Freezing temperatures have arrived about two weeks earlier than normal this fall in northern Iowa. The tops of soybean plants in Dean Coleman's fields near Humboldt in north central Iowa showed some damage Thursday morning Sept. 15, after the temperature at his farm dropped to 30 or 31 degrees early that morning.

"As we went around looking at our fields on Thursday morning, we saw that the tops of the plants were wilting," says Coleman, president of the Iowa Soybean Association. "The soybean plants were fairly close to being fully mature, so it may not be a large yield loss, but there will be some yield loss."

In Clarion in north central Iowa, gardens with tender annuals and perennials weathered that early morning chill. "It was cold last night," says Rick Rasmussen, owner of a garden center. "But the warmth from the ground was enough to keep the flowers warm so they didn't suffer frost damage."

Mason City set a record low of 26 degrees early Thursday morning

State climatologist Harry Hillaker says readings show Clarion was below freezing for several hours. The temperature there dropped briefly to 27 degrees. Mason City, farther north in north central Iowa, set a record low of 26 degrees early Thursday morning, says Hillaker. The old record low for that date was 28 degrees set on Sept. 15, 2007.

Hillaker says preliminary data shows Onawa in western Iowa set a record with 30 degrees, two degrees cooler than the previous record of 32 degrees in 1916. Algona tied a 1916 record with 32 degrees on September 15, 2011.

The National Weather Service had predicted a hard freeze for parts of Iowa for Thursday September 15 and that proved to be a correct forecast. But warm ground temperatures apparently helped offset the unusually cool morning in some areas. Hillaker says a few spots south of Interstate 80 dropped to freezing or below. Two of the exceptions were Chariton in south central Iowa and Atlantic in western Iowa, where the mercury reached 30 degrees and 32 degrees respectively. "The weather stations there are located in low areas so they are known for being a little colder than some of the areas around them," he says.

A look at low temperatures across Iowa on morning of September 15

A Canadian high pressure system brought temperatures to unseasonable lows for just the one night, notes Hillaker. While Mason City set a new record low early in the morning on September 15, here are the other low temperatures around Iowa that were recorded that morning:

* Northern Iowa: Spencer recorded 31 degrees F, Fort Dodge 30 degrees, Mason City 26 degrees, Waterloo 31 degrees, Decorah 36 degrees and Dubuque 36 degrees.

* Central Iowa: Sioux City dipped to 32 degrees F, Marshalltown had 31 degrees, Cedar Rapids 32 degrees, and Iowa City 37 degrees for a low.

* Southern Iowa: Council Bluffs had a low of 36 degrees F, Clarinda 34 degrees, Des Moines 41 degrees, Ottumwa 36 degrees and Burlington 40 degrees.

Effect of frost not bad in Iowa, but soybeans hurt in Minnesota, Dakotas

"We had widespread sub-freezing conditions in eastern South Dakota, Minnesota and even in northwest and north central Iowa," notes Iowa State University Extension Climatologist Elwynn Taylor. But despite the temperatures below freezing or right at freezing, it doesn't sound like there was severe crop damage. "We may not be able to tell until we get some sunny days again to see how widespread and severe the damage is to crops, particularly soybeans."

South Dakota's state climatologist, Dennis Toddy, checked with the horticulture folks in his area, and there doesn't seem to be much damage to vegetable and fruit crops either. "Judging from my own garden and others there doesn't seem to be too much damage here in southeast South Dakota," he says. "But there is some yield damage in places as it did get down to freezing."

Some people's gardens took some shots as you might expect with subfreezing, if they are located in lower spots. "But with 30 or 32 degree temperatures in a large area, we stayed a little warmer than was forecast," says Toddy. "That helped some crops get through this with little or no damage. This may have been a case where we had such warm temperatures recently, and still had warm enough soils to help keep air temperatures a little bit warmer than expected."

Chance of rain this weekend in Iowa, then look for dry harvest weather

Toddy and Taylor point out there were high clouds over the western Corn Belt the night of September 14 going into early morning September 15,  and those clouds coming in helped in protecting crops from frost damage. "When you're close to the freezing temperature, anything you can do to buy a degree or two can make a difference between just scraping by and avoiding yield damage--and taking a hit from frost and losing everything," says Taylor.

It was near the end of the growing season for corn and beans in the Dakotas and Minnesota. "But there were certainly beans up here that were plenty green when the frost hit on September 15," says Toddy, "and they still had some maturing left to do before the frost hit and stopped them."

Looking ahead for harvest 2011 weather prospects, a ridge over the western U.S. will continue to impact the Plains States and much of the western Corn Belt. There are chances of rain over the next 4 or 5 days or so. "After that, we look for the weather to be dry again for awhile and temperatures to get back on the warm side," says Toddy. "The locations that were not hurt by the September 15 frost probably are going to continue to have a chance to complete the maturity of their corn and soybean crops and then drydown in the field."