September 24, 2018

Super Convergence Zone Dumps Heavy Rain

One of the most striking peculiarities of western Washington weather is the Puget Sound convergence zone  (PSCZ), which can produce a heavy band of precipitation across northern Puget Sound and into the Cascades.

The PSCZ  results when low-level winds on the coast are westerly (form the west), move around the Olympics, and then converge together over Puget Sound, producing upward motion, clouds and precipitation.

This is exactly what occurred Saturday evening, as illustrated by the plot of surface observations at 7 PM (a large scale and close up view is shown below)

The air was relatively unstable Saturday evening and an upper level trough was producing additional the convergence zone really revved up.    

Now let me impress is a radar image around 7 PM Saturday.  Red indicates very heavy rain, and yellow is moderate to heavy. Intense convergence zone band.  Note the sharp southern edge of the rainfall, which is very typical of strong convergence zones.

There was substantial convection and thunderstorms in that heavy precipitation band, and lots of lightning was noted by the regional lightning sensors (see below-24h lightning ending 1 AM Sunday)

Seattle RainWatch combines radar and observations to provide a good estimate for rainfall.   Here is the RainWatch 24-h total ending 5 AM Sunday.  A substantial area got 1-2 inches, which even more in limited regions.  Pouring over Snohomish County but downtown Seattle was dry.   You got to love living around here.

Local rain gauges had 24-h rainfall totals that were consistent with RainWatch, with over 2 inches around Monroe.

I know someone that had a wedding celebration near Monroe...thankfully they had tents, but it was a soggy affair.

Now the really exciting part for me is how well this event was forecast....our high-resolution models was very skillful, days ahead of time.  To illustrate, here is the forecast from the UW WRF high-resolution (4/3 km grid spacing) model for the 24h precipitation ending 5 AM Sunday.  

Nice convergence zone.  It underplayed the total amounts a bit...but it was clear that Snohomish County was going to be drenched, which downtown Seattle would be dry.


  1. My cousin's son was married during this "event" at the Edmonds Yacht Club. We decided to run down by boat and spend the night in the marina where it was a simple 4 minute walk to the wedding. Sounded like a great idea... After surviving SCA conditions on the run down, we were safely moored in the marina… Then… The rain was "biblical"... I was wearing a suit (rare event for me... ) and my wife had a nice dress on.... After assessing the situation we decided to wear our Grunden foul weather gear over our nice clothes to survive the elements.... Actually I thought it was a very impressive PNW fashion statement.... :) Others were very envious... :) I think it was one of the longest lasting heavy CZ’s I can remember….

  2. Is that another convergence zone running SW-NE through the lower mainland?

  3. Edmonds certainly got hit pretty hard. There was foot-deep water running on some streets (exciting to drive in). Shell Creek shifted its course a few feet in spots inside Yost Park, with much evidence of unusually strong flow.

    Can't imagine what it was like in the very bad areas.

  4. We are contemplating a move to Astoria from here in Kingston and everyone (including you Cliff) said that Astoria definitely has more rain than Seattle. But under the PSCZ? I am not so sure.

    Are there any reliable rain gauges in North of Seattle from about 145th Street to Everett? Paine Field perhaps?

    Everyone nicknames Astoria "Dismal Niche" after a comment Lewis and Clark made when they first arrived, and stayed at a little cove on the north side of the Columbia a few miles east of the Astoria Bridge. They arrived in November during one of November's rainy and stormy periods. Dismal Niche is at the base of a south facing mountain on the north side of the river that caught all of the precipitation.

    I propose renaming the PSCZ to Dismal Puget Sound Convergence Zone. I can't wait to be out from under it.

  5. At least the trees and shrubs in the very localized PSCZ got some water on Saturday. Lucky for them.

    In other areas, including Sequim, it's so dry that I'm going to have to turn my irrigation back on this weekend if we don't get some rain. We've had only 0.63 inches in the last month. Not nearly enough.

    I love this dry, warm, sunny Fall, but my trees and shrubs are showing more and more signs of stress. The Rex block is definitely doing the job of keeping us dry, but we need enough moisture to wet the soil down deep. Inches.

    I hate it when what I want and what my lovely trees need are so different. But that's what irrigation is for. Just don't recall having to turn it back on at the end of September before.

    1. Fall just began and it was a dry summer no doubt but usually September is also a relatively dry month. In some areas this month has actually been been wetter than normal. In SW BC they have had several inches of rain so far this month. If it's late October and you still have severe dryness issues than that's a big deal but I wouldn't fret about it in late Spetember. In some years we have had epic dry spells at this time of year. Remember 1987,1993,1998,2002, 2012?

  6. "They had tents" !
    They need a Boat !

  7. Winters coming and skiers want to know what this El NiƱo forecast might mean. Can you do a post on it Cliff?

  8. John said - "In other areas, including Sequim, it's so dry that I'm going to have to turn my irrigation back on..."

    John - I am curious sometimes when I read your posts talking about it being dry in Sequim... isn't that why you live in Sequim? It is a beautiful, sunny and dry location in western WA due to the rain shadow of the Olympics, and averages less than half the precip in Seattle annually (37" in Seattle vs. 16" in Sequim).

    We don't get a lot of rain in an average September in any part of the state. For the month of September, the avg at Seatac is only 1.63". So just a ballpark here (since I don't know the Sept average in Sequim - John maybe you can help me out): half of 1.63 is .82, and you say you are at .63"? That doesn't seem like a huge anomaly to me in the big picture (ie < a quarter inch difference over an entire month). What am I missing?

  9. Nice post Cliff! I saw the ominous black clouds to the W & N @ ~ 3 pm on 9/22 (in W Lynnwood), & figured it was going to get wet & wild. In Seaview Edmonds, we had 1.8 inches of rain in less than six hours!
    While my older son & I were outside working on a kitchen drain, during one of the many downpours, a nearby 15-ft W Hemlock branch snapped (due the heavy rain) and tumbled 40 ft to the ground!


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