• Can a flooding disaster like Harvey happen in Western Washington?

    By: KIRO 7 Pinpoint Meteorologist Morgan Palmer


    An incredible set of circumstances has created what will likely be the costliest natural disaster in United States history, and a meteorological catastrophe on the extreme end of our expectations.

    Pondering the devastation hundreds of square miles flooded in Southeast Texas impacting many thousands of people, can a Harvey-like disaster happen in the Pacific Northwest?

    Bottom line: The geography, topography and meteorology of the Northwest simply won't allow for flooding on the same scale of area and population like they're seeing in Houston.

    But that does not mean we don't experience major river flooding.

    Tropical systems are the most efficient rainmakers

    Tropical systems like Harvey gain their strength and their incredible amounts of moisture from very warm waters like the Gulf of Mexico, where the water temperature has been approaching 90 degrees in spots.

    The warmer the air in a storm, the more water vapor that air can hold. Think of it as a sponge that can hold much more water, eventually dumping it out over land.

    Our atmospheric rivers -- sometimes called a "pineapple express" -- can deliver copious amounts of moisture from the subtropical Pacific near Hawaii to the Northwest, but still cannot compare to the overall amount of water a storm like Harvey can produce in an hour or a day over such a large area.

    Atmospheric rivers do deliver impressive rainfall amounts at times -- with record rainfall in Washington over 16 inches in 24 hours. But this will always happen at localized spots in the mountains, where the moisture locked up in the higher altitude winds gets wrung out the most.

    Scroll down to keep reading about drainage ability in Texas versus Washington. 



    Drainage ability of Southeast Texas and Western Washington vastly different

    Hurricanes will come and go but more and more people are living in very flat areas along the coastline of the United States -- particularly along the Gulf Coast and Florida.

    When you spill water on a table or flat surface, it will eventually find a way to run off, but not necessarily quickly.

    The flatness of Southeast Texas -- with so much of the area at or just above sea level -- simply cannot allow for the water to drain fast enough to keep up.  While they have set up dams, canals and levee systems to direct the water to the bays and the Gulf as quickly as possible, massive floods have happened in that area from tropical systems before and they will again.  It's just a matter of topography.  It's flat there.

    Homes are surrounded by floodwaters from Tropical Storm Harvey Tuesday, Aug. 29, 2017, in Spring, Texas. (AP Photo/David J. Phillip)

    Here in the Pacific Northwest and Western Washington, our highly-varied topography has allowed nature to cut deeper and more effective river channels from the mountains to the Sound and the Pacific.  Over millennia, this has allowed heavy rain (particularly in the mountains during atmospheric river events) to drain rather quickly to the ocean.

    When there is too much rain in Western Washington, our rivers do flood, of course.  However, since the elevation rises relatively quickly surrounding our rivers, that greatly limits the number of people that can be affected.

    Areas like the White and Green rivers, the Puyallup and the Chehalis rivers have more of people living near their banks but agencies like the Army Corps of Engineers have created effective levee and dam systems to control runoff, and limit major flooding as much as we can.

    But can we get 50 inches of rain over our cities like Harvey?

    Because of the fact that our Pacific airmasses are cooler -- and thus they can hold less water -- it's just impossible for any storm we could see in the Pacific Northwest to create that kind of rainfall in our lowland locations like seen in Houston.

    Interstate 69 is covered by floodwaters at the San Jacinto River bridge as floodwaters from Tropical Storm Harvey caused the river to overflow Tuesday, Aug. 29, 2017, in Humble, Texas. (AP Photo/David J. Phillip)

    Even with the heaviest rains imaginable in lowland locations, spots that aren't close to our major rivers and spots with a little elevation above those waterways won't have inundation like we see in the Houston area.

    Water will still always flow from high spot to lower spots.  The problem in Houston is that there aren't any high spots!

    Will climate change bring more floods to the Northwest?

    This is what we should pay close attention to!

    While a warming climate won't make our Pacific waters like the toasty Gulf of Mexico, we do actually project a greater threat of river flooding in the Northwest because of climate change.

    And it's not necessarily because storms and atmospheric rivers will be "juiced up" or have more water vapor to wring out as rain. They will, but that's not the primary focus in Western Washington.

    The belief is that warmer winter seasons later in the 21st century will actually bring us higher snow levels.

    That's the elevation line at which rain turns to snow in our mountains.

    A higher snow level would naturally mean more precipitation falling as rain and running off instead of being locked up in snowpack.  

    If this occurs with regularity, more frequent and severe river flood seasons would be expected.

    It would also likely have impact on our water supplies during summer seasons, possibly reducing spring runoff and requiring more reservoir capacity for summer months with a rising population. 

    I'll be tracking Harvey and our Labor Day Forecast on KIRO 7 News, streaming always at kiro.tv/LiveNews and our KIRO 7 apps.

    Follow Morgan on Facebook, Twitter and Instagram.

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