November 20, 2011

Northwest Flood Myths and A Major Flood Threat

As we gird ourselves for a period of heavy precipitation and probable flooding on some local rivers, it is probably a good time to talk about some "myths" regarding Northwest flooding events.  Some of these I have discussed previously in this blog, but recently I read a very nice summary of these misconceptions prepared for the USGS by Joseph Jones.  Here are a few:

MYTH:  A "100-year" flood only happens every 100 years on average.
FACT:  a "100-year" flood happens about every 4.5 years on rivers draining into Puget Sound

How many times have you heard on the news about some locale in the region having a "100-year" flood?   A few I bet.  When there is more than one over a relatively short period (5-10 years)  some people suggest something is wrong with the way meteorologists/hydrologists decide such things.  Others suggest that this is PROOF that global warming causing more extreme precipitation (I have a collection of such statements by activist groups and NGOs s that I will spare you right now).  How many 100-year floods have we had lately? January 2009 for the central WA Cascades, December 2007 over southwest Washington and Northwest Oregon, October 2003 over Northwest WA, February 2006 for northern Oregon and southwest WA.  And there are more.

Believe it or not, having frequent "100-year" floods over the region is just what one would expect.  Let me explain. When we use the term "100-year flood" we mean a 100-year recurrence interval...a flood we would expect to be observed once in 100 years at some location.  Or to put it another way, a flood with a 1% chance of being exceeded at a point in a given year.

Now the first thing to consider is that if there is a 1% chance of something happening in a year, it is possible for two events happening two years in a row.  Just like getting two heads in a row when an unbiased coin is flipped.   But there is something else that is more important in this case.   The term 100-year flood is talking about the chances of getting a flood at some location.  As the distance between points increase, the flooding at the points becomes less and less correlated.  You can get a flood at one location and not another.  Think of it this is like flipping coins at more than one location and so the chances of getting two heads in a row are increased.   Thus, if one considers a  region, the chances of getting a 100-year flood somewhere in the region is greatly enhanced over a single point.   A careful evaluation of the probabilities (e.g., Troutman and Karlinger 2003, Water Resources Research) reveals that for the rivers draining into Puget Sound one would expect a 100-year storm every 4.5 years!   If you widened your viewpoint to the all of western Oregon and Washington, the chances would even be better.

So if we have an 100-year flood every year or so around the region..that is exactly what one would expect and you don't need climate change to get it.

MYTH:  Rain-on-snow events, where warm rain falls on and melts a lot of snow, is a major cause of severe flooding.  In other words, melting snow is a critical component of major floods.

FACT:  Rainfall intensity and duration are the keys elements for all major local floods.  Melting snow might contribute in a minor way, but is not required.

If one analyzes the major floods in western Washington, all have been associated with extreme precipitation that could explain the vast majority of the flooding (reference:  hydrological expert Professor Dennis Lettenmaier of the UW, chief US Army Corps of Engineer meteorologist in Seattle--Harry Schick).  Several major floods early in the season occurred when there was little if any snow to melt--just as October 20, 2003.

This Week's Situation

There is a very serious heavy rain and flooding event setting up this week.  Originally it looked like the Olympics would be be in the center of the plume of moisture, but the latest model runs suggest the heaviest precipitation could be over southwest WA, northwest Oregon, and the southern WA Cascades.  Even the Puget Sound region will get a lot of rain and it would be worse for Portland.  Here is the 48-h total precipitation ending 4 PM Wednesday.  The reds are 5-10 inches--and most of this occurs over 24h (see second graphic for 24-h precipitation ending 4 AM on Wednesday). 

 Here is a blow-up 48h rainfall over Washington--Portland is going to get hit hard if this is true with some isolated areas of over 10 inches of precipitation.  It is going to fascinating (and scary) to watch this unfold with the new coastal Doppler radar.  This is the kind of situation it was installed to help with.

My Lost Dog Situation
       Leah, our black and white female cockapoo, is still on the loose in Mountlake Terrace...we have had some sightings and one person even gave her a hamburger before she bolted.  Our biggest problem is that the City of Mountlake Terrace is removing our signs, even ones on private property.  Last sighting was near the Moose Casino off of SW 220th.  If you live in that area, please let us know if you see her.


  1. Cliff its unbelievable that they are taking your signs down. Do you know what department is doing that? Id like to give them a stern but polite piece of my mind on Monday. :(

  2. Thank you for the rain update. I live near Spanaway/Frederickson and although we don't have to worry about river floods (living on a hill), our backyard is a low point and already is saturated almost year round. It will be interesting to see how the rain turns out, from the images we are in the black (heaviest rainfall) the entire time.

  3. Oh, Cliff, I am so glad your little doggy is alive, but I was really hoping she would be in your arms by now. Maybe for Thanksgiving...

    Thanks for the heads up for SW WA. I live in Clark County, so I will be paying close attention to the commonly flooded areas.

  4. Re 100 year floods:
    But aren't the weather events that lead to flooding in a region correlated? Adjacent rivers will flood due to the same rainy weather.

  5. I grew up with one of our many fresh water streams in my back yard. Hylebos Creek down a little north of Tacoma. Many years during Thanks Giving the creek would flood our street. My house was at the top of a hill, down below in the back yard were wetlands. It was neat to see the 2-3 feet of standing water through the bare trees & bushes during the wintertime. As an adult I'm not that sickly thanks to the endless amounts of mud and muck any kid could ask for :)

  6. Cliff,
    Good to bee you using the progs beyond 72 hours. As you know I have longed believed that could add value.
    Hope you are reunited with your pooch! All I can say is that is one tough Cockapoo. Fingers crossed!

    sitting here in San Diego at the moment. Moderate rain as the front pushes through. Nice.

  7. Any one taking down lost dog signs deserves to lose their own.

    I sure hope you have Leah back soon.

  8. See:
    High Winds and Heavy Snow especially for Bellingham, but did the Model Chart miss the Corresponding Rain ...or it won't be AS DRAMATIC as the Wind and Snow is depicted? You said it was going south and Portland seems to be in the heaviest rain band, that is the red on the chart. So it is only producing the highest winds and heaviest Snows way up north, but not as much rain as is heading towards Mt. St. Helens!

  9. Thank you Cliff for the rain update. We are driving down to Manzanita OR, on Wednesday. Keeping an eye on the developments.
    As to your dog, I am willing to call the city tomorrow about the signs. There is at least one on my street, you can put one inside my front yard if you want. We are south of the places where Leah was reported but if that can help. We keep an eye out when moving around. Good luck.

  10. In my opinion, climate does not cause ANY weather event. Climate simply sets parameters that weather can operate within for any local environment. By adding greenhouse gas to the atmosphere we are adding energy to the system that can manifest itself in any number of ways. It evaporates extra water to fall as rain or even snow in the winter. It can enhance low and high pressure systems to intensify storms or droughts. Even extend the seasons, heat the oceans, disrupt familiar circulation patterns and intensities. Melt polar ice, shift migration patterns and breading seasons for species. Raise freezing levels driving species on heat islands to extinction. All of which has been observed and documented by scientists around the world and verified.

  11. So I think I will not drive to Portland for Thanksgiving! And I have heard and even been involved in some amazing dog searches and returns of the dogs to their homes. I trust your friend will get home.

  12. I would suspect that the relationship between snow melt and flooding is different for different river systems. We live on the Skagit and it definitely floods more when warmer. I don't know if it is snowmelt or increased rain w/ the warmth of the "pineapple express"-type event that 'drives' the flood. The rising altitude of the snow level on the hills during the warming implies that some melting is happening though.

  13. Would love to hear your thoughts on the Snoqualmie Valley flooding, which in my opinion (and in my backyard) has gotten worse since 2005. As communities out here grow, more people are impacted by these flood events.

  14. Is it possible that you are metrically challenged? Given that your blog is for the PNW, which could be construed as extending into Canada (which is *gasp* a metric country), it would be awesome if you translated inches of rain into the (more sensible) metric system.

    .... I'm not going away ....


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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