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For the second straight year, the weather phenomenon La Niña is likely to affect the U.S. weather pattern this winter.

In an advisory issued earlier this month, the U.S. Climate Prediction Center (CPC) says the cooler-than-normal waters in the equatorial Pacific are likely to linger through at least early spring. Though forecast to be short-lived, forecasters believe the La Niña will likely affect temperature and precipitation patterns across the United States during the upcoming months. This will include Florida and it will likely be noticeable.

 

THE STATUS

The El Nino Southern Oscillation (ENSO), which is a measure of temperature and air pressure variability in the tropical Pacific Ocean, has been neutral for much of the fall. However, the recent cooling trend has been significant and long enough for the La Niña to be declared. The ENSO typically cycles between an El Niño and La Niña state every two to seven years.

An La Niña Watch was issued by the CPC on October 12 after forecast data suggested a 55-65% chance of development during the Northern Hemisphere Fall and Winter. The Australian Bureau of Meteorology (BOM) issued a similar advisory on October 24, stating there was at least a 50% chance of a La Nina forming by November 2017.

Last winter, the U.S. had a weak and short-lived La Nina that formed in November and was gone by February. This year, La Nina is expected to be weak and short-lived again, with sea surface temperatures warming by March 2018.

THE CAUSE

Global weather patterns are extremely complex, and even subtle shifts in temperature or pressure can produce large and notable changes over time, sometimes thousands of miles away. The Pacific Ocean is the largest body of water on the planet, and coincidentally one of the largest contributors to weather norms. The episodes of warmer and cooler water over the equatorial regions of the Pacific have been well-studied and linked to numerous extreme weather events around the globe.

The fluctuations in sea surface temperatures are related to variability in trade winds, winds that blow from east to west near the equator. When the trade winds are stronger, it allows for more up-welling of cooler subsurface ocean water (La Niña). When the trade winds are weaker over these areas, warmer ocean water is the result (El Niño).

Ocean temperatures, and their inherent variability, are the single biggest contributor to atmospheric patterns and resulting wind flow. Steering winds aloft, often referred to as the jet stream, transport warm and cool air across the planet. During a La Niña, this river of air is displaced further north across the continent of North America. Conversely, during an El Niño event, the jet typically dives further south. Both patterns result in areas of abnormality with regards to temperature and precipitation, most notable during the winter months.

THE EFFECT

La Niña events typically bring dry and warm conditions to the Florida peninsula from October to December, and even drier and warmer conditions (compared to normal) from January to March. The forecasts from NOAA’s Climate Prediction Center are no different for the winter counting period, with a greater than 50 percent chance of abnormally dry conditions from December to February, and a 40 to 50 percent chance of warmer-than-normal temperatures during the same period.

La Nina events generally occur every 3 to 7 years and can last anywhere from one to three years in duration. Long range forecast data suggests the sea surface temperatures over the eastern Pacific Ocean will warm closer to normal during the spring and summer months of 2018.


Meteorologist Jeff Huffman contributed to this report.

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