June 20, 2024

La Niña makes a curtain call

CHAMPAIGN, Ill. — When it comes to moisture in the fall, the lack of rainfall has been a good news/bad news story.

Dry conditions helped farmers in the Midwest complete harvest and fieldwork faster than they have in other years.

“The problem has been that the dryness went on about two or three weeks too long,” said Trent Ford, Illinois state climatologist.

The drought that has persisted in some form or fashion since September has been a national story, as well, as the nation’s major inland transportation waterway, the Mississippi River, has dried up to the point that barges were prevented from moving on the lower Mississippi.

The drought that impacted the Mississippi also impacted the major rivers that feed into it.

“Parts of the Ohio Valley from Cairo, Illinois, up to Louisville, Kentucky, had, in some cases, less than an inch or half an inch of rainfall from mid September to late October,” Ford said.

“They picked up a little bit, but they have been very, very dry and it added to the drought in the Missouri Valley. That led to the issues along the Mississippi we’ve seen.”

It wasn’t just the lower Ohio River that was impacted. Ford said drought in regions around most of the major tributaries of the Mississippi, including the upper Ohio River, the Illinois River and the Missouri River have impacted the flow into the Mississippi River throughout its length.

“The whole lower Ohio River basin, between Sept. 1 and Oct. 21, saw less than three-quarters of an inch of rain total around that whole basin. That’s a large area that isn’t super wet this time of year, but it is much wetter than that. That’s about three inches below normal,” he said.

“The lack of significant water from the Ohio River has added to the dryness in the upper Mississippi, as well as the Missouri basin, and, of course, the Illinois River basin is pretty dry or has been pretty dry this whole time.”

As late fall moves toward the first day of winter, Dec. 21, thoughts turn from rainfall to snowfall.

Ford said weather models are showing the return, for the third season, of La Niña.

“When you look at winter, December and January together, the models are really painting a very La Niña-esque signal. We’ve gone into the third consecutive cool season of La Niña, which is fairly rare,” he said.

“La Niña, the last two seasons, has played to type. It generally means wetter than normal conditions for a lot of the Midwest.

“This year, when we are looking at the outlooks for December and January, the models want to paint that whole Ohio Valley, from southeast Missouri all the way up to the Great Lakes as wetter than normal for that three-month period.”

If La Niña follows the traditional pattern, it would be welcome news for the Midwest and the Ohio Valley.

“Since that is a very common pattern for La Niña winters and the models have fairly good confidence in that, that is good to see,” Ford said.

“Hopefully, as we move into the winter season, we get some good moisture returned to the Ohio Valley. That will help that area, but it will also help the flow along the Mississippi River.”

Winter rain or snow in the lower Ohio Valley could benefit the river in the near term.

“That area, unlike the upper Midwest, doesn’t necessarily freeze for the entire winter. We get ephemeral freeze/thaw cycles, so it means that the Ohio River is flowing a lot,” Ford said.

“The soil frost is pretty limited, most of the time, so any rain we do get or snow will actually get into the soil column, unlike winter rain in the upper Midwest, in the middle of January, most of that is just going to run off.”

Long-term trends add to the scoreboard for winter moisture making up for deficit moisture in the summer and fall.

“One thing that is sort of working for us, in this instance, is the trends. The long-term trends in winter and spring rainfall are robust. Winters and springs across Illinois, in the past 100 years, have gotten wetter,” Ford said.

“What that does in any given year is its own manifestation. But what climate change will do is it will increase the odds of what we consider a wetter than normal winter or spring.”

Ford said temperature models for the winter ahead are mixed, with equal chances of above and below normal temperatures.

“There’s not really a strong La Niña signal when it comes to air temperature in Illinois in the wintertime, so it’s really the precipitation signal that’s the strongest,” he said.

Jeannine Otto

Jeannine Otto

Field Editor