A Nice Example of a Puget Sound Convergence Zone

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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.
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