Will this coming winter be a repeat of last winter's colder and snowier than normal weather? State climatologist Harry Hillaker has combed through the records and thinks this winter could be quite cold in Iowa. Precipitation is more difficult to predict, but he's betting on a snowier than normal winter.

A La Nina event is now occurring, and it tends to bring below-normal winter temperatures to the Upper Midwest. La Nina is a weather pattern that develops when the temperature of the surface water cools in the Pacific Ocean along the equator. It's the opposite of El Nino, which occurs when the water warms. The current La Nina developed this past spring and summer.

Iowa enjoyed unseasonably warm weather without much rain in October 2010 --great weather for harvesting. "During a La Nina, we usually have a warm, dry fall as a run-up to winter," says Hillaker. "The fall months, all the way through November, very frequently are warmer than normal in Iowa when La Nina is occurring. But that changes as we get farther into winter."

Colder than normal winter predicted for the coming months

Colder and snowier weather tends to occur in the winter during a La Nina event - especially in January, February and March. "Also, Iowa tends to be a little wetter than normal in winter during a La Nina weather pattern like the one we're in now," Hillaker adds. "Iowa has recorded four consecutive snowier than usual winters and three colder than usual ones. Thus, if this outlook materializes it would not represent much of a change from recent winters."

What does the recent strength in the Southern Oscillation Index mean regarding upcoming weather for this winter and next spring? The SOI during this past month has been running 2.2 on the scale, the highest it's been in a while. Is there any significance to that as farmers wrap up the 2010 harvest season?

Southern Oscillation Index indicates a La Nina is occurring

"It is significant, although the value of the SOI has stayed approximately the same the past 30 days," says Elwynn Taylor, Iowa State University Extension climatologist. "I don't know if that means SOI has peaked or not. But it does indicate that we are in a strong La Nina weather event."

However, strong doesn't really mean much as far as weather is concerned in Iowa, explains Taylor. As long as it's a La Nina, it's a La Nina. A strong one seems to have the same weather effect on Iowa as a weak one. It means a lot if you are in California or located down along the equator--the strength of a La Nina makes a big difference in the weather that occurs in those two locations.

The strength of the SOI might make a difference as to how long a La Nina period will last. "That's something of interest to us—whether this current La Nina event will still be with us when we get into the 2011 growing season," says Taylor. "Meanwhile, La Nina is definitely here for this fall and we are now seeing weather that is really typical of a La Nina. The National Weather Service forecast for the next 30 days and for the next four months is for typical La Nina weather."

What is typical La Nina weather in Iowa during winter months?

What is typical La Nina weather in Iowa during the winter months? It's usually colder than normal in northern Minnesota, north of the Twin Cities. And it's usually warmer than usual in the area south of the St. Louis to Kansas City line. So in Iowa, which is between Kansas City and Minneapolis, the winter weather can be either of these possibilities.

What about the growing season for 2011? If the current La Nina is still in place in April or early May in 2011, "Iowa could expect to have a rather dry spring and that's not bad. Except that's what happened in 1988 and it wasn't a La Nina event yet then. It was still a period where the weather was switching from an El Nino to a La Nina weather pattern," says Taylor.

"But in 2011, since we have been in a La Nina for awhile, it would be more like what happened in 1974," he explains. "In fact, that is the anxiety we have with this current El Nino as we are entering the end of 2010. The current pattern of weather is much like 1973 going into 1974. That's the most like the weather going into this fall, if you look at the weather records of the last 60 years. It doesn't mean we will have a dry weather pattern in 2011. This is not a forecast. But looking at how things are shaping up, the possibility of drought bothers us a little bit. Also the strength is of the current La Nina event is like it was in 1955 going into 1956 and 1973 going into 1974. The years 1956 and 1974 were not desirable years for corn production in Iowa and the U.S. Corn Belt."

Watch La Nina this winter for clues to 2011 growing season

How do we assess the current La Nina? If it starts to weaken in the winter, what are some signs to watch? "What we watch is the Southern Oscillation Index," says Taylor. "The National Weather Service puts out an update every two weeks on what's going on with the SOI. You must watch that because usually a change in the SOI and thus a change in the weather doesn't reach the Midwest for at least 30 days and sometimes 45 or more days. That's how long before a change in the La Nina and thus a change in the weather hits Iowa."

The Benner drought cycle, based on over 100 hundred years of crop yields and weather in the U.S. Corn Belt, shows droughts of major proportions coming every 18.6 years on average. "Tree ring studies over the last 800 years also tell us that the cycle of 18.6 years hold true," says Taylor. What it shows is that there is an average of approximately 18.6 years of relatively good growing weather in between the times when a major widespread drought will occur in the Corn Belt.

People are currently paying attention to the fact that 2011 is at the end of the period in that cycle when drought is likely to occur. "Also, I have studied the tree ring history of the past 800 years and 23 years is the longest time we've gone without a major drought in the Midwest. And right now we are at 22 years," says Taylor. "So if we can make it through the year 2011 without a major drought occurring in the Corn Belt, we are going to break an old record--an 800-year old record. This is one record I'd especially like to see set."