The Great Atmospheric River Irony: Simultaneous Extreme Precipitation and Extreme Dryness

Life is full of ironies and contradictions, and our weather situation of the past month is a wonderful example.

A series of atmospheric rivers have moved into the Northwest this month, bringing above-normal precipitation on the western slopes of the Olympics and Cascades.

Ironically, the SAME conditions that produce bountiful precipitation on the western slopes of our mountains cause drying over the downwind and lee slopes, resulting in profound rainshadows.   Huge precipitation contrasts reign.

But to understand why this is true, you must understand the nature of atmospheric rivers.

As I have mentioned in many blogs, atmospheric rivers are associated with plumes of large amounts of moisture from the tropics and subtropics that project into the midlatitudes.   For example, a well-defined tongue of moisture will be heading into our region this afternoon and overnight, as shown by a plot of such moisture at 10 PM tonight.

This is a plot of integrated water vapor (IWV), which is the total water vapor in a vertical column from the surface into the stratosphere.

But moisture is not enough to produce heavy precipitation.  You need to lift the air to saturation and to continually bring in new moisture.

That takes wind.  Wind can bring in more moisture.  And strong winds pushing up mountain slopes produce the upward motion that converts water vapor into precipitation.

Thus, meteorologists who need to evaluate the precipitation-producing potential of atmospheric rivers also plot the product of water vapor and wind, a quantity known as integrated water vapor transport or IVT for short.   

Below is the plot for 1 PM today, with blue colors indicating the largest values.  Wow...a potent atmospheric river is approaching, one that is moving huge amounts of water vapor towards our region (note the arrows indicate the direction and magnitude of this water vapor intrusion).

Strong atmospheric rivers in our region are generally associated with strong southwesterly flow (winds from the southwest).  A NOAA coastal radar wind/temperature profiler at Astoria shows the increasing winds today, which are increasingly from the southwest (see below, yellow and red colors indicating stronger winds).  The profiler is a type of weather radar that can provide wind speed, temperature, and more above a location. 

The strong southwesterly winds will produce massive precipitation on the western sides of our regional terrain.   But as this strong flow descends the northeast side of the Olympics and the eastern sides of the other region terrain, the air will sink, producing warming and declining relative humidity.   Such sinking air is unfavorable to the production of precipitation, resulting in a rainshadow.

Consider the forecast precipitation totals over the next 48 hours (below).  Precipitation ranges from 8-10 inches over "favored" locations on the upper windward slopes to virtually nothing near Port Townsend on the NE side of the Olympics.   Heavy upslope precipitation on one side, little precipitation on the lee side. 

Let me put it another way: there will be a roughly 100 to 1 ratio between precipitation on the western slopes (10 inches) compared to the downslope/rain shadow area near  Port Townsend (.1 inch).   A very small area may have a 500 to 1 ratio.  

And we have not even mentioned the dry conditions in the sinking zone east of the Cascades.

The strongest atmospheric rivers often have the strongest rainshadows, with their powerful winds being a key contributor.    

And there is another story regarding atmospheric river events:  a huge contrast in wind speed and direction that occurs as they interact with our terrain.  But that will wait until a future blog.

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!