A La Niña winter is on the way for the US

The global average temperature for September was a new record.

Enlarge / The global average temperature for September was a new record.

September apparently wasn’t feeling like doing anything unusual, so it ended up being the warmest September on record for the globe. That’s been something of a trend this year, with each month landing in its respective top three. It has become increasingly clear that 2020 will likely be the second warmest year on record, if it isn’t the first.

Unlike in August, the contiguous US didn’t set a record in September, though it was still above the 20th century average. A high-pressure ridge dominated over the West Coast again, leading to even more warm and dry weather for much of the Western US. But a trough set up over the Central US in mid-September, bringing cooler air southward.

Two more hurricanes—Sally and Beta—led to above-average rainfall in the Southeast. Total precipitation for the contiguous US was a touch above average as a result, but the average as usual masks local differences. Drought conditions have expanded and worsened over much of the West, and there has been little relief for wildfire conditions.

Speaking of those hurricanes, they brought the number of named storms making landfall in the contiguous US to nine for the year. That tied 1916 for the most on record, but Hurricane Delta’s landfall in Louisiana has since added to 2020’s dizzying tally.

September also saw the number of billion-dollar-plus disasters in the US climb to 16—tying 2011 and 2017 for the most in a year since the start of this (inflation-adjusted) metric in 1980.

  • Some of the notable events in December.

  • Each of these disasters caused at least a billion dollars in damages.

  • September wasn’t as warm for the eastern half of the US.

  • Total precipitation was nearly average, but the regional differences are huge.

  • Over 40% of the contiguous US is now in drought.

What next?

NOAA released its winter outlook on Thursday. These long-range outlooks are based on a combination of observed trends, important slow-changing patterns, and model simulations. NOAA typically discusses the next-month and next-three-months outlook, but this round includes the December-January-February seasonal window.

If you caught last month’s update, this will look pretty familiar. The biggest factor in play is the La Niña conditions in the Pacific Ocean, which are likely to persist at least until spring. La Niñas tend to have a pretty defined impact on US winter weather, though the variability of weather doesn’t disappear. But the cold surface temperatures in the eastern equatorial Pacific generally promote a shift in the US storm track that leads to more cold and wet weather across the northern tier of the country, with warmer and drier weather across the south.


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