Seasonal Outlook: looks like La Niña

Weather

NOAA released the long-range December-February outlook on Thursday, and the pattern looks a whole lot like La Niña for the eastern United States:

Those two maps show the odds of the entire three-month period being above/below average for temperatures (left) and precipitation (right).

Here’s what the usual La Niña pattern looks like:

“La Nina refers to the periodic cooling of ocean surface temperatures in the central and east-central equatorial Pacific.  Typically, La Nina events occur every 3 to 5 years or so, but on occasion can occur over successive years.  La Nina represents the cool phase of the El Nino/Southern Oscillation (ENSO) cycle.  La Nina weakened last spring into early summer but began to show signs of strengthening again late this summer as negative temperature anomalies returned to the equatorial Pacific Ocean.”

NATIONAL WEATHER SERVICE

A pattern like this does not tell us everything we need to know to determine whether or not it will snow or how many waves of severe weather will happen.

Those individual weather ‘events’ (record cold, heavy snow, ice, tornadoes) can happen whether a particular general pattern is in place or not; however, we do typically see an increased risk of severe weather with La Niña.

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 (2008 and 2011 are two example of that).

What about snow?

No year in my 20 years of forecasting has ever been a slam dunk either for or against snow. We just don’t get enough of it in Alabama and Tennessee to really forecast an above-average or below-average season.

Huntsville’s average annual snowfall is 2.2″ (30-year climatology).

Four of the past ten years had above-average snowfall (2010, 2011, 2014 and 2015). The rest of them had little or nothing at all.

Climatology tells us very little about impacts on winter weather around Alabama; there’s no real difference between the La Niña years and other years because we see so little anyway.

All La Nina winter composite

Love snow and want some encouragement? In spite of La Niña years typically being lean in the snow department in the East, 2010 and 2011 were both in strong La Niñas and had big snows! We’re currently trending into a strong one.

That doesn’t guarantee anything, though. Other strong years left us with nothing like the 2007-2008 seasons.

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.

What we learn from this is that we’d typically expect fewer exceptionally cold days and more stormy days.

Either way, we’ll be here for you as always: keeping track of it all!

Track heavy storms with WHNT.com’s Interactive Radar or swipe over to the radar feature on Live Alert 19! You can also get up-to-date, location-based alerts wherever you are on Live Alert 19. Download it today for iOS and Android.

Looking for the rest of the forecast? It’s always online at WHNT.com/Weather and in the “Daily Forecast” section on Live Alert 19!

-Jason
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