September 05, 2021

Could Seattle Experience Urban Flooding Like New York?

 Several of you have emailed me, with essentially the same question:

Could Seattle experience massive urban flooding such as the New York metropolitan area experienced during the past week?    Flooding that blocked roads, resulted in several dozen deaths, and costing billions of dollars?

There is bad news and good news to tell you about this.

The Bad News

The bad news is that Seattle has experienced serious urban flooding events.  Consider December 14, 2006, immediately before the Hanukah Eve windstorm.  A band of very heavy rain (roughly 1 inch per hour) hit Seattle, with roads flooded and cars/trucks inundated (see image).

Seattle's Mercer St.  Picture courtesy of Ricky Fischer

During the same event, a woman drowned in her basement in the Madison Valley of central Seattle. 

Urban flooding also occurred over northeast Seattle near Thornton Creek during the winter storms of 1996/1997 and October 2003.  Houses were destroyed and roads blocked.  There are many other examples of major urban floods around the city I could describe.

A major flooding vulnerability for Seattle is engendered by the city's substantial terrain, which creates bowls and low spots for heavy rain to accumulate (see terrain map below).  The Madison Valley is one of the most vulnerable areas, but there are others.  New York City has less terrain for such a concentration of water.

The terrain of central and northern Seattle.  Courtesy of GoogleEarth Pro.

    Seattle's urban flooding can usually be traced to one of three meteorological origins:
  • Atmospheric Rivers that bring warm, moist air into the region from the southwest.  The "Pineapple Express", with moisture streaming from near Hawaii is the most well known.
  • Intense Pacific fronts, whose intense rainbands can stagnate over the city (what happened on December 14th before the Chanukah Eve Storm).
  • Strong convective systems (thunderstorms), that although rare, can produce localized heavy rain.
River flooding or flooding from high levels of Puget Sound or Lake Washington are generally not a problem.

The Good News

Although Seattle is known as a very wet place, we generally do not experience extreme short-period rainstorms as observed in much of the U.S. To illustrate this, here is a plot from a NOAA/NWS Atlas of extreme precipitation around the U.S. Specifically, the figure shows the 100-year one-hour rainfall around the country.   To put it another way,  considering rainfall falling over one hour, how much rain would fall in that hour during a once in a hundred-year rain event?  You can expand on the figure by clicking on it.  

For New York, a once in one hundred year rainstorm would bring a bit over three inches, almost exactly what fell last week.


For Seattle, as shown in the blow-up below, the 100-year one-hour event is approximately one inch, very close to what fell on December 14, 2006 in Seattle.

I went through the records of the rain gauge records over Seattle and could not find any exceeding 1 inch in an hour.

So Seattle does not get the intense, short-period rains observed over the eastern U.S., lessening our vulnerability to extreme, rapid urban flooding.

But why?  The key reason is that we don't experience intense thunderstorms, embedded in tropical moisture, that remain in place long enough to produce extreme one-hour totals.

Extreme precipitation from thunderstorms requires very moist air, generally of tropical origin, great instability (the tendency for the air to connect), and something the focuses and maintains thunderstorms in one location.

These are EXACTLY the conditions that occurred in New York during the big event last week.  Extremely moist and unstable air was pulled northward by the remnants of Hurricane Ida.  A stationary front was set up by the storm, which created and focussed intense thunderstorms across the New York metro area (see radar imagery below).   A "perfect storm" of a confluence of the necessary factors and one that is exceedingly rare.


Seattle simply can not get this level of intense, stationary thunderstorms for several reasons, including:

  • Seattle is near cool water that discourages thunderstorms by lessening instability and results in less water vapor in the lower atmosphere (cool air can hold less water vapor than warm air).
  • Seattle never experiences tropical storms because of the cool water
  • Seattle does not have enough terrain to release intense, stationary thunderstorms, something that can happen near the Cascade foothills.

Will global warming change this situation?  From the regional climate simulations we are running at the University of Washington (and these are gold standards of such work), there is no suggestion of increases in summer precipitation intensity in Seattle as the Earth warms.

The Bottom Line:   Seattle can and will get heavy hourly precipitation, and we have our topographic vulnerabilities, but our extreme rain threat is clearly less than experienced in NY and the eastern U.S.

Final note:  A key aspect of protecting Seattle citizens from heavy precipitation is to know what is happening and what will happen during the next few hours.  The City of Seattle, with the University of Washington, developed one of the most advanced systems of this kind to do so, called Seattle RainWatch (see graphic).

Unfortunately, Seattle Public Utilities decided to no longer support RainWatch, leaving the city without an important tool for proactively dealing with heavy precipitation.



17 comments:

  1. I do remember a strange, long-term episode of Lightning storms, that hovered over Seattle, sometime in the mid-90s....Two days worth of lightning strikes, and over 3,000 of them!...it was also quite humid...maybe others can remember this event, as whenever I talk about it with friends, they basically do not believe me!.But I remember driving to work, worrying that I would get zapped at any time!

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  2. I remember the 2006 event certainly, finding my car parked at the edge of a pool of water 20-30 yards wide. Fortunately, it was positioned on slightly higher ground than some, preventing damage. Of course, I did not consider urban flooding possibilities that morning when parking. And of course, I vividly recall the howling winds all night and wide-spread power outages.

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  3. I tried Googling “Seattle Rain Watch”. It says that the City decided to pull funding in 2018 and it was discontinued?

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  4. SR 99 right above the Mercer street underpass used to turn into a deep water situation every time there was a heavy rain. Hopefully, the new roadway has fixed that…

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  5. I had to do some thinking but yes, I do recall that storm as I was on Broadway on Capitol Hill in my truck, stuck in traffic when that storm hit late in the day, early evening if I'm not mistaken.

    As to the unknown above who swore we had several days of thunderstorms, well, as someone who lived in the area, I certainly don't recall anything of the sort, but then again, I was living here in Tacoma in the north end with my parents at the time, but do recall a line of thunderstorms one summer day where it came up the hill from the Narrows bridge and got awfully close and stuck a house, across the street, and down 3 houses while at it, but it was not a 3 day event though, just one afternoon as we do get the occasional squall from time to time.

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  6. There was a two or three day event, with low grade rain and lightning that just kept going on and on here in Port Townsend about ten years ago. It was centered out on the straits west of town, but one bolt struck a large Sequoia tree in Chimacum—the tree exploded, and you can still see the trunk standing on Center Road a few hundred yards from the Chimacum intersection. It wasn’t constant lightning, but it stuck around off and on for a couple of days and evenings, possibly three. It could have been longer than ten years ago, but certainly wasn’t the 90’s. There wasn’t all that much rain associated with it, however.

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  7. I remember the 2006 rain event. I was fortunate to have biked into work that day. "Fortunate" because I was able to get home in my normal time (~45 minutes), while it took my wife almost two hours to drive approximately the same distance. I was passing lines of stopped cars in the bike lane, my feet splashing in the water at the bottom of every pedal stroke.

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  8. How about telling the full story about RainWatch, Cliff?

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  9. On the night of August 31, 2016 an intense band of rain moved across western Skagit County. The WSU extension rain gage at Sakuma Farms near Burlington measured over 1 inch per hour from 9:45 PM to 10:45 PM. The WSU West Mount Vernon gage measured over 1 inch per hour from 10:30 to Midnight and peaked at 1.9 inches per hour from that night at 10:45. 1.9 inches in an hour! This event was under reported in the media because most of Washington State received little to no rain that night.

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    Replies
    1. Yes, I remember that storm. But it seems that August storms have gotten less common. Just my impression.

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  10. Cliff,
    You've predictably downplayed climate change again.

    Summer precipitation is expected to decrease slightly, and winter precipitation is expected to increase. This is from the same work you are citing. Rainstorms are expected to increase in intensity in the cold season. You know this.

    Also, you fail to mention or consider sea level rise. There is a group at King County or seattle that has investigated urban flooding and relationships to climate change. flooding is expected to worsen, especially at the urban/coastal interface, where stormwater intersects sea level.

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    1. Colin...there is no downplay of climate...please read my blog more carefully. I have published on all these issues. Summer precipitation will slightly decrease over Seattle...so no climate change contribution there. Slightly more winter precipitation overall, but modest increases in atmospheric river. These don't provide the high intensity short-period rainfall amounts over Seattle...you need convection or a strong front to do that. Sea level rise is not a major issue for Seattle...the terrain rises quickly enough to avoid major issues in general. And you might to know that there is isostatic rebound from the glaciers in significant sections of the region.

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    2. Cliff, if I remember the report correctly, there are low lying pockets of Seattle where problems are expected, especially the Duwamish.

      Your only messaging around climate change is that its not a problem. Why isnt your message "We dont get big thunderstorms and wet tropical systems, and climate change wont change that. Summers, when these kind of systems could happen, will be trending dryer with climate change. The flooding we get is associated with seasonal precipitation in the late fall, winter, and early spring, and that precipitation is expected to become more intense. So, while we wont get extreme urban flooding like in NY, we do expect urban flooding to get worse, and that could cause problems. Seattle is doing/should be doing x about it."

      ???

      ... instead of another "Climate change wont affect x" by reputable weather scientist Cliff Mass.

      https://seattlecitygis.maps.arcgis.com/apps/webappviewer/index.html?id=531658b7209e46acbaed730574214353

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  11. In 1993 (our "year without a summer") I was living in Kenmore. Sometime during the summer on a weekend, I think, the skies opened up and after it was raining for a few minutes I set out a Tupperware container with vertical sides as a crude rain gauge. Within less than an hour there was almost 2" of water in it. Considering that I set it out late, I think I got at last 2 inches of rain. I am always suspicious of "once in a hundred year (fill in the blank)" proclamations. We haven't been recording weather long enough.

    But, Cliff, I disagree that our lack of hard rain is a good thing. You like storms, I know so OK: A good hard rain not only douses the fires that are burning all around, it also makes life interesting.

    As for basement apartments, I tried it once. They are chilly in this climate and, unless you are a vampire, too dark!

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  12. My wife is from Nagasaki, Japan. On her 19th birthday, when her company was taking her out to a restaurant, a cloud-burst occurred over the city, resulting (it seems) in up to 6 inches of rain in an hour. 300 died. Stone bridges hundreds of years old were destroyed over the city's little river. It seems that one feature which Nagasaki shares with Seattle, lots of hills, was especially fatal. But Seattle seems to require many days or even weeks of build-up for the mudslides to come down.

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  13. Cliff, have you looked at whether the frequency of actual occurrence of these "100-year events" is increasing? This is what I hear anecdotally from, for example, the Northeast U.S. Has the NYC area received something like 4 of these in the past 15-20 years, vs. the expectation of less than one? Of course, a coin can come up heads infinitely many times in a row in theory. Just curious your take. Thanks!

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  14. you still use global warming instead of climate change. How are them icecaps doing?

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

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