The strongest cold front since last spring will sweep cool air from Canada into much of the U.S. late this week, giving some northern areas including the Northern Plains and the Midwest a first frost by the end of the week.

A large area of high pressure will build south from northern Canada this week, and cooler air will pour into the Northern Plains on Wednesday, before spreading into the Midwest and Northeast U.S. by week's end. But areas from the eastern Dakotas to Minnesota, northern Iowa, western Wisconsin, and Michigan will get the brunt of chilly air. Combining with forecast clear skies, the areas could get a first frost.

In Jackson County, Mich., for instance, the coldest temperature of the late summer season so far has been 47 degrees. But by Friday morning the area is likely to reach the freezing mark, with widespread frost and patchy frost. While frost by mid-September isn't the norm, it isn't that unusual. The area has recorded many daily lows since record keeping in the 20s in mid-September over the years.

Areas east of Chicago will face some risk of frost by week's end as the cold air spreads, but the odds aren't as high.

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, Dale Mohler, agricultural meteorologist at Accuweather, said in a report on the service's Web site.

The official start of fall remains nine days away, yet the cool Canadian air will have a widespread impact through nearly two-third of the U.S. beginning late Wednesday and into the weekend. The cold front will drop through the Northern Plains and Great Lakes, increasing winds with some chilly showers as it moves through.

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