Monday, April 18, 2011

The convergence zone

There is a natural tendency to think of weather as a region-wide phenomenon.  And in the Midwest where I grew up, this approach is a good approximation (except for thunderstorms and tornadoes, of course).

But not here in Seattle.  With the influence of the Sound, the Cascades and the Olympics, we have several "micro-climates" - areas where the local climate is significantly different.  One in particular that is frequently mentioned by the TV weather folks is the "Convergence Zone".

The Convergence Zone is pretty easily explained.

Directly to the West of Seattle, where most of our weather comes from, sit the Olympic Mountains.  Now the Olympics are not a chain of mountains like the Cascades or the Rockies...  instead they are best described as a "patch" of mountains.

So Seattle is guarded from the incoming weather by the Olympics.  Sort of.  The incoming weather is forced to split by the Olympics, with some going South and some going North.  But things being as they are, those two streams of air rejoin again on the far side of the Olympics, exactly like a river parting to flow around a rock and then rejoining again downstream.  The area where the two streams of air rejoin is the Convergence Zone.  (This, by the way, explains why we can have a Southerly wind in the South Sound while at the same time a Northerly in Admiralty Inlet).

Frequently, the Convergence Zone is a place of nasty weather.  It is pretty much the case that if it is raining anywhere in Puget Sound, it will be raining hard in the Convergence Zone.

So, where is it?  Well, it is not in a fixed location.  Considering how it is created, it will be located pretty much diametrically opposite the direction the weather is coming from.  Under normal conditions, it seems to be at Everett.  But if the weather is coming from the Southwest instead of the West, the Convergence Zone will move North, to say Camano Island.

It exists in good weather too.  Have you ever been sailing down Admiralty Inlet, returning from the San Juans and then suddenly become becalmed just about at the Southern tip of Whidbey Island?  Yup, you guessed it - you're in the Convergence Zone (and just about across from Everett, aren't you?).

The idea for this post came to me yesterday, as we drove from Camano Island to Seattle.  On Camano Island, it was cloudy, and in Seattle it was sunny (really!)... but in Everett, ah in Everett, it was pouring rain so hard that the windshield wipers couldn't keep up.

If you are moving here from out of the area, it would be a good idea to give consideration to our many micro-climates, including the Convergence Zone.  In fact, you might just want to consider renting for a while until you have a better idea how the weather plays out over the course of a year.  Is there such a thing as a Consulting Meteorologist?  If there is, realtors are missing a bet.  I call dibs on the idea.
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4 comments:

imiloa said...

Interesting stuff, thanks for the post. I like the consulting meteorologist idea, now if I can only find one in the SF Bay Area... We are planning to move down and I have been researching marinas and micro-climates to minimize fog/maximize sunshine.

bob said...

I wonder if you could just do the bold thing and email one (or several!) of the SFBay TV meteorologists - they are undoubtedly listed on the TV station websites - and plead for help...

What would you have to lose?

bob

Bevinne from hevinne said...

Thanks for explaining the Convergence zone - I wondered if it stretched to Camano Island? You answered my question well and I never knew that it was the convergence of the split air streams. My question is whether Camano Island is sunnier over all than say - Everett where I live now. My Mom called it the cheap man's Sequim ... LOL! But I haven't lived there long enuf to answer that question - maybe someday. And it is true that cars cost less in Puyallup but is it sunnier than here in Everett - it seem it is - but I have not watched the weather report or lived there at all to find that out. I guess my major question is - is it sunny ANYPLACE from Olympia to Bellingham ... probably not... Thanks for explaining that. ;)

bob said...

Bevinne -

Having driven between Camano and Seattle a *lot*, I'd have to say that most times, Camano is north of the convergence zone. And more often than not, Everett is smack dab in the middle of it...

bob

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