air temperature is above freezing? Even if the air temperature is slightly above freezing, surface temperatures may be at or below freezing. When water vapor in the air comes in contact with these surfaces, they form ice crystals.

Rural valleys are most likely to experience frost. Cities and towns gobble up more heat during the day and give it off during the night. Hilltops are also warmer as they are exposed to more sun during the day. Click on any of the images to view fullscreen. -ERIC