One of the biggest players in North American weather patterns returns to the scene soon. NOAA’s Climate Prediction Center issued a forecast for a ‘likely’ return to La Niña conditions in the Pacific for the upcoming fall and winter seasons.
La Niña means cooler-than-usual water temperatures across the equatorial Pacific Ocean (blue areas in the map below).
Part of NOAA’s description of La Niña from Climate.gov says:
“The atmosphere over the tropical Pacific exhibits changes commonly associated with La Niña, including one or more of the following:
- stronger than usual easterly trade winds,
- an increase in cloudiness and rainfall over Indonesia and a corresponding drop in average surface pressure,
- a decrease in cloudiness and rainfall in the eastern tropical Pacific, and an increase in the average surface pressure.”
And the diagnostic discussion from the Climate Prediction Center released Thursday reads like this:
…the latest forecast model runs from the NCEP CFSv2, many of the models from the North American Multi-Model Ensemble, and some models from our international partners indicate the onset of La Niña during the Northern Hemisphere fall, continuing into winter 2021-22. The forecaster consensus favors these model ensembles, while also noting the historical tendency for a second winter of La Niña to follow the first. In summary, ENSO-neutral is favored through the Northern Hemisphere summer and into the fall (51% chance for the August-October season), with La Niña potentially emerging during the September-November season and lasting through the 2021-22 winter (66% chance during November-January; click CPC/IRI consensus forecast for the chances in each 3-month period).NOAA CLIMATE PREDICTION CENTER ENSO DIAGNOSTIC DISCUSSION
What does this mean for Alabama and Tennessee?
First of all, you have to think of this in terms of long-term weather patterns: not specific weather events at a specific location. We had a La Niña last fall/winter, and it’s not entirely unheard of to have them back to back like this. That doesn’t mean an exact repeat of the weather in late 2020 and early 2021, though!
- HURRICANES? La Niña in the autumn can enhance the Atlantic hurricane season by reducing wind shear around the Gulf, Caribbean and western Atlantic.
That affects the number and strength of storms; however, it does not give us a specific point to be more/less concerned about a storm strike. In fact, Colorado State University’s newest outlook released Thursday now expects 20 named storms: well above average.
- SNOW? It was 2,184 days between 1″ or greater snowfalls in Huntsville. That drought ended on February 17, 2021 with 3.1″ of snow at Huntsville International: part of a series of winter storms that shut down Northwest Alabama, North Mississippi and much of Middle and West Tennessee for days.
La Niña years have provided some serious snow in the past.
In spite of La Niña years typically being lean in the snow department in the East over-all, 2010 and 2011 were both in strong La Niñas and had big snows. Last winter was also pretty robust in the snow and ice department.
That doesn’t guarantee anything, though. Other strong La Niña years left us with nothing like the 2007-2008 seasons.
- SEVERE WEATHER? How much does it influence severe storms?
Research published in 2015 shows a decreased tornado risk in El Niño but a statistically significant increase with La Niña over the Tennessee Valley west toward eastern Oklahoma: Dixie Alley.
In the four maps below, the two on the left show (top left) tornado frequency decreasing and (bottom left) hail storm frequency decreasing in El Niño years. The two on the right show a different scenario: (top right) increasing tornado frequency and (bottom right) increasing hail storm frequency during winter and spring months.
It has been noted over the last few decades that some of Alabama’s worst tornado outbreaks have come as the La Niña pattern weakens/ends in the spring. That was the case in 2008, 2011 and again in 2021 (mainly in Central Alabama and Mississippi).
So when it comes down to it, the individual, high-impact events are influenced by these larger drivers like La Niña, but it does not guarantee them or deny them completely.
Either way, we’ll be here for you as always: keeping track of it all!
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