From WP By Matthew Cappucci
The atmospheric and oceanic pattern will have a bearing on the Western drought, the end of hurricane season and the forecast for winter and spring
After a months-long period of relative atmospheric balance between El Niño and La Niña, the National Oceanic and Atmospheric Administration announced Thursday that La Niña has returned.
The intensifying La Niña should peak in magnitude, or strength, by the end of 2021, having bearings on the drought in the West, the end of hurricane season and the upcoming winter.
It’s one of many drivers in our atmosphere, but it is often among the most important given the extent to which it shuffles other atmospheric features key in determining how weather evolves over the Lower 48.
In brief, here are some of the key impacts La Niña could have in the coming months:
Extending favorable conditions for Atlantic hurricane activity this fall.
Worsening drought conditions in the Southwest through the winter and potentially elevating the fire risk through the fall.
Raising the odds of a cold, stormy winter across the northern tier of the United States and a mild, dry winter across the South.
Increasing tornado activity in the Plains and South during the spring.
La Niña is the opposite of El Niño, which often makes headlines for spurring powerful southern storms that can generate beneficial rains in California and track across the entire nation.
About six weeks remain until the start of meteorological winter (Dec. 1), and forecasters are already looking ahead to what may be in store.
What is La Niña?
La Niña begins with a cooling of waters in the eastern tropical Pacific.
During La Niña winters, high pressure near the Aleutian chain shoves the polar jet stream north over Alaska, maintaining an active storm track there.
That keeps the northern United States anomalously wet, while the South is left largely warm and dry. This is bad news for California and other parts of the Southwest, which are enduring a historic drought. The persistence of warm, dry conditions would cause the drought to worsen and potentially prolong the fire season.
La Niña arrived in fall 2020 before fading away in May 2021. Neutral conditions, bridging the divide between La Niña and El Niño, prevailed through the early fall before the NOAA’s declaration of La Niña’s return Thursday.
La Niña tends to exert a slight cooling effect on global temperatures, but recent La Niña years are warmer than El Niño years were just a decade ago because of the warming influences of human-caused climate change.
Impact on hurricane season
Conditions that have begun to skew slightly toward La Niña have already helped amplify the effects of the 2021 Atlantic hurricane season, and were in large part responsible for supercharging the record 2020 season, during which 30 named storms occurred.
La Niña patterns reduce wind shear, or a change of wind speed or direction with height, over the Caribbean and western parts of the Atlantic’s Main Development Region (MDR), the strip of territory between the Lesser Antilles and the coast of Africa.
La Niña conditions also influence the Walker circulation, or a horizontal overturning circulation in the tropics, and induce broad upward motion over the Atlantic with some subsidence, or sinking, in the Pacific.
This season is running about 54 percent ahead of average in the Atlantic, but 28 percent behind typical norms in the Pacific.
Though it’s impossible to predict specific storms or periods of active tropical weather, it looks like something called a convectively coupled Kelvin wave could favor more upward motion in the Atlantic beginning around Oct. 22.
Winters are notoriously difficult to predict because of the complexities of pinpointing storm tracks, rain-snow lines and precipitation amounts more than a few days in advance.
Weather.com published a broad winter outlook for temperatures that is commensurate with typical La Niña expectations, depicting below-average temperatures in the northern United States and above-average warmth in the South.
Parts of the Front Range, High Plains and Columbia River Basin in the Rockies, as well as the Ohio and Tennessee Valleys and Midwest, are included in AccuWeather’s prediction of above-average precipitation; the Southwest and Southeastern United States are in line to see unusually dry conditions.
Through the end of 2021, AccuWeather calls for little of the rain needed to ease the drought and fire risk in Southern California.
The National Weather Service Climate Prediction Center isn’t slated to issue its updated December through February forecast until Oct. 16, but its mid-September outlook indicated that virtually the entirety of the Lower 48, save for the Pacific Northwest, should see near to above-average temperatures. Its precipitation forecast is similar to AccuWeather’s, with drier conditions probable in the South and an uptick in precipitation for northerly regions.
Winter forecasts depend on far more than just La Niña, though, as evidenced by the record-shattering cold blast of February that wrought havoc on Texas’s electrical grid.
“The most impressive atmospheric feature [lately] has been this ridge of high pressure over Eastern Canada,” he wrote in a Twitter direct message. “It has acted like an immovable boulder in the jet stream, and if that feature stayed park over Eastern Canada for much of the winter we would all be saying ‘what winter?’ ”
He does think that could change, but a transition like that is something that weather models struggle to anticipate.
“Where that block relocates will could be potentially critical to how the winter begins and may even set the tone for the winter,” he wrote.
The behavior of the polar vortex, the zone of frigid air surrounding the Arctic, will also play a crucial role.
“Once the polar vortex weakens, it could be predisposed to further weakening in the coming weeks or months and we have a more severe winter,” Cohen wrote.
But Cohen also said there are influences that could halt any vortex weakening.
Severe weather season
If La Niña lingers into spring, it could enhance the upcoming severe weather season in tornado country across the Great Plains and Deep South.
La Niña amplifies south-to-north temperature contrasts across the central Lower 48, which sets the stage for repeated clashes of the seasons.
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