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