January 08, 2009

A Nice Example of a Puget Sound Convergence Zone

A very nice Puget Sound convergence zone occurred yesterday, with a well defined band in the radar (see image).
A classic aspect of the zone you can see is its continuation into the Cascades, where it enhances precipitation in the mountains. Not infrequently, the CZ increases snow for Stevens pass, with less for Snoqualmie. You also notice the enhanced drying to the north and south. I have also included a high-resolution visible satellite image. You can see the convergence band and the clear zones to the north and south. This is a very good thing to keep in mind...if you are in convergence rain and clouds...head north and south and you might have perfect weather for outdoor fun.
Convergences zones occur 12-25 times per year...so if you live in the area...better know about it! A map of the surface observations at 2PM really shows the converging winds of the zone. Winds on the coast are northwesterly...as we often see. If you are observant you may notice another convergence zone over th Strait of Georgia...and that often occurs at the same time as the PSCZ. In fact, look at the satellite picture...you will see the clouds associated with it.

Interesting to look at the observations at the UW as it passed that location around 25 GMT (or 3 PM PST)--the wind direction switched from southerly to easterly, there was a spike in precipitation, and the temperature dropped in the cool, precipitating side (see images).

Finally, I have included a time-height cross section from the Seattle profiler..which show how conditions changed aloft. Time is on the x axis and height in meters on the y axis. Red is temperature and blue are winds. As the convergence zone went through around 23 GMT the
winds abruptly switched from NW to N and temperatures dropped in the clouds and rain. Then the winds turned easterly. I will explain that in another blog.

PS: I am happy to report that FINALLY copies of my weather book from the second printing are becoming widely available. UW bookstore has a big supply--and I will have a book signing there at 1 PM on Saturday, January 17th. Barnes and Noble.com have it and Amazon now has the book in stock. Elliot Bay and Powell books just got copies...and I assume the rest of the local bookstores will be getting theirs soon, if the don't have them already. For links of these, you can go to my web site (click on the book icon).

PSS: There should be a special on the snow and floods on KIRO TV on Sunday...I believe after the football game.


  1. Cliff, is that cold Alaskan Air mass going to make it down here or stay east of the rockies?

  2. In your book, you show air from the south arriving at the CZ climbing over the air arriving from the north. Is that always the case, and if so why? The book and and the blog are really great, thanks!
    - Steve

  3. Cliff, please answer our question about the cold Alaskan air! Thank you.

  4. Devin...no Alaska Air..the ridge will protect us..cliff

  5. Like Cliff said, the cold Alaskan air will not go to the PNW. It is going to the midwest and East. Chicago will be below zero, and even NYC may have a morning low below zero for the first time in 14 years.

    But, winter is still young, and a lot of experts are saying we could have another arctic outbreak in late January or early February. One of them is Ed Berry, who correctly forecasted arctic air for us four weeks before we got the cold in December.

  6. Cliff, I'm curious to learn, and I'm sure someone has done the calculation, how many acre feet of water fell during this storm and fall on average from these pretty much annual events. I grew up in Issaquah in the 1950s and '60s and it seemed that our farm was swept with these events at least once a year, and frequently twice. A lot of rain would fall. I suspect that the number is in the hundreds of thousands, but that's obviously a simple "guest-imate."

    Does anyone know what the record setting event has been? Some of the events of my childhood seem in memory larger than recent events, but records have frequently shown my memories to be wrong and I suspect the same is true in this respect.


  7. Well we go from classic CZ yesterday to classic "dirty" ridge today. 30.62" is pretty impressive for a high of non-arctic origin.

    I have to agree with the people that suggest that more cold weather may follow later this month or in February. Historically, there have been quite a few Seattle winters that have had a cold and/or snowy start,a mild uneventful respite in the middle,then another--although briefer-- cold spell in February.The winter of 1955-1956 is probably the most outstanding example.
    By the way,for those who like me are interested in historical weather events and climatology, this week is the 100th anniversary of one of the most severe arctic air outbreaks in Seattle since records began in 1890.From Jan 5th through the 12th in 1909,the temperaure remained at or below 25 degrees F for eight consecutive days,a record that still stands.This event is even more impressive by the fact that the weather station at that time was on the roof of the Alaska Building on 2nd and Cherry, near the balmy waters of Elliott Bay.

  8. Climo Man, do you ever post at Westernusawx.com? If not, you should, as a lot of people like you post there.

  9. Thanks, anonymous--I`ll check that site out.

  10. Devin-

    You asked about whether it's always the case that air from the south rises over the air from the north.

    I just got Cliff's book and haven't had a chance to find the page you're talking about - but I believe the answer would be that the air coming from the south is generally warmer than the air coming down to us from the cold north. Since warmer air is more buoyant, it will be the air mass to rise up over the cooler air.


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