The 2017 Atlantic hurricane season,already among the top 10 most active on record,may get a small boost during its final months from what may be a developing La Niña.
In their monthly diagnostic discussion issued Sept. 14,scientists at NOAA's Climate Prediction Center said there was a 55 to 60 percent chance a La Niña could develop during the fall or winter.
(MORE:Hurricane Central)
The latest sea-surface temperature anomaly analysis shows a rather distinct cooler-than-average pool of water already in the equatorial Pacific Ocean. Despite that,the magnitude of the anomaly over a three-month period has yet to officially reach La Niña criteria.
La Niña,a counterpart of the more famous El Niño,is the periodic cooling of the equatorial eastern and central Pacific Ocean,which can shift weather patterns over a period of months,bringing the possibility of more sustained warm,cold,wet or dry weather in parts of the world.
However,the tropical atmosphere has been behaving as if La Niña had never left since last fall,thanks to warmer water collecting in the western equatorial Pacific Ocean,according to Dr. Michael Ventrice,atmospheric scientist for The Weather Now 24,an IBM Business.
Clusters of thunderstorms tend to be more numerous over pools of warmer ocean water,which then affects the atmosphere downstream.
(MORE:Eight Straight Atlantic Hurricanes a First Since 1893)
Less Hostile Atlantic Environment
There's one important takeaway from all this La Niña talk,put simply by Dr. Ventrice:"That means reduced vertical wind shear over the Atlantic."
Wind shear,the change in wind speed and/or direction with height,acts to either inhibit tropical cyclones from forming or can rip them apart.
So far in the 2017 hurricane season,wind shear has generally been lower than average. However,wind shear tends to increase by October and especially November.
While La Niña is only one of many influences on the atmosphere at any time,if it helps reduce wind shear,that would contribute overall to a healthier environment for tropical cyclones to form and intensify in the fall.
While admittedly a small sample size,there are a couple of recent years during which a La Niña had developed by fall.
(MORE:Fall/Early Winter Temperature Outlook)
Just one year ago,the October-through-November period was almost four times as active,featuring not only Category 5 Hurricane Matthew but also Category 4 Hurricane Nicole and Thanksgiving landfalling Hurricane Otto.
In 1995,October-through-November was more than twice as active because of six named storms,four of which were hurricanes,including Category 4 Hurricane Opal,and Category 3 Hurricane Roxanne.
According to the National Hurricane Center,a typical October-through-November period sees another three named storms develop,two of which become hurricanes and one of which reaches at least Category 3 status.
Activity Shifts West in Fall
By October,we typically see the formation zones of tropical storms and hurricanes shift back west toward the western Caribbean Sea,eastern Gulf of Mexico and far western Atlantic Ocean as the "Cape Verde"portion of the hurricane season,featuring development of African easterly waves in the eastern Atlantic Ocean,fades.
So far in 2017,the western Caribbean Sea has been a relatively spared area,with the exception of Hurricane Irma's pummeling of parts of Cuba.
As a result,some of the largest sea-surface temperature anomalies and ocean heat content in the tropical Atlantic Basin are in the very part of the Caribbean Sea that is typically a hotbed for late-season development.
(MORE:Beware of Late-Season Hurricanes)
Roughly one-fifth of all U.S. hurricane landfalls have occurred in October and November.
In South Florida, October is the month with the most hurricane direct hits. According to NOAA's Best Track Database,23 October hurricanes have passed within 100 nautical miles of downtown Miami since 1851.
With roughly one-quarter of an average Atlantic hurricane season left, a number of tropical storms or hurricanes may still form before season's end. Stay aware of what's brewing in the tropics on www.weathernow24.com,and make sure your hurricane plan is ready in case a fall hurricane threatens.
Jonathan Erdman is a senior meteorologist at www.weathernow24.com and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7. Follow him on Facebook and Twitter.
