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home : current news : current news December 15, 2017

11/21/2017 12:23:00 PM
Nothing extreme about winter forecast
The Three Sisters have donned their winter blankets. Odds are on a cool winter, particularly chilly in December and February  but nothing in the forecast is extreme. photo by Ceili Cornelius
+ click to enlarge
The Three Sisters have donned their winter blankets. Odds are on a cool winter, particularly chilly in December and February but nothing in the forecast is extreme. photo by Ceili Cornelius

By Ron Thorkildson

As if on cue, the arrival of September's autumnal equinox brought with it an abrupt change in the weather. Earlier in the month the heat and smoke of summer was still in play as temperatures soared into the 90s. Then, on September 19, the snow level dropped to 5,500 feet and blanketed the central Cascades with its first snowfall - thanks to a shot of chilly air from the Gulf of Alaska.

And just like that, the summer of 2017 was history.

The cool and wet weather continued through October, with the temperature bottoming out at 18 degrees F on Halloween morning as snow continued to fall in the mountains. The first half of November saw normal temperatures, but rainfall was more than 0.75 of an inch below normal.

On September 14, the Climate Prediction Center issued a La Niña watch. It stated there was a 55-60 percent chance of a La Niña to develop during the fall and winter 2017-18 season. At that time the Oceanic Niño Index (ONI) was 0.2, making the El Niño Southern Oscillation (ENSO) signal neutral.

The La Niña watch was updated on November 9, declaring that the ONI value is currently near negative 0.5, putting the ENSO on the cusp of the expected La Niña. There is now a 65-75 percent chance that La Niña conditions will continue at least through the 2017-18 winter.

The outlooks generally favor above-average temperatures and below-median precipitation across the southern tier of the United States, and below-average temperatures and above-median precipitation across the northern tier of the states.

Wintertime La Niñas usually tip the odds in favor of cooler and wetter conditions here in the Pacific Northwest.

The Oregon Chapter of the American Meteorological Society held its annual Winter Weather Forecast Conference on October 28 at Oregon Museum of Science and Industry in Portland. Kyle Dittmer, hydrologist/meteorologist for the Columbia River Inter-Tribal Fish Commission in Portland, gave his outlook for the period December through February. Temperatures should be slightly cooler than normal in December and much cooler in February, but a shade above normal in January. Precipitation is expected to be normal in December and February, slightly above average in January.

Rod Hill, meteorologist for KGW TV in Portland, believes temperatures will be cold in December and February, slightly above normal in January. He thinks precipitation totals for the three-month period will be normal at best, and wouldn't be surprised if it turned out on the dry side.

Though not in attendance at the conference in Portland, Pete Parsons of the Oregon Department of Forestry has issued his own forecast for the three-month period November through January. Parsons anticipates temperatures in November and December to come in somewhat above average, colder than normal in January. Precipitation levels are expected to be above normal for the three-month period, with ample snow in the Cascades.

Parsons further projects that, unlike the past two seasons, the second half of winter will be colder than the first half.

The Climate Prediction Center's revised outlooks with regard to temperature and precipitation came out on November 16. For the three-month period December through February, there is a 70 percent chance of below-normal temperatures in western Washington extending into extreme northwestern Oregon. Odds for cold temperatures fall to 55 percent in Central Oregon.

There's a 55 percent chance of above-normal precipitation levels in Washington and northeast of a line extending from about Astoria to southeastern Oregon. The remainder of Oregon stands an equal chance of getting greater, less or normal amounts of precipitation.

For access to these charts, visit predictions/long_range/two_class.php.

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