December 05, 2012

Major Weather Transition Ahead

During the past week or so, the Northwest coast of the U.S., from the central California to Seattle has pummelled by one Pacific Storm after another.  Looking at the percent of normal precipitation (see image below) for the last 7 days, you can see northern California has been particularly hard hit, with some places getting 800% of normal precipitation.  The reason:  a persistent trough over the northeast Pacific and an amazing frequency of atmospheric rivers extending into the northern part of the Golden State.
But there is a very positive side to this deluge:  with large precipitation additions recently, both snowpack and reservoir levels are near average over most of California, for a good start to the water season. California needs water.  Eventually, when human-forced global warming hits, they will have  real water problem.  But not this year.

But things are going to change soon, since virtually all the major models are forecasting a major flow transition this weekend, with a high-amplitude  upper-level ridge developing over the eastern Pacific (see example for Saturday at 4 PM).  This ridge is far enough offshore so that we will be in cool, occasionally showery flow, but it will be WAY drier than the last week. 

 This is a favorable pattern for snow, with far lower freezing levels.  Finally, it should be cool enough so that Snoqualmie will get enough white stuff to open.  Here is the 72 h snow total from the UW WRF model.  Over a foot in the Washington Cascades, a feet over some favored locations.

This pattern continues into early next week.  Take a look at the upper level map for Monday at 4 PM... an even bigger ridge.  But don't expect that we will be cloud free and completely dry...there is plenty of flow over us and weak disturbances will southward down the eastern flank of the high.

Anyway, this will be an opportunity for things to dry out a bit and winds should be more modest for the entire region.


  1. Dear Dr. Mass,
    I just finished reading an article on atmospheric rivers on the Scientific American website. Besides looking at megastorms that come every 200 years it pointed out that much of California's rainfall and snow pack derives from atmospheric river events that add up to only a few days per year. I would be very interested in a blog where you discussed the article in respect to the NW.

    Thanks for all of your efforts in public education. Scientists who can communicate effectively with the general public are all too rare.

  2. Baker has had close to two feet during the day today.

    As I was travelling from Baker to Sequim via Port Townsend ferry, the rainshadow was evident, but appeared to be over the southern Kitsap Penninsula, rather than Sequim. I guess that means direct Westerly flow.

    My weather station in Sequim just picked up .14 inches of rain in about 1/2 hour! Serious weather!

    Love it!

  3. Oof. My work wants me to go to Lake Wenatchee on Monday to delineate wetlands. I was hoping that the snow would melt off, but it doesn't sound like we will be so lucky. Thanks, Cliff!

  4. Huge hail on the east side of Orcas Island today! (1:32PM)


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