NOAA DECLARES LA NIÑA, WHICH IS LIKELY TO AFFECT HURRICANE SEASON AND DOMINATE WINTER ~ The Washington Post

La Niña patterns favor enhanced hurricane activity over the Atlantic.

Current sea surface temperature anomalies in degrees Celsius. Note the cool waters in the eastern Pacific commensurate with La Niña. (Tropical Tidbits) 

By Matthew CappucciSeptember 10, 2020 at 2:15 p.m. MDTAdd to list

The National Oceanic and Atmospheric Administration declared Thursday that a La Niña pattern had become established, having bearing on the remainder of the hurricane season and the upcoming winter. La Niña conditions are likely to continue through at least wintertime, potentially returning to a more relaxed “neutral” state by spring.

Seven tropical systems to watch, including longer term U.S. threat, on peak day of hurricane season

La Niña, which means “the girl” in Spanish, is the opposite of an El Niño. La Niña features unusually cool ocean waters in the equatorial tropical Pacific Ocean and can influence weather patterns beyond the Pacific.

The expectation of a La Niña pattern was a contributor in NOAA’s early August forecast of an “extremely active” hurricane season.

La Niñas and El Niños, which represent opposite phases of ENSO, or the El Niño Southern Oscillation, are major drivers of weather and climate trends in North America.

NOAA's estimated likelihood of La Niña to continue in the coming months. (NOAA/NWS)
NOAA’s estimated likelihood of La Niña to continue in the coming months. (NOAA/NWS) 

Much of late 2018 through early 2020 had skewed a bit more toward the weak El Niño side. Now, the pendulum is swinging in the other direction, which will have major implications in the months ahead.

What is a La Niña?

A historical chart of La Niña events (blue) and El Niño events (red). (NWS Tampa Bay)
A historical chart of La Niña events (blue) and El Niño events (red). (NWS Tampa Bay) 

A La Niña pattern is characterized by anomalously cool sea surface temperatures in the eastern Pacific. That’s the opposite of an El Niño, during which the east tropical Pacific is atypically toasty.ADhttps://1bb65c05e6dd38c35ad8c1060e923df2.safeframe.googlesyndication.com/safeframe/1-0-37/html/container.html

With a significant change in ocean temperatures ongoing and further in the offing, a La Niña alters several key circulation patterns in the atmosphere and can influence weather across the globe.

Most La Niña events last for at least several months but can occasionally stretch for years. Strong La Niñas occurred between 1973-1976, 1988-1989 and from 1998 to early 2001.

Winter weather

A typical wintertime La Niña pattern. (NOAA/NWS)
A typical wintertime La Niña pattern. (NOAA/NWS) 

NOAA forecasters have stated there is a 75 percent chance that La Niña will stick around for the entirety of winter. Generally speaking, La Niña typically increases the odds of above-average snowfall in the Pacific Northwest, northern Plains, Great Lakes region and northern New England. However, every La Niña is different, and other weather patterns can overwhelm its effects.AD

In the Mid-Atlantic, South, southern/central Plains and Southwest, snow may be more scarce. 

The impending La Niña is bad news for Central and Southern California, where potentially reduced snowpack at the higher elevations and fewer winter storms may prolong this year’s fire season, already a record, and set the stage for a challenging 2021.

Severe weather impacts 

A look at typical wintertime storm tracks during La Niña versus El Niño years. During La Niña years, the jet stream is typically farther north, with fewer storm systems passing directly over the South. (NWS Tallahassee)
A look at typical wintertime storm tracks during La Niña versus El Niño years. During La Niña years, the jet stream is typically farther north, with fewer storm systems passing directly over the South. (NWS Tallahassee) 

Because La Niña patterns shift the location of the jet stream farther north, there is generally less unsettled weather in the wintertime across the southern United States. This means drier, and subsequently warmer, conditions are likely for the Desert Southwest, South and Southeast.

It also limits the number of wintertime severe weather events in Florida and the South, commonly known as “Dixie Alley” by storm chasers. Disturbances in the jet stream can cause severe weather and tornado activity across the Interstate 10 corridor, particularly from Louisiana and Mississippi to Alabama, Florida and Georgia, between December and February. But with the jet stream retreating farther north, those chances are significantly diminished.

In fact, tornadoes in this region are only half as common during La Niña winters as compared with El Niño winters, welcome news for residents who live there.

A look at severe weather tendencies from March through May during El Niño and La Niña years. (Climate.gov)
A look at severe weather tendencies from March through May during El Niño and La Niña years. (Climate.gov) 

Between March and May, however, a La Niña pattern can increase tornado and severe thunderstorm incidence for portions of the central and southern Plains, as well as into portions of Arkansas, northern Louisiana and the central Mississippi Valley.

It is unclear whether this La Niña will still be in place by then, though.

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