Whether or not NOAA’s winter weather outlook is accurate is highly dependent on what happens with La Niña. La Niña is a coupled ocean-atmosphere phenomenon that is the counterpart of El Niño as part of the broader climate Southern Oscillation climate pattern. During a period of La Niña, the sea surface temperature across the equatorial Eastern Central Pacific Ocean is much cooler than usual, with average water temperatures dropping by 3-5 degrees Celsius below normal. The La Niña phenomena typically persists for at least five months. Water temperatures over the Pacific have significant, extensive effects on the weather in North America.
NOAA’s Climate Prediction Center’s Winter Outlook suggests La Niña will return for the second year in a row, however, they believe there’s only a 55-65% chance of it developing before winter sets in. Should La Niña not set in, the Winter Outlook could be very inaccurate. With the chance not high (>75%), there isn’t a high level of confidence in the Winter Outlook at this time. “If La Nina conditions develop, we predict it will be weak and potentially short-lived, but it could still shape the character of the upcoming winter,” said Mike Halpert, deputy director of NOAA’s Climate Prediction Center. “Typical La Nina patterns during winter include above average precipitation and colder than average temperatures along the Northern Tier of the U.S. and below normal precipitation and drier conditions across the South.”
While water temperatures in the Pacific play a key role in how the winter will pan out, NOAA also looks at other ingredients in the forecast. Other factors that influence winter weather include the Arctic Oscillation (AO), which influences the number of Arctic air masses that penetrate into the South and is difficult to predict more than one to two weeks in advance, and the Madden-Julian Oscillation (MJO), which can affect the number of heavy rain events along the West Coast, according to NOAA.
According to the Winter Outlook, NOAA believes much of the United States could see milder weather than usual, moreso than colder weather than usual.
Warmer-than-normal conditions are most likely across the southern two-thirds of the continental U.S., along the East Coast, across Hawaii and in western and northern Alaska.
Below-average temperatures are favored along the Northern Tier of the country from Minnesota to the Pacific Northwest and in southeastern Alaska.
The rest of the country falls into the equal chance category, which means they have an equal chance for above-, near-, or below-normal temperatures and/or precipitation because there is not a strong enough climate signal in these areas to shift the odds.
According to the Winter Outlook, NOAA also believes Hawaii and the southern United States, especially the Southeast, will be drier than normal, while areas to the north, especially around the Great Lakes, will see more precipitation than usual. When combined with near-normal temperatures, this could translate to a very snowy winter for the Great Lakes and Northeastern US.
Wetter-than-average conditions are favored across most of the northern United States, extending from the northern Rockies, to the eastern Great Lakes, the Ohio Valley, in Hawaii and in western and northern Alaska.
Drier-than-normal conditions are most likely across the entire southern U.S.
According to NOAA, despite the outlook favoring above-average precipitation this winter, drought is likely to persist in parts of the northern Plains, although improvement is anticipated farther West. Because portions of the Gulf Coast saw significant precipitation from the 2017 Atlantic Hurricane Season, the drier than usual forecast winter should balance out with recent heavy rains and not push places in Texas or Florida into drought.
NOAA plans to update their US Winter Outlook November 16. By then, a better understanding of La Niña should be known which would in turn lead to a higher confidence forecast.