The National Oceanic and Atmospheric Administration now predicts a 65 to 75 percent chance of weak La Nina conditions to continue at least through the winter.
The NOAA announced Thursday that La Niña has arrived for the second winter in a row.
The southern regions of the U.S. will have a drier winter. From the mid-Atlantic and through the southeast into Texas, winter will be drier and warmer, according to NOAA. The northwest and northeast will be wetter, and across the country in the northern regions, people should expect colder temperatures than usual.
Typically, cooling sea-surface temperatures in the equatorial Pacific Ocean lead to cold, snowy winters for most of the northern half of the United States, with the opposite conditions seen in the south.
This year is slightly different, and the recent warming trend is expected to dominate. While it’s likely precipitation will stay above normal this winter, since areas such as Ohio will trend on the warmer side, it’s possible some of that precipitation will be shifted toward rain, rather than the normally very heavy snow during La Nina years.
La Niña is expected to be weak this year, as was last year’s La Niña. Last winter, the west and upper Midwest had one of the wettest winters on record, reported the Weather Channel. The east, south, and Midwest, however, had one of the warmest winters recorded.
NOAA said oceanic and atmospheric signals in October and early November are consistent with a weak La Niña. You can see the strip of cooler-than-average sea-surface temperatures near the equator as of early November in the graphic below.
The black box highlights the cooler-than-average sea-surface temperatures near the equator indicating La Niña conditions on Nov. 9, 2017.
El Nino and La Nina are the warm and cool phases of a recurring climate pattern across the tropical Pacific, called the El Nino-Southern Oscillation, or ENSO. ENSO conditions are usually felt heaviest in the fall and winter, which is why snow and chilly temperatures are most talked about with a La Nina event.
With La Nina, strengthening winds across the Pacific push the warm surface water away, bringing up cooler water below. This cooler water initiates sinking motion in the atmosphere over the Pacific, which dries things out in that region. The cool water can also shift the position of the jet stream, shifting the threat for severe weather and wet conditions in the United States north.
El Nino is essentially the opposite of La Nina. In this phenomenon, weakened surface winds lead to an unusually warm surface temperature in the Pacific Ocean. The increasing heat in the ocean triggers rising motion in the atmosphere above, which intensifies storms and rain over the Pacific.
Warm Pacific Ocean temperatures also shift the jet stream south, making the atmosphere more unstable there. The instability then makes the southern portion of the United States more susceptible to severe storms and tornadoes. So, during El Nino, the southern portion of the United States sees wetter than normal conditions, while the northern half of the states see a drier winter.
This is the fifth “double dip” La Niña in historical record—which is an unofficial term indicating when neutral conditions briefly prevail in between the La Niña winters, according to a climate update from NOAA’s ENSO blog posted on Wednesday. In October, there were signs of a La Niña winter coming as a result of cooler sea surface temperatures in the east-central tropical Pacific and more precipitation over Indonesia. But it wasn’t quite enough to make the call until this month, when all the signs for La Niña were clear and indications of less rain over the central Pacific and more over Indonesia had strengthened.
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