February 23, 2021

Double Convergence Zone

One of the key elements of western Washington weather in the Puget Sound Convergence Zone, a band of clouds and precipitation that is produced by air moving around the Olympics Mountains and converging...or coming together...over Puget Sound (see schematic below).


As the the airstreams come together, air is forced upwards, producing clouds and precipitation.  The Puget Sound convergence zone usually forms when the low-level air from off the Pacific is from the northwest.

Under these conditions, another convergence zone often forms downstream of the mountains of Vancouver Island.   Perhaps we should call it the Georgia Strait Convergence Zone or the Victoria Convergence.  Few people talk about it.   Canadians are more restrained perhaps.

Anyway, tonight we have had a spectacular example of a DOUBLE convergence zone, with great examples of both happening simultaneously.

Let's start with the weather radar.  Two convergence zones are obvious, one heading just north of Everett and the other over Seattle.


One thing you should notice is that these convergence zones can extend WAY downstream, ascending and crossing the Cascades, and can be big snow producers in the Cascades.

You can see this by viewing the visible satellite picture around 5 PM today (see below). And also notice the enhanced clearing to the north and south of the convergence zone clouds....good to know if you want to find some sun.


If you would like a convergence zone "crib sheet" indicating their locations with dashed ovals....here it is:

We are now coming into convergence zone season, since northwesterly flow is more prevalent during spring.  And La Nina years like this one have more northwesterly flow...and thus convergence zones-- than normal.

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14 comments:

  1. I've always referred to it as the Vancouver Island Convergence zone. Fun weather day today!

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  2. Wet slushy snow at 260ft elevation in Port Townsend. Bit of it sticking on cooler surfaces.

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  3. Cliff...the northern convergence zone had hail coming down..also major wind change..before it came thru it was blowing at about 30mph from the west, after passing light wind.

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  4. Quick dusting of really large snowflakes. Temp dropped from 39 to 33 in just over an hour in North Puyallup around 300' elevation. Has been interesting weather the past few weeks.

    Thanks for sharing your expertise Cliff!

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  5. Cliff: A blog post please about why some conditions result in rain on the Olympic Peninsula plus a rain shadow in Sequim or East of the Olympics while similar conditions "run around" the mountains and result in a convergence zone to the East. Seems sometimes winds out of the Southwest would cause a convergence zone around Sequim, Whidbey, Camano, etc. Thanks.

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  6. Didn't notice anything last night (wasn't looking) but this morning (2/24/21) at 6am, there was a bit of the white stuff mostly on non pavement and on cars and I can just notice it in my patchy grass.

    Otherwise, it's bare everywhere else, maybe 2 inches, at best of thte white stuff and 32F out there apparently. This being Tacoma. We did get patchy clouds so the sun was in/out much of the day yesterday so got a walk in.

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  7. The Puget Sound Convergence Zone is the same approximate area that was rain shadowed just a few days earlier.

    I think it could be interesting to discuss what factors drive whether the Olympics cause a rain shadow in that area, vs. a convergence zone with extra precipitation.

    To add one more effect, during the lead-up to the snow a 1-1/2 weeks ago, the UW GFS model was consistently showing the normal convergence zone area getting little to nothing. My understanding is that was actually due to dry east winds coming out of the valleys in that area evaporating the precipitation before it could reach the ground.

    Fortunately for those of us hoping for snow in that area, the GFS over-estimated that effect. Here's the forecast from a mere ~12 hours before the snow started falling:
    https://a.atmos.washington.edu/wrfrt/data/2021021212/load.cgi?images_d3/ww_snowacc.84.0000.gif

    It showed 1 to 2 inches forecast in my area. Sunday afternoon I measured 11 inches.

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  8. trying to figure out why west winds sometimes go around the Olympics to create a convergence zone and sometimes go over the Olympics to create a rain shadow - there must be something different in the nature of the westerlies?

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  9. The northern convergence was very apparent in the Bellingham-Ferndale area yesterday afternoon. The south side of Bellingham was shrouded in dark clouds while Ferndale was clear, except for a brief hail shower in the mid-afternoon.

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  10. Weather is beautiful. And your excellent presentations - as usual - make it even more beautiful. Thank you Cliff!

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  11. Monthly Weather Review Volume 125 Issue 8
    Interaction of a Warm-Season Frontal System with the Coastal Mountains of the Western United States. Part II: Evolution of a Puget Sound Convergence Zone:

    https://journals.ametsoc.org/view/journals/mwre/125/8/1520-0493_1997_125_1730_ioawsf_2.0.co_2.xml

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  12. Cliff, are they going to have the weather workshop this year (and I hope not virtual!) I look forward to it. It is time to put this Covid behind us. If we had it in May, many or most will be vaccinated.

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  13. Cliff, why do high winds develop centered off Vancouver Island/Seattle?

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  14. Cliff, why do the high winds develop centered off Vancouver Island/Seattle?

    Windy illustrates the general area in 'maroon': https://www.windy.com/?48.311,-122.632,5

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