La Niña watch: Mild fall with closer to average winter ahead?

A weak La Niña event could signal a closer to average winter for Minnesota. But it's fighting the trend of milder winters with climate change.

NOAA CFS2 winter temperature outlook
The National Oceanic and Atmospheric Administration’s CFS2 winter temperature outlook.

The National Oceanic and Atmospheric Administration’s Climate Prediction Center has issued a La Niña watch for the upcoming winter season.

La Niña events historically favor colder, snowier winters in Minnesota.

Typical La Nina winter pattern
A typical La Niña winter pattern.

But La Niña events now have a powerful competing effect to overcome to produce colder than average winters. Climate change is driving a strong trend toward milder winters in Minnesota overall.

La Niña watch

Tropical Pacific Sea Surface Temperatures (SST) are forecast to cool as we move through fall. If tropical Pacific SST’s average minus 0.5 degrees Celsius, that’s the threshold for a La Niña event. Right now, NOAA and other forecast models favor a weak La Niña event for this upcoming winter. Here’s more detail from NOAA’s monthly climate diagnostics discussion.

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At this time, forecasters anticipate La Niña to be of weak strength (seasonal average Niño-3.4 index values between -0.5ºC to -0.9ºC). In summary, a transition from ENSO-neutral to La Niña is favored in the next couple of months, with a 70-80% chance of La Niña during the Northern Hemisphere winter 2021-22.

La Niña events often produce winter jet stream patterns that statistically favor lower than average temperatures across the northern U.S.

That translates historically into a 70- to 80-percent chance of a colder than average winter in Minnesota.

La Nina temps
La Nina historical winter temperature trends.

Climate change driving milder winters

Any La Niña event this winter will be fighting against the strong trend of milder winters in Minnesota and the Upper Midwest driven by climate change. Overall Minnesota winters have warmed by about 5 to 6 degrees since 1970.

Winter warming trend in the Twin Cities since 1970
The winter warming trend in the Twin Cities since 1970.
NOAA data via Climate Central

So if the weak La Niña event materializes this fall and winter, it will have to overcome the strong trend of milder winters in Minnesota.

Looking ahead: Mild October, closer to average winter?

NOAA’s monthly outlook for October strongly favors a warmer than average October for Minnesota and the Upper Midwest.

October temperature outlook
October temperature outlook.

The three-month outlook through December favors equal chances of above and below-average temperatures overall.

3-month temperature outlook
The three-month temperature outlook.

Right now, NOAA’s CFS2 seasonal temperatures product favors a colder than average winter for Canada and northern Minnesota, with equal chances of a cold or mild winter across most of Minnesota. Most of the U.S. south of Minnesota favors a milder than average winter in La Niña years.

NOAA CFS2 winter temperature outlook
NOAA CFS2 winter temperature outlook.

So this could be one of those winters where Minnesota records near average temperatures overall, with a couple of polar vortex outbreaks and some prolonged mild spells during the winter season.

Stay tuned.