If historical weather patterns continue, Iowa, the nation’s largest corn producing state, may see another year of moisture-stressed corn yields at a time when supplies and prices still are recovering from the drought of 2012.

“Historically, severely deficit precipitation years of the magnitude of 2012 do not recover to normal annual precipitation in a single year,” Elwynn Taylor, Iowa State University extention climatologist and professor of agronomy, said in a report in early December, which included soil moisture observations from other extension agricultural specialists. “Accordingly, an additional year of significant moisture stress is considered to be not unlikely and a fourth consecutive year of below trend U.S. corn yield a distinct possibility.”

The 2012 Midwest drought was a continuation of a weather anomaly — a La Niña event — that began July 22, 2010, and peaked on Oct. 23, 2010, and was second in strength only to one in the mid-1950s based on more than 100 years of records, according to the I.S.U. report.

“The young but potent La Niña resulted in an abrupt change in weather on a planetary basis that included record flooding in Montana, North Dakota and adjacent Canada,” the report said. “The event ended a several-year drought in the northwest United States and ended (with drought) a six-year continuous string of above trend U.S. corn yields.”

Average U.S. corn yield exceeded the projected trend for six consecutive years from 2004 through 2009 but has been below trend since 2010, according to data from the National Agricultural Statistics Service of the U.S. Department of Agriculture. The record high U.S. average corn yield was 164.7 bus an acre in 2009. Average yield fell to 152.8 bus an acre in 2010 and to 147.2 bus in 2011 before plummeting to an estimated 122.3 bus in 2012, the lowest average yield since 113.5 bus in 1995, according to the U.S.D.A.

“Drought in 2011 was much like the drought of the mid-1950s: developing in the south central United States with rain becoming scant in the Corn Belt after early July,” the report said. “Midwest crops depleted the subsoil moisture to the extent of rooting depth, and over winter precipitation did not bring full recharge to western Corn Belt soils. Rooting conditions in 2012 were near ideal and observed corn and soybean roots to depths greater than eight feet were reported in numerous locations. Deep rooting provided sufficient water to enable a greater-than-anticipated crop yield in numerous Corn Belt locations, but resulted in about eight feet of moisture depleted soil and a resultant requirement of 16+ inches of moisture needed to replenish subsoil moisture.”

“It is not likely that subsoil moisture will be fully replenished by the beginning of the 2013 planting season,” I.S.U. said.

Subsoil moisture in southeast Iowa in November exceeded observations from November 2011 and likely will be near normal values with spring rains, the study said, but northwest Iowa observations were below year-earlier levels in November. The greatest concern may be in central Iowa where 2012 was one of the three driest years since 1950 (1956 and 1988 the other two).

“In both cases, the subsequent year also received below normal precipitation and experienced below trend yields in Iowa,” according to the I.S.U. study.

“Moisture deficit in the subsoil increases the risk of crop yields being below trend and prevents the recovery of river, pond and well water to normal levels,” according to the I.S.U. report. “The probabilities will become more definitive in the early weeks of 2013.”

The study appears to explain much of what happened to the Midwest corn crop during 2012. The crop began with promising condition ratings early in the growing season but then deteriorated quickly as the summer progressed. Initial projections of a record high yield of 166 bus an acre and a record large crop of 14,790 million bus plunged to the latest estimates of 122.3 bus an acre and 10,725 million bus, the smallest crop since 10,531 million bus in 2006. The final 2012 corn yield and production estimate will be released by the U.S.D.A. on Jan. 11, 2013.

Initial projections from private analysts already are calling for increased area planted to corn and the possibility of a record large 14-billion-plus bu corn crop in 2013, not unlike early ideas in 2012. But those forecasts depend on a return to near trend levels for average yield. The current record large corn crop was 13,092 million bus in 2009.

The pattern also provides insight into 25-year low November winter wheat condition ratings, pulled down by very poor ratings in Nebraska and North Dakota in the western Corn Belt.

Futures traders have somewhat bought into the early projections with new crop corn futures prices (September 2013 forward) trading from around $6.20@6.40 a bu last week, as much as $1 below old crop months trading around $7.20 a bu, which was down from record high corn futures prices near $8.50 a bu in August.

Worrisome is the 2013 corn carryover forecast, placed at 647 million bus in the U.S.D.A.’s Dec. 11 World Agricultural Supply and Demand Estimates. That number is down 35% from this year and 55% from the latest five-year average, leaving little leeway for another “bad” U.S. corn crop should the Iowa State report prove prophetic.