La Niña letup. What does it mean for hurricane season?

After an abrupt cooling of waters this spring in the eastern Pacific – heralding the demise of one of the strongest El Niño events on record – surface waters across the eastern Pacific have plateaued over the past month, begging the question whether La Niña will surface in full before the peak of the hurricane season.

La Niña – the periodic cooling of waters in the eastern Pacific – reduces storm-busting wind shear over the tropical Atlantic which can increase overall hurricane activity. It is one of the main factors driving forecasts for a potentially hyperactive hurricane season.

Illustration showing typical conditions during La Niña episodes. Upper-level winds relax in the Atlantic, as do the fast-moving east-to-west flowing trade winds closer to the ocean. The combination reduces wind shear and allows more organization and development of thunderstorms, promoting higher hurricane activity. Credit: NOAA.

But new forecasts issued today from government forecasters delay – though don’t dismiss – the onset of La Niña this fall. In their monthly discussion released Thursday, forecasters at the Climate Prediction Center note a slower rate of cooling favoring the start of La Niña by September rather than earlier expectations of July or August. They maintain high odds (70% chance) of La Niña taking hold this fall and persisting through the winter.

El Niño or lack thereof matters more than La Niña for Florida hurricanes

As we’ve covered in previous newsletters, studies show the number of landfalling U.S. hurricanes is nearly twice as high during La Niña years than during El Niño years.

Average annual mainland U.S. hurricane landfall by El Niño, Neutral, or La Niña phase. The odds of a mainland U.S. hurricane landfall is nearly twice as high during La Niña years than during El Niño years, but almost imperceptible between Neutral and La Niña years. Graphic from Klotzbach et al. 2017.

The difference, however, is muted between U.S. landfalls in La Niña years compared to neutral years – those years not meeting either El Niño or La Niña criteria. Additionally, while the risk of a hurricane landfall in Florida is over twice as high during La Niña years than during El Niño years, the hurricane risk is essentially identical for Florida whether La Niña conditions or neutral conditions are present in the eastern Pacific. The same is true for the Gulf Coast states.

The upshot is that while La Niña matters, it matters in comparison to El Niño. Once El Niño is out of the picture, the threat for more hurricanes and more U.S. landfalls – especially Florida landfalls – ramps up. As far as we’re concerned in Florida, whether La Niña is strong, weak, or nonexistent matters much less than whether El Niño is still hanging around.

El Niño kicked the bucket in June and the odds of El Niño returning this year are next to none. Officially, we’re in no man’s neutral land, but odds still favor a transition to La Niña by August or September, the peak months of the hurricane season. How soon it arrives, however, has less significance for hurricane season than the end of El Niño. Without El Niño in place, the odds for a U.S. or Florida impact are certainly higher, especially with waters in the tropical Atlantic at record or near-record warmth.

What’s behind the La Niña letup and when will it resume?

Stronger than average east-to-west flowing trade winds in the eastern Pacific are the catalyst for La Niña, but those winds were much weaker in June.

The Central American Gyre or CAG – the sprawling area of low pressure that extends from the eastern Pacific into the Caribbean – was particularly active in June, helping to spawn Tropical Storm Alberto, the season’s first named storm that caused major coastal flooding along parts of the Texas coast three weeks ago.

The Central American Gyre has since shut down and a return to strong easterly trades in the Pacific is forecast. Cooler than average waters are lurking beneath the top waters of the far eastern Pacific and the transition to La Niña should resume in the weeks ahead.

Tropical Atlantic on brief summer break for now

The NHC continues to note a broad and disorganized area of low pressure off the southeast U.S. this morning.

Upper-level winds remain unfavorable and development is unlikely. Regardless, the system will be moving toward the Carolinas and inland tomorrow, where it will enhance the heavy rain threat in eastern North Carolina.

Otherwise, the tropical Atlantic will stay quiet through at least the next week as more seasonal Saharan dust and wind shear blanket the basin.

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