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Frost Chances Overnight Threaten Growing Season

Now is the time to protect sensitive plants.
A First Alert Day has been issued into Friday morning for cold temperatures leading to frost...
A First Alert Day has been issued into Friday morning for cold temperatures leading to frost formation.(WMTV)
Published: Oct. 21, 2021 at 9:33 PM CDT
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MADISON, Wis. (WMTV) -What a difference a day makes across southern Wisconsin! From late season warmth in the 70s on Wednesday to the coldest air of the season spilling in Thursday night. The seasons first widespread 30 degree temperatures and frost will threaten the growing season as we wrap up the week. This has prompted a First Alert Day here at NBC15 as we prepare for the colder temperatures.

A Frost Advisory has been issued for all of southern Wisconsin late Thursday night into early Friday morning. Overnight lows are expected to dip down into the middle and upper 20s to the north and the lower and middle 30s to the south. Widespread frost is anticipated across the area with localized areas experiencing a killing freeze.

FROST ADVISORY issued for southern Wisconsin late tonight into Friday morning. Frost and freezing conditions could damage or kill sensitive vegetation left outdoors unprotected. Cover up or bring inside any plants that are vulnerable to the cold conditions.

Posted by Meteorologist Brian Doogs NBC15 on Thursday, October 21, 2021

You may be wondering how frost can form in areas that don’t officially fall below 32 degrees. The answer lies in in the fact that the official temperature is measured several feet off the ground. Right at ground level early in the morning, the temperature can run a few degrees cooler. That will put most places at or below the freezing mark and lead to widespread frost formation.

Ideal frost formation requires light winds, clear skies, and temperatures below 37 degrees. The wild card tonight will be some lingering cloud cover, especially south of Madison. Should some clouds linger a littler longer, temperatures may be a touch warmer near the stateline.

FROST CONCERNS: Temperatures tonight will be on either side of freezing, but everyone stands the chance of frost. The...

Posted by Meteorologist Brian Doogs NBC15 on Thursday, October 21, 2021

While widespread frost is likely into Friday morning, a widespread freeze is anticipated into early Saturday morning. This will likely end the growing season for the rest of Wisconsin and has prompted another First Alert Day through early Saturday. It is that time of year where colder temperatures become more and more likely and a reminder that keeping that ice scraper in your car might be a good idea.

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