March 29, 2011

Atmospheric Rivers and Rainshadows

In past blogs I have talked about atmospheric rivers---plumes of moisture approaching our region that generally start in the tropic and subtropics. These moisture rivers are generally associated with warm air (since warm air can contain more water vapor than cold air) and they can dump huge amounts of precipitation when they interact with the substantial terrain of our region.

You know the most famous of these atmospheric rivers well...the Pineapple Express, which begins somewhere near Hawaii.

Not all atmospheric rivers approach us from the southwest, and our region is about to be influenced by one from a more westerly direction. The direction of the flow is very important. A nice video produced by NOAA is found below:

Here is an image showing you the total water vapor in a column predicted for 8 PM on Wednesday. A nice plume of moisture heading to us from the west-southwest. Not the equivalent of the real primo atmospheric rivers....but good enough to supply plenty of moisture.

As we will see the direction of the flow will be important.

In many a blog I have talked about local rainshadows and their brethren, the windward precipitation enhancement. Flow approaching a barrier (particularly moist flow) will produce copious precipitation, while descending air dries rapidly. Normally, our major precipitation events and primo atmospheric rivers are associated with southwesterly or south-southwesterly flow and the rainshadow is over Sequim and Port Townsend, home to many retirees and golfers. But these folks don't always get the shadow...when coastal winds turn more westerly the rainshadow shifts southward over Puget Sound and the lavender farms NE of the Olympics get wet.

That is what will happen tomorrow. Here is the 24h rainfall predicted for the period ending 5 AM on Thursday. The rainshadow will move southward and north Puget Sound will be virtually rain free, while the Cascades and the Olympics get pounded with as much as 5-10 inches of rain. Rivers will rise rapidly and some flooding is possible (see the NWS web site for flood watches and warnings). There is also a substantial avalanche threat with all the snow we have had lately gets hit by heavy rain and warm temperatures. Sequim looks soggy too!

Here is the 72-h rainfall ending 5 AM on Friday. The north Cascades will really get drenched with almost the whole area predicted to get 5-10 inches. Add to this all the snow that will melt implies very serious flooding potential.

So if you want to stay dry tomorrow...head to southern Snohomish County or the northern Kitsap....or drive over to Vantage on the Columbia River....


  1. This is off topic, sorry. Do you have any insight as to what this summer will be like in the PNW? If so, could you do a post? We're up for jul/aug/sept and are wondering if it will be as bad as last year. Thx!

  2. Nicely written tonight (you must be getting rested up from your trip). I like the map of the atmospheric river. I was thinking today while out on a walk that often I have trouble with the legends on the charts you use. The one tonight on 24 Hr rainfall projections had a scale on the right that appears to be in "cin". What is that? It didn't seem to match the 5-10 inch projections you were making. Often the other references are also opaque to me(GFS 4Km domain). Is there a general legend reference guide for the terminology and techniques of these different charts that I could access? thanks, rkburk

  3. This river is kinda different. Its all orographic. You can tell because the models have little precip. out over the ocean.

    And speaking of models, this event is occuring but how can the GFS and NAM be off from another in the exact placement of the heaviest rain bands.

    I live in rainshadow country and I'm fascinated by it. The rainshadow is so strong that radar returns are much further up the west slopes and slopping over farther along the east slopes like tonight its 50/50. Its always more around 80/20.

  4. I have heard that a large snowpack will mitigate the flood threat; that loosely speaking the snow acts as a "sponge" and prevents much of the rainfall from draining into creeks and rivers. I don't know if our current healthy snowpack would play a role in reducing the flooding during this storm or not.

  5. Another nice article, Cliff. Because of the westerly flow, it sure does seem like West Seattle is getting the Olympic rain shadow effect thus far!

    Much windier here above Alki Point than I expected, though!

  6. Rk burk, the legend does indeed say "cin", which I believe is hundredths of an inch. If you look at the top you'll see black represents 512 - 1024 hundredths which would be 5 - 10 inches.

    The thing I'm confused about is how the chart can be either 24 or 72 hour when it says 48h in the corner. Thanks for all your work though, Cliff. Otherwise I wouldn't have any charts at all to look at.

  7. I had the same question as RK Burk. What is the measure to the right of the rainfall projection chart? CIN=cumulated inches? centimeters?

  8. When we finally came back to the PNW after 17 years in Hawaii we got our guitars out, sat on our deck with that stunning view of the Olympics across the bucolic former dairylands of the "Dungeness Valley" (it's not a valley you know) and sang :
    I'm being followed by a rainshadow...rainshadow...rainshadow!

    Mahalo for the blog. We love it.
    Mike a
    Mike and Carol

  9. Is today's, Wednesday's radar a perfect example of how precipitation stops on the other side of the Cascades, or is there just no radar coverage over there.
    The precipitation starts up again at the Eastern Washington mountains.

  10. The scale is in centiinches (cin) i.e. units of 100th of an inch. So 128 cin is 1.28"

    The scale on the rainfall plots is logarithmic (i.e. log base 2 - doubling at each band). This helps to show both very high amounts and details in small amounts on the same plot. That means you see detail in the low lands with very small amounts of rainfall in the rain shadow. For example, the pale blue (less than 1 cin or 0.01" ... less than a trace just enough to wet concrete) and light blue (2cin or 0.02" just a little bit more). Very little rain. But on the peaks with high levels of rain the bands are much larger e.g. 512 and 1024 cin (5.12" and 10.24").

    The 48h storm total amounts shown on those plots is in the dark red between 512 and 1024 cin (5.12" and 10.24") as Cliff mentions.

    Snow level is expect to rise above 8000 feet during the storm.

  11. The Middle Fork of the Snoqualmie is coming up faster than I have seen it come up in years. I can literally watch the rocks disappearing under the volume of water that is coming down. Usually you look out and the water has risen a bit, you look out 15 mins. later and it has risen a bit more. I am watching right now and the rocks are disappearing before me eyes. I know these are not scientific observations, but the stage in feet, according to the Snoqualmie River Flooding page has gone up well over 5' in less than 6 hours and it is still coming down in buckets. Even my ducks are staying out of it!

  12. Fascinating video in teeny tiny window, and no audio at all. What am I doing wrong? How to get large window for these cool videos, and some sound? Thanks for my favorite blog!

  13. Interesting day here on Sinclair Island. We were on the edge of a rainshadow formed by the mountains of Vancouver Island. The rain came in fits and starts, the sky always appearing brighter to the west but repeatedly dashing our hopes of a real break. By the looks of it San Juan Island was pretty dry.

  14. Are there archival records of barometric pressure?

    Has anyone done or is anyone doing research or comparisons of rate and range of barometric changes?

    I have only a decorative barometer but in the >25 years I've lived here on the SE shore of Maury Island it seems to me I'm seeing more events of a greater range &/or rapid rate of change?

    Is there evidence for what I think I've been seeing?

  15. The MM5 forecast form Monday morning looks like an April Fools joke, is it?

  16. Chains required on Snoqualmie and snowing hard! April? Really...tells us warmth is on the way :)

  17. Read in the UK Daily Telegraph today that West Coast and expect lots of Tsunami debris from Japan.

    Does your expertise run to predicting where the currents might take the majority of it? There is a huge 60 mile island of 'stuff' coming our way.


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