DULUTH, MINN. — The first frost of the season is expected tonight across parts of Wisconsin, Minnesota and the Upper Peninsula of Michigan, with BAM Weather warning that clear skies and calm winds overnight will allow ground temperatures to drop cold enough for frost even though air temperatures are expected to stay above freezing.
First Frost of the Season Arrives Tonight
Tonight marks a notable seasonal milestone for parts of the Upper Great Lakes, with the first frost of the fall expected across a wide swath of northern Minnesota, northern Wisconsin and Michigan’s Upper Peninsula. Low temperature projections for 7 a.m. Monday, September 14, show widespread readings in the low to mid 40s across the highlighted frost zone, cold enough to allow frost to form despite air temperatures staying technically above freezing.
For gardeners, farmers and anyone with sensitive outdoor plants across this region, tonight represents the first real test of the season for cold-sensitive vegetation that has made it through summer.
Why Frost Can Form Even When Air Temps Stay Above Freezing
BAM Weather specifically notes that while air temperature is expected to remain above freezing tonight, ground temperature will run colder due to calm winds and clear overnight skies. This happens because of a process called radiational cooling, where the ground surface loses heat rapidly into a clear night sky, allowing surface temperatures to drop several degrees below the air temperature measured at the standard height of about 5 feet.
Calm winds play an equally important role in this process, since any breeze would mix the colder air near the ground with slightly warmer air above it, preventing the surface from cooling as dramatically. Tonight’s combination of clear skies and calm winds creates ideal conditions for this kind of ground-level cooling, even with air temperatures reported in the low to mid 40s.
International Falls, Ely and Rhinelander See the Coldest Readings
Among the specific locations included in tonight’s forecast, International Falls and Ely are both projected to see lows around 41 degrees, while Rhinelander is also expected to bottom out near 41 degrees. Virginia, Wausau and Marinette are each projected in the low to mid 40s as well, placing these communities squarely within the zone where frost is considered likely by early Monday morning.
These readings, all comfortably within the range where ground-level frost becomes likely under clear, calm conditions, represent some of the coldest spots within tonight’s highlighted frost zone.
The Frost Likely Zone Stretches Across Northern Minnesota, Wisconsin and the UP
The area BAM Weather has specifically labeled as carrying a likely frost risk stretches from around International Falls and Duluth in Minnesota, east across northern Wisconsin near Rhinelander and Wausau, and into Michigan’s Upper Peninsula, including Marquette, Escanaba and Hancock. That broad footprint reflects a widespread first-frost event rather than an isolated cold pocket confined to just one small area.
Communities throughout this zone should treat tonight as their first real opportunity to see visible frost on the ground, car windshields and exposed vegetation as temperatures bottom out in the early morning hours.
Protecting Plants and Crops From Tonight’s Frost
With frost likely across this stretch of the Upper Great Lakes, BAM Weather specifically cautions that plants and crops remain vulnerable to damage tonight, encouraging residents to take steps to protect sensitive vegetation as needed. Common protective measures include covering garden plants with sheets, blankets or specialized frost cloths, bringing potted plants indoors overnight, and watering plants before nightfall, since moist soil holds heat better than dry soil.
For farmers with crops still in the field, particularly those sensitive to frost damage, tonight’s conditions across the highlighted zone warrant close attention as harvest season continues across the broader region.
Minneapolis and Points South Stay Milder
While the frost zone covers a large stretch of northern Minnesota and Wisconsin, communities farther south, including Minneapolis, Saint Cloud, Lakeville, Rochester and Winona, are all projected to see low temperatures around 48 degrees, comfortably outside the range where frost becomes likely. That contrast illustrates how localized tonight’s frost risk actually is, concentrated specifically across the northern tier of the region rather than affecting the broader Upper Midwest uniformly.
Residents in these milder, southern communities can expect a chilly but frost-free morning, even as their neighbors farther north wake up to the season’s first visible frost.
