December's continued dry spell runs counter to typical La Niña weather pattern

The Portland area is experiencing the driest and one of the coldest Decembers in recorded history.

Through Thursday, Dec. 15, Portland International Airport recorded just 0.06 inch of precipitation. That is more that 2.5 inches below normal for the first half of the month.

'Rainfall amounts like this are unprecedented in the month of December. This is more like something we would see in the first half of August, not December,' says Steve Pierce, president of the Oregon chapter of the American Meteorological Society.

Recording Portland weather began in 1871, according to Pierce.

Portland also experienced the eighth coldest December on record through Dec. 15. The average temperature for the first half of the month was 36 degrees, which is more than five degrees below normal for that period.

'So far, Portland has recorded below normal temperatures every single day during the first half of this month. Additionally, Portland has recorded sub-freezing morning lows on 12 of the first 15 days of December. The highest number of December sub-freezing overnight lows in Portland for the entire month is 25 set in 1985,' Pierce says.

According to Pierce, the rainfall records run counter to predictions. The region is in a La Niña weather pattern, which is supposed to be both wet and cold. But a ridge of high pressure has anchored itself near the Pacific Northwest since the start of the month, and it is steering storm systems well north into Canada - something not normally associate with a typical La Niña pattern.

'It appears as though the wet half of our wet and cold La Niña is taking a vacation and she shows no sign of returning anytime soon. Several cities across the Pacific Northwest are currently experiencing their all-time driest start to December on record,' says Pierce.

At this rate, 2011 could set a record for coldness, Pierce says.

"With 15 days remaining in 2011 and no significantly warmer weather on the way, the Portland International Airport is very likely to set a new all-time record for the least number of 60 degree or higher days in a calendar year,' he says. 'So far in 2011, Portland International Airport has reached 60 degrees or higher just 169 times, which eclipses every other year on record at that location. The next closest year is 1950 and 1955, which each recorded 172 days at or above 60 degrees.'

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