Ag Weather Forum

It felt like summer left a large majority of the country last week as fall-like temperatures spread through most of the country east of the Rockies. Humidity dropped as well, and temperatures have been largely comfortable since. Another pair of cold fronts are moving down through North America this week and will reinforce the cooler air. But will it be cold enough for a frost in some locations?

The potential is certainly there. The fronts are tapping into air directly from northern Canada where early morning temperatures have been below freezing already. One front is already working through the Central Plains and Upper Midwest on early Wednesday morning. Temperatures broke below 40 degrees Fahrenheit along the Canadian border and across Saskatchewan on Wednesday morning. The second is forming over northwestern Canada and will be quick to move down into the U.S. on Thursday and Friday.

Temperatures usually moderate on their trip southward, picking up stored heat from the ground and getting a boost from a higher sun angle to bring temperatures up a bit. But there may still be some potential for breaking down into the 30s for morning lows through Sunday morning, Sept. 7. The areas most at risk would be across the Dakotas, Minnesota and Wisconsin. Parts of northern Iowa and Illinois would also be at risk if the right conditions occur, and Michigan may see temperatures below 40 F on Sept. 7-8.

To get temperatures that low this time of year requires not only the access from the cold, arctic air in northern Canada, but also clear skies at night and calm winds. The fronts will be producing showers and cloud cover in their wake and that will complicate the forecast. Calm winds will be tough to come by between the fronts on Thursday morning, or directly behind the second front on Friday morning, but winds are likely to diminish during the weekend with high pressure settling into the region.

Spots that are prone to being colder than the surrounding areas, including small dips between hills or in river valleys would be most at risk. Given the complications and time of year, the risk for a frost is still pretty low, but not zero.

If frost does occur, it would be significantly early. Though frosts have occurred this early in parts of the Northern Plains and Upper Midwest before, it is very rare. The median date for the first frost is in the last few days of September or early October across the Dakotas, Minnesota, and Wisconsin, and early-to-mid October across Iowa and northern Illinois. You can find more specific information here: https://www.ncei.noaa.gov/….

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John Baranick can be reached at john.baranick@dtn.com

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