November 27, 2021

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.


  1. Is there a way to relate atmospheric rivers to terms of regular rivers like cfs? I suppose inches of precipitation per unit area could give something like acre-feet/time?

    1. Yes there is. Scott Sistek has an excellent blog on this on his Q13 Fox weather blog. Example: "The scale uses 250 kilograms of vapor per square meter as the minimum criteria for an atmospheric river."

  2. Cliff, do you have any idea why the torrent of rain from two weeks ago hit so hard in what is usually a rain shadow in SW BC? The towns of Princeton, Merritt, Spences Bridge, Lytton (of 50 degrees C fame), and Lillooet all are in the dry belt on the lee side of the Cascades/Coast Ranges. Sagebrush and cactus and other xeric species are the natural groundcover. All these places were hammered, with loss of life and some highways being so badly damaged government spokespeople say it may take years to be able to access some ranches and homes. The current atmospheric rivers are behaving normally as you describe in this post - huge amounts of rain on the windward side, very little in these interior communities. What was so different about the mid November storm??

    1. I'm not Cliff, so take my answer with a small grain of salt, but the reason that places like Princeton, Merritt, etc have been hit so hard recently has to do with the direction the precipitation comes from with these kind of atmospheric river set ups. You are correct that they are on the lee side of the mountains, with one key exception. The Fraser River Valley provides a fairly significant opening between the North Cascades and the Coast Ranges. Normally, this gap doesn't make much of a difference, as our typical fall & winter rainstorms tend to approach from the Northwest and move off in an easterly or southeasterly direction. When this happens, inland areas like this are extremely well sheltered from the rain, as there are hundreds of kilometers of fairly significant mountains between there and the Pacific Ocean in that direction.

      Atmospheric rivers, on the other hand, approach us from the southwest, which just so happens to be the exact same orientation as the gap in the mountains created by the Fraser River valley. On top of that, the mountains that do still exist in and around the Fraser River valley are generally quite a bit lower in elevation than the Coast Range to the NW, and there is a LOT less terrain between Princeton and the coast when heading SW instead of NW. So for the relatively unusual events where we get precipitation approaching from the SW, like with an atmospheric river, suddenly these places are in much less of a rain shadow than they are when storms approach from the NW. These areas only maintain their dry climate and desert-ish landscape because it's rare for precipitation to move in from the SW. Hypothetically, if atmospheric rivers were the norm for this region instead of storms coming in from the North Pacific, then these areas would actually be rather green & lush like it is on the coast.

      Also, since the raw model output maps like Cliff has shared above don't have cities & towns labeled, it's kind of hard to tell, but most of these places you mention are not actually in much of a rain shadow with this coming atmospheric river, just like last time. Unfortunately, this event doesn't look much different than the last, and many of the hardest hit areas will probably be soaked yet again.

      PS If my rambling wall of text doesn't make much sense to you, here's a crudely annotated terrain map that might visualize it better, lol:

    2. Thanks for that Ben, and I can see that might explain the southern towns, but Spences Bridge to Lillooet are on the Fraser/Thompson above the Canyon, and tucked well in behind very tall ranges. THe other thing is the atmospheric rivers, back when they were the Pineapple Express, DID come from the SW (the reference to Hawaii), but seldom hit the southern towns that are more open to the Fraser Valley (the Alison Pass and Coquihalla and their flanking mountains are pretty substantial themselves, though not like the Coast Mt. north of Vancouver that Lytton, Spences Bridge and Lillooet are behind).

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

    But in order to actually achieve anything like an understanding would require a journalist to actually read about this phenomenon, instead of publishing yet another ranting screed about AGW and the oncoming Armageddon. That's much easier to publish, and in addition feeds their clickbait machines.

  4. I love these compact bites of information. Clear, concise, and adds wonderfully to my knowledge of the weather. And over time, they are greatly enhancing my understanding of weather phenomena as well as helping me debunk well-meant, but false comments by friends about all kinds of weather, including climate change.

  5. Interesting that these strong rain shadow contrasts aren't associated with an exceptionally strong PSCG. Any reason as to why the convergence zone doesn't seem to be materializing? FWIW, I'm happy that the my area seems to at least be partially rain shadowed and not at risk of convergence zone precipitation.

    1. I have the same question, which I wish Cliff would write a detailed blog about.. under what conditions does the zone form? It's not just "when the wind if out of the west". There's more to it than that. I'm guessing there are specific details in the profile of the atmosphere that must be in place. What are they?

    2. PSCZ typically sets up in post frontal environments. Ahead of fronts, and with atmospheric rivers, the flow is more laminar. Or stable. In a post frontal environment the flow becomes much more unstable, and as the air rushes in behind the front to fill the lower pressure, the convergence is strong and creates the lift necessary.

  6. (trouble posting with Firefox)...
    Not really rain shadow related but this part of the southern Willamette Valley has not received nearly enough rain since mid month. Oregon Cascades are well below normal for snow amounts. Hope this changes soon as the drier than normal winters last two years have led to early fire seasons here.

  7. I also find it interesting that temperature fluctuations are minor during these events. Can you please do a blog on the longest period of minor temperature fluctuation in the Northwest?

  8. Read this twice and can't find an explanaition of the thesis - stronger storms, dryer on the east side.


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