Thursday, February 6, 2020

Amazing Murk, Very Wet on the Cascade Slopes, and an Unusual Atmospheric River

The murky, wet conditions during the past few days are some of the worst I have seen in years for February in our region.  Unbelievably dark and wet.  For example, below is the view from the SpaceNeedle PanoCam around 9:30 AM today (left) compared to four days ago.   It was like the city was not there.


And, of course, there is the heavy rainfall along the western slopes of the Cascades that is producing flooding and landslides.

The origin of all this water is a moderate atmospheric river that is coming from an unusual direction.

An atmospheric river is a narrow current of warm, moist air from the tropics and subtropics that extends northward into the midlatitudes.    A NOAA satellite two days ago, shows the tongue of moisture passing northwest of Hawaii and then bending eastward toward the Northwest.


A model simulation of the water vapor added up in the vertical (known as column-integrated water vapor) for 4 PM Wednesday shows the tongue of moisture reaching our coast from a west-northwest direction.


This is somewhat unusual, with most atmospheric rivers in our region coming from the south or southwest.

Such an orientation to the atmospheric river (roughly east-west) has major implications for us:  it tends to put bountiful precipitation over the western side of the Cascades. 

In fact, the last time we had a major atmospheric river from this direction was in January 2009, an event that almost took out the Howard Hanson Dam.

The Howard Hanson Dam located east of Tacoma

Why is a west-east or northwest-southeast atmospheric river orientation so potent for producing heavy rain on the windward (western) side of the Cascades?   

Because it is associated with westerly wind (wind from the west) that moves rapidly up the mountains, cooling and releasing the large amounts of water vapor in the atmospheric river.

Consider the diagram below.  If an atmospheric river is from the south, with attendant southerly winds, there is not much upslope flow since the air is running parallel to the terrain.  But a westerly atmospheric river has westerly winds that move directly up to terrain slope, condensing massive amount of water vapor into liquid water.



To put it another way, imagine you are on a hike.  If you walk straight up hill, you work hard and gain lots of elevation.  But turn 90 degrees and walk parallel to to crest line, it is no effort at all (assuming you don't bang into a tree, of course).

With a westerly atmospheric river, there is not only LOTS of precipitation on the western side of the Cascades, but a rain shadow downstream of the Olympics and on the eastern side of the Cascades.

Want proof?  Here is the National Weather Service precipitation analysis for the 24-h ending 4 AM this morning.  Huge precipitation contrasts with a half foot on the western side of the Cascades.


The western slopes of the Cascades have been getting hammered with a large volume of precipitation.  A number of rivers are flooding now, with water over nearby roadways.  Be careful.

The good news:  a major change is in store, with dry conditions on Sunday.


8 comments:

  1. This atmospheric river is also evident in SE Washington as a NW flow moving upslope in the Palouse and Blue Mountains.Unusually heavy precip in those areas with many local rivers at or near flood stage.

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  2. 10 day RFC verification also shows it well - including Wallowas/Clearwater region:

    https://www.nwrfc.noaa.gov/weather/verif_fcst.cgi?date=20200207

    Verification shows more fell than forecast.

    Also check out the Grande Ronde response:
    https://www.nwrfc.noaa.gov/river/station/flowplot/flowplot.cgi?tryo3
    https://www.nwrfc.noaa.gov/river/station/flowplot/flowplot.cgi?lgno3

    2 days ago, the Grande Ronde at Troy was 2.5 kcfs - now just passing 30 kcfs.

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  3. "almost took out the Howard Hanson Dam." A bit of hyperbole for effect I hope.

    I moved many tons of sand bags that year to protect my work site in the Kent valley for the event that never occurred. High flow rates on the Green River for many months, yes. Levies temporarily raised as a precautionary measure, yes. The reservoir drawn down to allow repairs to be effected safely, yes. The dam almost being "taken out", no. Anything approaching the Oroville Dam event, no.

    Please leave the hyperbole for other, less reputable news sources.

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    Replies
    1. I remember the sandbags well as my wife and I frequently cycled the Green River trail. I think you are correct that the dam did not come close to failing, so that was a major overstatement. There was considerable concern that leakage from the dam and the need to reduce dam levels could cause flooding.

      https://www.yumpu.com/en/document/read/38434307/howard-hanson-dam-interim-risk-reduction-final-seattle-district

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  4. Cliff, are we seeing a higher incidence of mudslides due to the wet weather in WA. I dont remember seeing so many in such a short amount of time but im interested to know whether there is mudlside occurence data over time anywhere? This article peaked the thought: https://www.nrk.no/chasing-climate-change-1.14859595

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  5. Cliff,

    I am ready for a dry Sunday! Especially so after all the rainy, colder conditions. No raincoats, wind in our hair and smiles on our faces. Let's do it!

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  6. On 2/5, I measured 0.00" of evapotranspiration at my location in NW Bellingham. Daily evapotranspiration (measured in inches and most easily understood as negative rainfall) is equal to the sum of the amount of water vapor released to the atmosphere by a particular type of vegetation during the metabolic process of transpiration and the amount of liquid water (dew) evaporated into the atmosphere from the surface of that particular type of vegetation over the course of of a 24-hour period. Evapotranspiration is a parameter which is calculated/estimated rather than measured, the equation for which includes variables for temperature, humidity, solar irradiance and wind speed in addition to a constant which is determined experimentally and corresponds to a particular type of vegetation cover. All of the terms of the equation are directly related to evapotranspiration except humidity in that lower temperature, irradiance and wind speeds tend to inhibit evapotranspiration while lower humidity tends to promote evapotranspiration. Thus, a cool/cold, rainy/humid, cloudy/overcast day with calm winds is the prime candidate for especially low evapotranspiration values and 2/5 was just such a day. The temperature range that day was 34F-39F, the relative humidity ranged from 96%-97% (the dew point ranged from 33F-39F), the maximum solar irradiance value was 62 W/m^2 (for reference, 62 W/m^2 was the lowest irradiance value I had measured in at least 25 days and on 2/3, I measured a maximum irradiance value of 566 W/m^2 - nearly an order of magnitude greater then the 2/5 max value). Finally, the maximum wind speed on 2/5 was just 5MPH from the NW and I measured 0.68" of precipitation to boot. This was truly a cold, nasty day and a textbook example of a day during which the weather conditions were ideally suited to minimize evapotranspiration.

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  7. I would like to read your analysis on the low level wind shear we had last night. Flights landing in Seattle experienced the feeling of riding on ocean waves except in the air.

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