Weather Blog

The great Convergence Zone trade-off

The great Convergence Zone trade-off

I just received some interesting weather data from the people at SkunkBayWeather.com, who run a 24-hour web camera and weather station from their home's waterfront perch in Hansville.

It helps shed light on the interesting trade-off that those who live in western Snohomish and northern Kitsap County get by living both in the Puget Sound Convergence Zone area and also get fringe benefits from the Olympic Rain Shadow.

Those who live up there know that there are many a day when the Puget Sound Convergence Zone is making it rain up there while the rest of the area remains dry. But you might be surprised to know that despite those rainy days, that area gets about the same amount -- or in many cases, less amount -- of rain than their Seattle counterparts?

Here is a chart comparing annual rainfall in Hansville to Seattle from 2009 to 2011 (also adding in Paine Field in Everett):

Annual Rain 2009 2010 2011
Hansville 23.68 28.06 25.40
Everett 29.96 35.47 30.78
Seattle 38.44 46.99 36.40

Now here is a chart comparing number of days with measurable rain:

Number of days with measurable rain (>0.01") 2009 2010 2011
Hansville 147 208 190
Everett 152 215 192
Seattle 146 190 167

Note how it rains more often in Hansville and Everett than Seattle, courtesy of the Puget Sound Convergence Zone, but that the annual rainfall amounts are somewhat less, especially in Hansville, from the rain shadow.

What this data would indicate is that it rains more frequently in Hansville and Everett than Seattle, but on the days when it really rains, they get less accumulated rain up north than in Seattle.

For example, on a good Convergence Zone day, Everett and Hansville might get 0.30" of rain while Seattle stays dry. Then, later in winter, a soaking all-day storm rolls through that might bring 1.00" of rain to Seattle, but thanks to the edges of the rain shadow, Hansville and Everett might only get 0.50" or so.

Now you've got two rainy days in Everett and Hansville to Seattle's one rainy day, but only 0.80" of rain up north compared to 1.00" in Seattle.

So it's a trade off: if you live up north, it rains more often but on many days it rains lighter than other places.

If you really want to get the best of the Convergence Zone without the rain shadow effects, head further east in Snohomish County. If you want the best of the rain shadow without the convergence zone, head north and west, out toward Camano Island, the San Juan Islands, or the Sequim-Port Townsend area.

By the way, Seattle itself does get a little bit of the rain shadowing effect itself, although it's weaker than its northern counterparts. But go further south toward Olympia and they get much more accumulated rain per year at about 51" on average (versus about 37" for Seattle) for roughly the same amount of days of rain.