When Will Killing Frost Hit Most Of Iowa?

ISU climatologist foresees it hitting at normal time—October 15 to 17 in much of the state.

Published on: Oct 4, 2008

A normal killing frost date will likely occur this fall in Iowa and then we'll have a colder than normal winter. That's what Iowa State University Extension climatologist Elwynn Taylor foresees happening, based on weather indications as of early October. He expects the first 28 degree F killing frost to likely hit around October 15 to 17 over a large part of the state.

It's surprising the weather has gone as late as it has this year, by holding the first killing frost off this fall. That is, considering the unusual weather events Iowa had with wet weather in spring and flooding in June. "There has been some luck involved in getting the nice weather this fall--as well as a change in the general weather pattern," says Taylor. "But it's good that we're looking at near normal frost dates—that's what we are currently expecting. Of course, if the first killing frost would hold off until the end of October, which would be even better."

It's a tale of two crops in 2008

Corn and soybeans planted in a timely fashion in Iowa this past spring are now maturing and being harvested. And, there are fields that were replanted or planted quite late due to flooding and extreme wetness. Those crops still need a first killing frost to hold off longer—so they can have more time to mature.

As of October 3, there is still a lot of corn in Iowa that hasn't yet reached black layer stage or physiological maturity—which is when it is safe from frost. What does Taylor see ahead for temperature in the next couple weeks?

"The normal temperature this time of year in central Iowa is about 71 degrees during the day and 45 degrees at night," he says. "The 45 degrees at night gets soybeans to come to an understanding that it is time for them to mature. When temperatures fall below 50 degrees soybeans get the message it's time for their leaves to change color and for the beans to mature—whether they have a lot of viable grain seed set in their pods or not."

Hard killing freeze occurs at 28 degrees

"We consider a hard freeze—one that really puts a stop to adding weight to corn and soybean harvests—when the temperature hits a low of 28 degrees," he says. "We don't see that happening for awhile yet for most of Iowa. We know it's been cold the past few days. And on this date (October 2) in history it's been as cold as 25 degrees in Ames back in 1961. It's been as high as 91 in 1971."

Taylor adds, "We're not expecting either of those extremes during the next two weeks. We expect temperatures to be near normal, 71 degrees as we get into the weekend of October 4 to 6, and then it'll likely turn a little bit cooler with some rain next week."

The outlook from the National Weather Service for October 6 to 10 is for warmer than usual for Iowa, Nebraska, South Dakota and Minnesota and around the Great Lakes. That's good news to help finish up maturity on crops. It's going to be a little bit on the wet side of usual, for the northern third of Iowa especially, and up into Wisconsin and Minnesota and we don't need that right now, he says.

Look for killing frost around mid-October

"Looking toward the middle of October, we'll be moving back to seasonal temperatures," predicts Taylor. "That means we expect to get a killing frost around the middle of the month--October 15 to 17 in much of Iowa."

Looking back at the year 2008, is this a strange enough year that Taylor would call it very unusual in the history of Iowa weather?

"It is indeed--it is," he says. "I don't think it's the most unusual year I've ever lived through—but 2008 has been an unusual year. We've had extremes. In the last two years Iowa has gone from El Nino conditions to La Nina to El Nino and now back to La Nina. La Nina is associated with extreme weather in Iowa. That is, extremely warm and extremely cold. Our past winter was one of the coldest in recent history. Nothing like we had in 1979 but this past winter was one of the coldest for needing to heat houses and buildings."

Weather in 2009 may be as variable as 2008

Taylor adds, "So last winter got people's attention. And here we are going right back into La Nina--and doing it two months earlier than we did it a year ago. So yes, we could have a repeat of this type of extreme weather like we had in 2008 all over again in 2009."

That means we could see these same effects next year? "Yes," says Taylor. But when we have a year like 2008—call it an exceptional year or whatever--isn't it fairly rare for it to repeat itself?

"I'd say it is rare because I haven't seen the weather do it before," he answers. "We had a La Nina, then went down to neutral conditions and now we're right back up. I looked at historical records and this has happened a couple of times in recorded history. So it's happened before and it looks now like it's doing it again."