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Half of December without rain? Wettest month of year shaping up dry

(SBG image)

EUGENE, Ore. -- December is Eugene’s wettest month, and rain probably won’t fall for the next 10 to 15 days.

“What has set up over the Pacific Northwest is a strong blocking pattern,” says KVAL Meteorologist Travis Knudsen. “A weather pattern like this usually sticks around for a full week or two before finally breaking down or moving.”

The center of a broad area of high pressure formed off the Oregon coast by a couple hundred miles. This pressure center persistently forces storms across the Pacific Ocean north into Canada, Knudsen says, carrying with them all the moisture that otherwise would fall in the state.

“Usually we see these types of blocking patterns in summer,” says Knudsen. “They’re usually fairly common then. In the winter, they’re much rarer and much more disruptive to the season.”

Though Knudsen says they do happen, the most recent dry spell in Oregon to last more than 10 days was in 2011 - between November 30 and December 13, no rain fell at the Mahlon Sweet Field in Eugene.

Roseburg and Coos Bay saw a period of dry weather that year as well, lasting 13 days for both.

“In place of the clouds and rain we get stagnate air,” says Knudsen, “meaning fog and low clouds are likely each morning, but the afternoons should manage to burn them away and provide a couple hours of blue skies before the sunset.”

Temperatures for the coming days will stay below average. Eugene’s average high in early December is in the middle 40s, but instead, daytime temperatures will struggle to get out of the 30s. Overnight lows—due to a lack of clouds early in the night—will drop below freezing most nights.

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Knudsen says morning fog and low clouds will become routine as long as the blocking pattern continues. An Air Stagnation Advisory is in effect for central and southern parts of the Willamette Valley, as well as most of Douglas County. The advisory lasts through Monday morning of next week.

“Most areas should manage to burn off the fog by 11 a.m. or noon, though places farther south such as Eugene will see the sunshine later than places like Corvallis and Salem,” says Knudsen. “Roseburg, for example, might not see sun until 1 or 2 in the afternoon on some days.”

The lack of moisture is likely to have an impact on snow levels in the Cascades. Snowpack readings from early December placed the Willamette Basin at 64 percent of normal. The Rouge and Umpqua Basins fared better at 85 percent of normal. These percentages will likely fall in the coming days.

“While it does stay cold in the Cascades,” says Knudsen, “temperatures will rise to above freezing each day with sunny skies. Expect snowmelt each day.”

2017’s December begins comparably to last year. The first four days of December 2016 brought 0.55” of rain versus 2017’s 0.58” of rain over the same period.

But with the dry weather pattern setting up over the Pacific Northwest, Knudsen says there is no immediate sign of a return of the ice storm which hit December 14 of last year.

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