Saturday, August 31, 2013

A Great Irony: The Pacific Northwest is Unprepared for Intense Rain

It happened again on Thursday.   A line of modest thunderstorms moved across western Washington, an event that would hardly crack a yawn in folks from the central or eastern U.S.  There was lightning!  Thunder!  Short, but intense showers!

And we are amazed, and unprepared.     Our streets flood, water fills basements, sewers overflow.  And this in a region supposedly web-footed and Goretex-ready.   What is going on?

First...the "big" event on Thursday.

 Marine Drive in Everett on Thursday evening

Local media had a field day

Between four and 6 PM that day, a line of moderate-intensity thunderstorms moved northward up Puget Sound.  Here is a sample radar image around 5 PM (reds indicate very heavy precipitation). Impressive for around here.


Based on rain gauges and radar, the heaviest totals from this event reached 1-1.5 inches , with the regions from Edmonds to Everett and Renton to Sammanish being particularly hard hit.  This is supported by the 24-h rainfall totals from CoCoRahs rain gauges (shown here), although keep in mind most of this fell within one hour on Thursday. 


 Seattle Rainwatch, which combines radar and rain gauge data, found a similar pattern.  (see below).  Note how non-uniform convective showers can be.
Many locations got most of their rainfall in 15-30 minutes...a real downpour for around here.

Now, time for irony.   We think we are rain-accustomed folks with moss between our teeth, but the truth is that we are heavy-rain wimps.   We rarely get intense rain, with a half-inch or more in an hour, and generally experience relatively gentle, but long-lived precipitation events.  As a result, our drainage infrastructure and roads have generally not been designed to handle intense short-period rainfalls, rain generally associated with thunderstorms.

To illustrate our lack of intense rain compared to the rest of the nation, here is an official NWS analysis of what should be the biggest 1-h rainfall over a period of ten years. If you can read the tiny numbers, you will see that the eastern portion of the U.S. gets FAR heavier precipitation...with their ten-year values reaching 2-3 inches, while ours is only about .5 inch.


The big reason for the difference is that they get lots of thunderstorms, including plenty of strong ones.   We get few thunderstorms and they generally are not intense.   Why fewer thunderstorms?   The cool Pacific is a major reason.

My general observation is that less than .2 inches and hour things are generally ok.  Hit .5 inch an hour and some urban flooding occurs.   Get to inch an hour and serious flooding and damage occurs.  This happened in some areas (e.g., near Everett) this week.  And it happened on December 14, 2006 before the Chanukah Eve storm.  Nearly an inch fell in a swatch from West Seattle to Kirkland and tragically a woman died from flooding in her basement.    In order to better prepare for such intense precipitation events, the City of Seattle maintains a dense rain gauge network over the city and has supported the development of the Seattle RainWatch application that combines weather radar and rain gauges to describe what is happening in real time, to predict the next hour, and to warn when necessary (see image).


With a heads up on heavy rain locations, the city can better prepare and is able to move personnel where they are most needed.  RainWatch, which is joint UW/Seattle project, will be undergoing a number of important enhancements this year...so stay tuned.

Finally,  considering that some of our region's infrastructure can't handle even current major precipitation events and the potential exist for even heavier events under global warming (but is uncertain),  future infrastructure projects should be built with better drainage in mind.





6 comments:

smokejumper said...

I just want to mention that I really look forward to this blog during storm season (fall & winter). But you've had some great posts this summer.

A lot has to do with our awesome but interesting summer. Yet again in E. Washington, a stormy July last year, we have had another active season overall. With a potential grand finale this next week. What's going on? No complaints, loving it.

Brad Tankersley said...

Thanks Cliff, the UW meteorological department and the weathermen for such an exciting summer, this one makes up for the endlessly boring one we had last year.

Houseboat guy said...

Well I can say that here on lake union for about 1 minute we had the hardest rain I have ever seen in Seattle. The water overflowed my clean gutters ( never seen that happen befor) and a boat and building exactly 100 meters away was not visible threw the rain. The surface of the lake was not visible threw spray/ splash. Most impressive was my guest on board from New Orleans said " damn, that's about as hard as I have ever seen it rain" ..... Mind you, it only lasted for about a minuet.

Josh said...

Completely unrelated to this post, but is there any chance you can talk to your publisher about releasing an e-book version of your book? Really want to read it, but I've moved away from paper books.

tamlee said...

Hi, Cliff, thanks for your informative blog!

Your post on Thursday's thunderstorms reminded me of a question about a phenomenon I noticed that night.

I live on the east side of Capitol Hill, near 23rd E and Aloha. I was initially surprised to hear train whistles at night in that location, and eventually figured out that on some cloudy nights, the sound gets reflected or refracted over the hill.
Last Thursday after the storms, the phenomenon was more intense than ever. Not only did the train whistle sound like it was coming down the street, but at one point I think I actually heard the ferry horn for the first time as well.

What is actually going on when this is observable, and what conditions set it up?

Thanks!

Chris said...

That Thursday seemed to be the start of the annual summer migration of the college student. We managed to get two loads from son's old house to our garage between storm bursts.

I got to sit on the porch of the house as I waited for them to unload the first trip. It was fun to see the downpour. It reminded me a bit of what I experienced as a teenager in the Panama Canal Zone during the rainy season.

The other family moving their college kids next door did not fare so well. They had two U-Hauls, but the three open bed trucks were drenched.

Fortunately we were able to finish the next morning with less rain, and move him mostly from the garage to different house with sunny skies.

(by the way, a drive through the streets north of the UW shows all of the stuff students dump when they move... le sigh)