Atmospheric River Update

The first of three plumes of moisture (atmospheric rivers) is upon us and moderate rain is currently observed along the coast (see radar at 8:30 AM).  If you are living in the interior of western Washington, NOW is the time to head out for some Thanksgiving exercise.  Rain will move in later this afternoon.  If you are in Oregon, you have the whole day.


This first plume of moisture is associated with the first "atmospheric river" in a sequence of three. You can see the "river" in the satellite-based atmospheric water vapor image shown below. Lots of water vapor over the tropics, with thin tendrils of moisture heading into the midlatitudes, including one approaching our region. As I have explained previously, when such moisture is forced upwards by our terrain, moderate to heavy rain results.

The media has recently discovered this term (atmospheric river), often hinting (or stating) that they are deadly or unusual.    

Let me clarify things.  Atmospheric rivers are normal features of the earth's atmosphere that have been known for a very long time, but were previously called other names (such as the warm sector jet or water sector moisture plume).   The image above has six atmospheric rivers, and a similar image over the Atlantic (below) shows two.
Like anything else, atmospheric rivers vary in strength, location, and longevity.  And if even a modest one gets stuck over a region for a period of time, unusually heavy precipitation totals can occur (such as happened earlier this month over Northwest Washington and southwestern British Columbia.

Let's examine the latest forecasts for our region.  The predicted accumulated precipitation over the 24-h ending 4 AM Friday....the total of the first atmospheric river...... is shown below.

The moisture is coming in from the southwest, so the western slopes of the Olympics, Cascades, and Vancouver Island mountains get quite wet, with totals reaching 2-3 inches.  But Puget Sound and Portland are rain shadowed, generally getting less than an inch.   But if you really want to be dry, head to the lower eastern slopes of the Cascades, where the descending flow will prevent any rain.  

Yes, you can stroll around Vantage or Yakima without a worry about getting wet.


A closer view reveals the profound rainshadow to the northeast of the Olympics.   Wow.  A spot south of Port Townsend should be essentially dry for the period.  No wonder so many folks retire there.  Palm Springs in western Washington.


Friday afternoon and Saturday morning should be dry over the region.  An opportunity to work off some of Thanksgiving excesses.

The next front/atmospheric river will spread precipitation over the region Saturday night and Sunday.  

To illustrate, here is the 24-h precipitation ending 4 PM Sunday.   More precipitation than the Thanksgiving storm, with 5-8 inches at some mountain locations.   But again, profound rain shadowing, particularly over Yakima and Portland.   And unfortunately, little precipitation in California where is it acutely needed.


What about the real rivers?

Below is the latest flooding forecast from the NOAA/NWS river forecast center.  The biggest predicted problem will be the Skagit River, where a moderate flood is predicted, with a minor flood on the Nooksack.


To give you more insight into the situation, below is the river stage level and discharge rate at the Skagit River near Mt. Vernon.  Time is on the x-axis and the current time is shown by the vertical line.  You will notice three peaks in the river level, the first on Saturday from today's atmospheric river and then a larger peak on Monday from the weekend event.   The red horizontal line shows the major flood level..which is barely reached.   We are not going to be close to the November13-16th event, which is also evident in the plot 


The same is true of the Nooksack River, which caused so much trouble recently.  The second event will bring things up to a major flood level, but not nearly as high November 16-17.  Can be thankful for that.


Enjoy Thanksgiving.  I have to start cooking!



Comments

Post a Comment

Popular posts from this blog

Why so little lightning in the Pacific Northwest? And a very nice weekend ahead.

Image
During the past 24-hours, there have been a few lightning strokes over the region (see below) but nothing to write home about (see below). It turns out that the Pacific Northwest and the remainder of the West Coast have some of the lowest frequencies of thunderstorms in the entire country (see below). But why?    My podcast provides the answer. And I also give the weekend forecast....and I think most will like what I have to say. To listen to my podcast, use the link below or access it through your favorite podcast service. Some major podcast servers:     Like the podcast? Support on Patreon 
Read more

The Time of Year You Can See the Air Move

Image
  Most of the time, you can not see the air move.   You can feel it, but the complex, turbulent motions are invisible to you, except when it interacts with leaves and branches on trees and plants. But for a few weeks of the year, the sky is full of small, cotton-like tuffs that like a natural MRI machine reveals the complex three-dimensional motions of the atmosphere.   Cottonwood season.  And we right in the middle of it.   Being cottonwood aware is of particular value this year.  It reveals the active dispersion and movement of air that makes the outdoors essentially COVID safe, and thus should be of some comfort for those nervous about transmission outside. Cottonwood seeds are embedded in small cotton-like fibers.   A million seeds weigh about 3 pounds.  Yes, each seed and tuff weights about .000003 pounds.   And that fact, plus the cottonwood tuff, means that these seeds fall VERY, VERY, VERY slowly.   So slowly that they are excellent markers for the three-dimensional flow of the
Read more

A Much Colder Than Normal April: But How Unusual?

Image
  April was a chilly month over the Northwest, but how chilly?     It turns out the answer depends on where you are located. The difference between this April's temperatures from normal across Washington State (below) shows that the entire state was well below normal.  The eastern side of the state was particularly frigid:  some areas were more than 6 degrees below normal.   Considering the west side of the state, the April mean temperatures at SeaTac Airport are plotted below.   This April was the coolest since 2011. April 2020 was in 15th place for the coolest April since the mid-1940s and was a bit more than 4F below normal.  So if you felt that you were cheated out of normal spring warmth...you are right. But if you really wanted to experience extraordinary April cold you had to be east of the Cascade crest. Consider Yakima.  Wow.  It had the third coldest April temperature since the mid-1940s.   And the all-time record in 1955 was only slightly colder. If you want a longer-te
Read more