For the second year in a row, the climate pattern known as La Niña has developed in the Pacific Ocean, which will likely prolong the severe drought in much of the Western United States this winter while bringing some relief to Northern California and the Pacific Northwest, government forecasters said Thursday.
In La Niña, lower than normal sea surface temperatures in the equatorial Pacific lead to changes in the jet stream, a high altitude river of winds that can affect weather elsewhere in the world.
Over North America, La Niña usually, although not always, shifts the jet stream to the north, bringing more storms to that part of the continent and fewer to the south.
That typical pattern was reflected in the National Oceanic and Atmospheric Administration outlook for the coming winter, released Thursday.
Jon Gottschalck, chief of the operational prediction branch of NOAA’s Climate Prediction Center, said that with the development of La Niña, which is expected to continue into spring, wetter-than-normal conditions are forecast for in the Pacific Northwest and northern Rocky Mountains, and in the Great Lakes and Ohio Valley regions. Below-normal precipitation is expected across most of the South.
In terms of temperatures, the outlook calls for cooler conditions in the Pacific Northwest to the Northern Plains, and warmer-than-normal conditions almost everywhere else: a broad swath from Central and Southern California through the central Rockies into Minnesota and across the entire Midwest and East.
“A major region of concern this winter remains the Southwest,” Mr. Gottschalck said. The region has been mired in drought, to a greater or lesser extent, for the past two decades, and there was only slight improvement this summer despite strong monsoon rainfall in Arizona and elsewhere.
With La Niña, Mr. Gottschalck said, drought was expected to persist in the Southwest and develop in the Southern Plains.
But the forecast of more precipitation for the Pacific Northwest bodes well for drought conditions improving, and in some cases being eliminated, in that region, Mr. Gottschalck said. Northern California may see improvement as well, which is good news for a region that has endured a severe wildfire season this year, in part because of lingering heat and dryness.
La Niña is the opposite phase of El Niño, when higher than normal sea surface temperatures in the Pacific tend to shift the jet stream to the south, bringing wetter conditions to Southern California and elsewhere in the south.
After last year’s La Niña, Pacific Ocean temperatures rose somewhat, but not enough for El Niño to develop. Temperatures have now declined again, leading to the second La Niña. But such a “double-dip” La Niña is not unheard-of, Mr. Gottschalck said.
He said the forecast for now is that this La Niña will be a moderate one, meaning sea surface temperatures will be about 1 to 1.4 degrees Celsius (1.8 to 2.5 degrees Fahrenheit) below normal. But Mr. Gottschalck did not rule out that temperatures could decline more, leading to a strong La Niña.
In general, with a stronger La Niña, the response of the jet stream is greater as well.
Mr. Gottschalck said that with La Niña, the likelihood of blizzards or other strong winter storms in the Northeast is reduced because the shift of the jet stream moves winter storms west of the Appalachian Mountains.
But as with every La Niña and El Niño, the typical conditions do not always materialize, Mr. Gottschalck said. He noted that NOAA’s winter outlook is a probabilistic forecast, meaning it reflects the likelihood of what will happen and is not definitive.