October 08, 2016

Atmospheric Rivers Hit the Northwest

The dry part of the year is over.  Done.  Finished.   Today and later this week, currents of moisture are impacting our region, bring substantial rain, particularly in the mountains.  Over the 24 h ending at 7 PM Saturday, a number of locations has received several inches of rain, particularly over the SW side of the Olympics and north Cascades (the graphic below shows places getting more than 1.2 inches).  A few locations have been drenched with 3-5 inches.

A narrow tongue of moisture, originating in the tropics, called an atmospheric river, is associated with this wet bounty, as shown by the figure below (which shows the total amount of moisture in column of air, predicted for 5 PM Saturday).  Blue and white are high values.

The radar image at 2:34 PM Saturday shows the precipitation explicitly.   I had to quit my gardening tasks around this time....too wet.

This band is slowly moving southward and will be positioned south of Seattle on Sunday.   Good for Seattle, bad for Portland and vicinity.

The UW WRF model run shows the 24h rainfall ending 5 AM Sunday, indicating 2-5 inches in places.

The forecast for the next 24hr (below) hits Portland and vicinity quite hard.  Amazingly, tight east-west band.  Very much the look of an atmospheric river.

But as interesting and impressive this event appears, a much more threatening situation will occur on Thursday, particularly over Oregon and northern CA.  Here is the forecast atmospheric moisture forecast for 2 PM Thursday.  Wow.  First class atmospheric river (blue colors) all the way to the coast.

The 24h predicted totals ending 5 AM Friday are substantial from northern CA to southern BC, with "favored" black areas getting 2 to 5 inches.

The 72 hr total ending 5 PM Saturday are huge, reaching 5-10 inches near the Oregon/CA border.

So we are in store for an active period.  October and November are primo atmospheric river months along the West Coast.  The subtropics are still warm, so the moisture supply is large (warm air holds more water vapor) and the flow pattern gets more active, with deep troughs in the eastern Pacific drawing the moisture northward in filaments of high water vapor values. When such moisture is forced to rise by West Coast terrain....watch out.  Very heavy precipitation is often the result.  You better find your Goretex jacket.


  1. So with this troughing period ahead of us throughout October and into November, what are the chances of that being coupled with cold air? I am a snow lover....

  2. I haven't done my diligence yet to check the forecast temperatures, but the recent plot verifies that we've been holding in a warm spell; rather I should say a "not cold" spell: The overnight lows have been above the normal lows, holding in the mid 50s (except one day).

    I look forward each year to that first day in October when I can sleep well, when it drops below 50F early enough in the evening that windows are enough to get the bedroom to 52-55F by midnight. I sure hope it still happens in October this year, or I'll fear for our snow.

    As an aside, I did a quick three-hour jaunt on west Rattlesnake Saturday morning in shorts and a T-shirt. It was so warm that there was no point in wearing GoreTex, not for the 0.03" of drizzle we got.

  3. The annual rite of Fall in my house is when my Labradors move from the cooler floor up on the bed to burrow into the comforter (I sleep with windows open). That happened two or three weeks ago when the Sequim temps started to drop well into the 40's most nights.

    I suspect this would happen much later if I lived in the urban heat island of Seattle.

    Before we moved back to the US and retire (returning after living/working in SE Asia for a long time), we searched the entire country for places that didn't get too cold in winter or too hot in summer. Weather was our highest priority in location. To our great surprise (but probably not for most PNW folks), the northern portion of eastern Clallam County was the only place within the borders of the US that fit our weather needs. Other than for a few too hot, too dry days in summer -- perfection.

    The best news is that most Americans think we're crazy living in this far corner of the map (under the coffee cup). I pretend to agree with them, hoping they will quickly return to Florida or Arizona or S California (or wherever) with horror stories about cold and rain and darkness. Now if we could just change the state flower from Rhodies to moss, and maybe replace the GoldFinch with some perpetually wet bird like the Rhinoceros Auklet, we'd complete the story.

  4. Any estimates on potential wind speeds during this period? Thanks.

    1. I'm wondering the same. I'm sure they won't have much as far as wind predictions go til it gets closer. But it looks like if Saturdays storm gets a bit closer to the coast and/or comes in a bit further south (central or southern Vancouver island instead of the northern part) it could get a bit crazy around here!

  5. Any information in north central Washington State?
    We are at a little over 1,900 feet.
    Above Grand Coulee,Wa

  6. Any information for north of Grand Voulee, WA
    We are at about 1,900 feet


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