The warmer-than-average forecast for much of the central and eastern United States fits into the long-term trend toward milder winters driven by human-caused climate change. But NOAA was careful to point out that cold, wintry surprises are possible because of the unpredictable behavior of the polar vortex.
Last winter, the polar vortex unleashed a historic, punishing blast of cold to the central states in February.
The pattern of temperatures and precipitation predicted by NOAA for this winter largely reflects typical conditions during La Niña events. La Niña conditions prevailed last winter and, outside the February cold snap, delivered average to above-average temperatures to a large portion of the Lower 48 states.
Last week, NOAA declared that La Niña had returned and that it should persist through the winter.
“Consistent with typical La Niña conditions during winter months, we anticipate below-normal temperatures along portions of the northern tier of the U.S. while much of the South experiences above-normal temperatures,” said Jon Gottschalck, the lead forecaster for NOAA’s outlook. “The Southwest will certainly remain a region of concern as we anticipate below-normal precipitation where drought conditions continue in most areas.”
La Niña’s influence on the winter
La Niña first manifests as a region of anomalously cool ocean water temperatures in the eastern tropical Pacific. That sets off a chain of events that nudges the jet stream toward the coast of Alaska before it dives southeast over British Columbia and eventually into the U.S. Central Plains. Cold air oozes south into the northern United States within the dip in the jet stream, while warmth can build to its south.
NOAA’s outlook calls for warmer-than-normal conditions everywhere south and east of the Pacific Northwest, the northern Rockies and the Upper Midwest.
The jet stream pattern, guiding storms across the northern part of the country, rather than the south, favors above-average precipitation across the Pacific Northwest and the Great Lakes but below-average amounts across the Southern states. In between, NOAA calls for equal chances of above- or below-average amounts.
At a news conference with reporters Thursday, Gottschalck said La Niñas typically reduce the chance of blockbuster snowstorms along the East Coast from Washington to Boston because storms tend to cut to their west, drawing in mild air off the ocean, producing rain or a mix of rain, ice and snow.
“We’ve had major blizzards during La Niña, but typically storm track is more inland, such that warmer air will work its way in,” Gottschalck said, noting that ocean temperatures along the East Coast are “way” above normal. “Snowfall is typically below average during La Niña in the Mid-Atlantic.”
Gottschalck did note that the chance of big snowstorms in New England may tick up late in the winter during La Niña years.
The polar vortex is a wild card
Outside of the Pacific Northwest and the Northern Rockies, where confidence is fairly high that conditions will be cold and stormy, Gottschalck said the character of the winter will depend a lot on weather fluctuations that can’t be forecast this far in advance.
The most important element in question is the polar vortex, a pool of frigid air and low pressure typically bottled up at high latitudes over the North Pole. A strong polar vortex is more self-contained and whirs around faster, keeping its frosty influence relegated to the Arctic Circle. If the polar vortex is weaker, however, there’s not as much to keep that cold air in place, allowing lobes of it to pinch off and spill to the mid-latitudes.
That’s exactly what happened in February, when a massive dip in the jet stream brought a once-in-a-generation deep freeze to Texas, wreaking havoc on the state’s electrical grid and water infrastructure. Every county in the Lone Star State was included in a winter storm warning, with crashing temperatures toppling all-time records.
Gottschalck stressed that extreme events such as that Texas cold snap cannot be forecast months ahead of time. He said it’s unlikely but not impossible that such an event would happen again this winter.
Such events “are just not predictable at these lead times,” he said.
He acknowledged that “there’s considerable research and a number of different hypotheses” focused on predicting the polar vortex up to months in advance, but noted that “there’s considerable spread as to how reliable those predictions are.”