The chilliest air since last spring will sweep into part of the northern U.S. and neighboring Canada, bringing the first frost of the season to some locations.
A large area of high pressure will build southward from northern Canada and will roll into the northern Plains, Midwest and Northeast U.S. this week.
The much cooler air will first pour into the northern Plains and Canadian Prairies today into Tuesday and will spread into the Northeastern U.S. during the second half of the week.
The forecast magnitude of the chilly shot and the overall high pressure area is a little early for the time of year but has happened before.
The average date of the first frost or freeze is just that, an average.
According to Agricultural Meteorologist Dale Mohler, the greatest risk of damaging frost is over the Upper Midwest.
"The core of the chilly air and necessary clear skies and light winds for frost lies from the eastern Dakotas to much of Minnesota northern Iowa, western Wisconsin, southern Manitoba and part of Northwestern Ontario Wednesday night and Thursday night," Mohler said.
"Farther east, there is still some risk of frost, but the number of hours necessary to damage tender plants and vegetables will be much less and temperatures will be very marginal," Mohler added.
Mohler was referring to areas east of Chicago to the mid-Atlantic and New England.
In general, the frost will not be heavy enough to kill problem weeds and grasses for allergy sufferers.
The warm waters of the Great Lakes modify temperatures this early in the season. So when an air mass moves in from southern Canada and across the northern Plains, it passes over the Great Lakes, picking up a bit of warmth.
"There will also be cloud cover issues and perhaps some wind working against frost farther east," Mohler added.
The risk of frost is generally for rural locations and open areas in the suburbs. In the larger cities, including those of the Upper Midwest, the heat given off by pavement and buildings should be enough to ward off the risk of frost.
Forecast low temperatures are made for a height about six feet above the ground. In clear, calm situations, the temperature nearest the ground, not including pavement or concrete, can be 10 degrees lower or more. This is why we mention frost when actual temperatures are forecast to be above freezing.
It is possible by next weekend that enough cool air is still around, winds diminish and skies clear to allow frost in the normally cold spots of the mid-Atlantic and New England.
The southward push of cool air across the Great Plains, combined with a storm brewing in the Southwest, could initiate areas of rain to some needy locations farther south, including part of Texas, Oklahoma, New Mexico, Kansas and Colorado.