NOAA: La Nina likely this winter
Drought likely to persist in northern Plains.
Forecasters at the National Oceanic and Atmospheric Administration’s (NOAA) Climate Prediction Center released the U.S. Winter Outlook this week, with La Nina conditions potentially emerging for the second year in a row as the biggest wild card in how this year’s winter will shape up. There is a 55-65% chance that La Nina will develop before winter sets in.
“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 the 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.”
Other factors that influence winter weather include the Arctic Oscillation, 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, which can affect the number of heavy rain events along the West Coast.
2017 U.S. winter outlook
According to NOAA, wetter-than-average conditions are favored across most of the northern U.S., extending from the northern Rockies to the eastern Great Lakes, the Ohio Valley, in Hawaii and in western and northern Alaska. The entire southern U.S., on the other hand, is expected to experience drier-than-normal conditions.
NOAA said 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, but 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,” NOAA noted.
Drought sticking around
Regarding drought, NOAA said 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.
NOAA’s “U.S. Seasonal Drought Outlook” reported that drought improvement or removal occurred during the past month across the Pacific Northwest, northern Rockies, northern and central Great Plains and western Corn Belt.
According to the “U.S. Drought Monitor,” as of Oct. 17, abnormal dryness (D0) and moderate drought (D1) expanded across parts of the eastern U.S. and lower Mississippi Valley.
NOAA Climate Prediction Center forecaster Brad Pugh said the highest confidence for drought removal over the Oct. 19, 2017, to Jan. 31, 2018, outlook period is across Washington and northern Idaho due to heavy precipitation forecasts in the short term and a wet time of year during the outlook period. Removal and improvement also are predicted for the northern Rockies.
“Although the seasonal outlook calls for increased chances of above-normal precipitation across eastern Montana, a relatively dry time of year favors persistence on a broad scale. This persistence extends east to include the ongoing drought areas of the Dakotas,” Pugh said.
Elsewhere, NOAA said drought could develop across scattered areas of the South, mainly in regions that missed the rainfall associated with the active 2017 hurricane season.
“Persistence is most likely for the ongoing drought areas across the Southeast, lower Mississippi Valley and east Texas -- consistent with the seasonal precipitation outlook,” Pugh said.
NOAA’s seasonal outlooks give the likelihood that temperature and precipitation will be above, near or below average and also how drought is expected to change, but they do not project seasonal snowfall accumulations. While the last two winters featured above-average temperatures over much of the nation, significant snowstorms still affected different parts of the country.
“Snow forecasts are generally not predictable more than a week in advance because they depend upon the strength and track of winter storms,” NOAA explained.