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NOAA Still Predicting La Nina Could Shape Coming Winter; Unrelated To Big Rain Hitting This Weekend
Posted on Friday, October 20, 2017 (PST)

A weak La Nina weather pattern is still predicted to settle in before winter, but that powerful atmospheric influence is unrelated to a “river of moisture” that is moving across the Pacific Northwest this week.


The NOAA Climate Prediction Center put out its monthly outlook Thursday along with a teleconference from College Park, Md.


The long-term forecast favors a wetter-than-average winter across the northern states, including Columbia Basin states, along with colder-than-average temperatures. That has been the outlook for the last couple months with a La Nina in effect — a designation that could be upgraded to an advisory in coming weeks.


“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 at 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 and drier conditions in the South.”


La Nina kicked in prior to the winter of 2016-2017, and the result was cold temperatures and record or near-record snowpacks in Oregon, Washington, Idaho and Montana, followed by a bountiful runoff in the Columbia River Basin and a return to hot-and-dry weather in the region after La Nina subsided.


Halpert and other NOAA forecasters caution one La Nina or El Nino pattern will differ from others — in other words, their influences on weather will differ. Weather can also be greatly affected by other powerful but short-term atmospheric forces, such as the Arctic Oscillation and “Polar Express” patterns.


The “atmospheric river” that stretched across a 5,000-mile region of the Pacific Ocean this week is unrelated to a La Nina pattern that has yet to become established, said David Bishop, a meteorologist with the National Weather Service in Portland.


“La Nina has a 55-to-60 percent chance of developing before winter sets in,” Bishop said. “This is a completely independent system.”


Bishop noted that the media has become somewhat captivated by atmospheric rivers in recent years, but “they are not out of the ordinary by any stretch of the imagination.”


He added that “It is, quite literally, a river of moisture in the atmosphere. It can bring a lot of rain and storm-force winds, which can lead to dangerous surf conditions on the seaboard.”


Satellite images show this week’s event taking shape in a stunning stretch of clouds and low-pressure pockets stretching from China to Washington and Oregon, prior to the system swiftly moving to the east.


Bishop said two or more inches of rain were expected to fall in western Oregon from Wednesday evening through late Saturday-early Sunday. The freezing elevation during that period in the Pacific Northwest was expected to reach a low of 4,000 feet on Friday night and up to 5,000 feet during the duration of the event. After it subsides, the freezing level is expected to be at elevations above 8,000 feet.


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