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 way....it 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.