One year ago, Beth Hall reported that it looked like the 2020-21 winter might start out mild, with increasing odds for more winter-like weather in the latter half of the season. Hall, the Indiana state climatologist, said that while climatological winter concludes at the end of February, winter-like weather might continue into March in 2021.

Winter a year ago played out very closely to what she expected. The season skirted along harmlessly until early February, with a few exceptions. Then big snows and cold weather hit parts of Indiana hard. And how about measurable snow and a late frost and freeze on April 20 and April 21? That’s certainly beyond climatological winter.

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So, what is Hall saying in late fall this year, with the winter of 2021-22 approaching? “Things are not quite as clear, but there are some indications that we could see a winter that is somewhat similar to last winter,” Hall says. “October was unseasonably mild, and that trend may continue. Many believe the early part of winter will be mild, with above-normal temperatures. If temperatures trend above normal, it often means you get more rain than snow.”

The latter part of winter could once again be a different story, she says. “There was a La Niña event in place, which affects air circulation patterns, a year ago, and there are strong indications that a La Niña event is developing again. If so, it might play a role in causing this winter to play out somewhat like last winter — mild early with a winter punch later.”

Two La Niña events back to back is rare, but it has happened before, Hall says. However, it’s uncertain how strong the La Niña event will be and how long it will last if it develops, she adds. Plus, other weather-forcing factors, including the Artic Oscillation, can also influence winter weather patterns. Sometimes, these other factors override the effects of a La Niña event.

More about La Niña

A La Niña is an event within the El Niño-Southern Oscillation, which weather people often call ENSO. Austin Pearson, a Purdue University Extension educator in Tipton County, Ind., who specializes in helping people understand weather terminology, recently explained the basics behind ENSO.

For starters, it’s a naturally occurring phenomenon located along the equator in the Pacific Ocean. “Scientists pay close attention to the sea surface temperatures in this region as the varying temperatures impact the positioning of jet streams in the winter and hurricane development in the summer,” Pearson says. He notes that there are three ENSO phases: ENSO neutral; El Niño, when sea surface temps are 0.5 degree F or more above normal; and La Niña, when temps are 0.5 degree or more lower than normal. In general, La Niña conditions are associated with cold, wet winters across much of the Northern U.S. and warmer, drier conditions in the Southern U.S.

Sea surface temperatures have this impact because they affect atmospheric pressures, which in turn affect atmospheric circulation patterns aloft across the globe.

Where predictions become more mirky, Hall says, is when you factor in how strong these trends are and when they start and end. Add in other factors affecting weather, and long-term weather prediction is not foolproof.