A forecast for La Niña conditions in the Pacific Northwest has been raised to an advisory, with colder and wetter weather seen in October now expected to continue throughout the winter.
During a Nov. 16 teleconference, NOAA forecasters showed how colder and wetter conditions prevailed through much of October, but not all of the Pacific Northwest. For example, those conditions did not extend into most of the Northern Rockies.
Temperatures in the Pacific Northwest states of Washington, Oregon, Idaho and parts of western Montana ranked in the top 10 coldest for the month of October.
Nationwide, however, October averaged out to be warm and wetter than normal. “It is unusual to see a warm year that is really wet,” said Jake Crouch, a NOAA climate scientist based in Washington, D.C. Much of the precipitation nationwide average can be attributed to very rainy weather in the Northeast toward the end of the month.
The long-range December forecast predicts colder and wetter-than-average conditions in the Pacific Northwest, and those conditions are expected to continue through January.
Weak La Niña conditions emerged in October, as reflected by below-average sea surface temperatures across most of the central and eastern equatorial Pacific Ocean.
Stephen Baxter, a forecaster with the NOAA Climate Prediction Center in College Park, Maryland, said a second winter of La Niña was somewhat unexpected. “Six or eight months ago we expecting El Niño and it actually turned out to be quite different,” Baxter said.
An El Niño influence tends to deliver warmer and drier conditions to the Pacific Northwest.
“La Niña is likely to affect temperature and precipitation across the United States during the upcoming months,” the Climate Prediction Center advisory states. There is now a 65 to 75 percent chance of that outcome, when the confidence for a weak La Niña was only at about 55 percent just in the last couple months.
Generally, the advisory translates to a prediction of warmer and drier weather across the southern tier of the U.S., and colder and wetter weather across the northern tier of the contiguous states.
Forecasters often caution that another La Niña does not necessarily mean this winter will be the same as last winter; but it is likely to be similar with some differences.
Meanwhile, scientists say there is a greater than 50-percent chance La Niña will also be in place February through April 2018, says NOAA.
This is the second winter in a row with a La Niña, and like last year, forecasters expect this one to be weak. Last year, this weather phenomenon extended from July 2016 to January 2017 before a return to neutral El Niño Southern Oscillation (ENSO) conditions.
La Niña (translated from Spanish as “little girl”) is a natural ocean-atmospheric phenomenon marked by cooler-than-average sea surface temperatures in the central Pacific Ocean near the equator, the opposite of El Niño (“little boy”).
Typical La Niña patterns during winter include above-average precipitation and colder-than-average temperatures along the northern tier of the U.S. and below-normal precipitation and drier conditions across the South.