Thursday, December 10, 2015

Pacific Northwest Deluge

It's been raining pretty much the entire month of December in the Pacific Northwest, but the last several days have accounted for the majority of the precipitation so far. This month so far rainfall has totaled from 16 to 25 inches in northwest Oregon and western Washington, with lower amounts north into British Columbia. The CoCoRaHS station at OR-TL-2 (Tillamook 16.1 NE) has measured 25.03 inches of rain so far this month. In Washington the CoCoRaHS observer at WA-GH-47 has measured 22.87 inches of precipitation through today. Over the weekend Portland, OR experienced its wettest 24 period on record with 3.33 inches.

Total precipitation for December 1-10, 2015 in the Pacific NW.
Percent of normal precipitation for December 1-10, 2015 in the Pacific NW

The persistent heavy rain has resulted in extensive flooding and at least one fatality. The flooding even impacted the Olympic Mountains Experiment (OLYMPEX), a ground validation field campaign designed to verify and validate satellite measurement of precipitation from the constellation of satellites known as the Global Precipitation Measurement (GPM) The Doppler on Wheels (DOW) was raised several feet off the ground to keep it out of floodwaters, and the DOW crew had to kayak in to operate the DOW.

A DOW operator kayaks out to the DOW which is surrounded by floodwaters.
Photo credit: OLYMPEX

On the positive side, the rain is helping put a dent in drought conditions. The biggest improvement has been in northwest Oregon into central and northern Washington.

However, significant drought still persists, though there will likely be more improvement after this week's rain and snow.

In an unusual twist today a tornado touched down in Battle Ground, WA. A damage survey team from the Portland NWS office found EF-1 damage from the tornado with estimated winds of 104 mph. There were two touchdowns along its two-mile path, and 36 homes and one business were damaged, as well as numerous trees.

Unfortunately snowfall has not been as prolific as the rain. There was snow in the Cascades earlier in the month and was at its peak (so far) the last week of November, but warm weather and rain melted much of that in the past ten days.

Snow Water Equivalent (SWE) for December 1 (left) and December 10 (right).

Mountain snow pack is important for warm season water supplies as well as for winter recreation. Last year 28.7 percent of the Pacific NW region was covered with snow compared to only 22.6 percent on this date last year. The storm now approaching the Pacific Northwest holds promise for some significant mountain snows for the Cascades and northern Sierra through this weekend. A foot of snow fell in the mountains today and snow is forecast in higher elevations through tomorrow. Heavier snow is expected over the weekend. It remains to be seen how long the snowpack can be retained.  

The long-term drought in the west has taken a toll on winter snowpack. Last season recorded some of the lowest snowpack amounts on record in the Cascades and Sierras. There has been some hope of chipping at the drought in California with the expected increase in western storminess as a result of the strong El Niño. However, so far the impacts we've seen from El Niño in the Pacific Northwest and California are nothing like the last strong El Niño in 1997-1998. In 1997-1998 the Pacific Northwest had a fairly dry December while southern California picked up above normal rainfall.

Clearly the weather this year is so far not behaving like the weather in the last strong El Niño in 1997-1998. Granted, there is a long way to go before this season is over, and it will be interesting to see how the pattern develops as the season progresses.

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