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A Wet Lesson in Seattle Weather for Newbies

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"It rains in Seattle, like, all the time." "No it doesn't." "Does too." Get a clue. Where you live means the difference between sunshine and clouds, rain and snow, and whether the wind is going to blow the leaves from your lawn and turn your trash can lid into a flying saucer.

Step one, apologize to the meteorologists. If you're used to a midwest forecast you've lived in luxury. From the middle of the country and east, meteorologists have dozens and hundreds of weather stations telling them what's coming. Seattle's weather is determined out over the Pacific where the population density is rather low. Surprises happen. Want better forecasts? Help them get more floating weather stations.


Image: Western Regional Climate Center
Raw data: Seattle's average annual rainfall is 37.41 inches. Really kind of useless when you look at the map.

What about your neighborhood? Geography makes the biggest difference. As if a great unknown series of storms weren't enough, when they hit land they run into an 8,000 foot high fence called the Olympics. After that splits and churns the air, the tumbled mass flows across various bumps of land and great flat swathes of nearly constant temperature water. It's a mess.

When the storms recollide they set up the pesky Puget Sound Convergence Zone, a line of clouds and rain that may only be a few miles wide, but is hard to ignore - especially, when everywhere else is laughing because their forecast for rain turned into blue sky. The PSCZ can be anywhere but it frequently forms up over north King County. Sorry, Montlake Terrace. Cliff Mass has an excellent and expert blog about all of these phenomena.

Even during a storm, blue skies happen. When the weather's just right, the Olympic Rain Shadow forms. Storms coming in from the southwest leave such a gap in the rain pattern that Sequim, on the northeast corner of the Olympic peninsula, gets less than 16 inches of rain a year. The commute time is a bit extreme, but it has been done! From the middle of Whidbey and north, the islands are in the same banana belt.

That banana belt comes at a price, though. The San Juans are far enough north that storms coming in from the west miss the Olympics and hit full force. Power outages happen.

As the weather moves past the city it rises as it hits the Cascades. A phenomenon called orographic lift is a fancy way of saying the higher you go the wetter you get. North Bend gets almost 60 inches of rain, and it is on the edge of the foothills. At the crest, Snoqualmie Pass gets 100 inches of precipitation. Go north or south enough and Mt. Rainier and Mt. Baker have each held the record for snowfall at over 1,000 inches (and don't expect it to be champagne powder.) Temperate rainforest is defined as over 55 inches of rain a year. Live in the foothills, live in a rainforest. Just don't expect monkeys and warm weather.

By the way, the foothills get their windstorms, too. Sometimes the air decides to switch directions and flow back downhill and hit the lowlands. At least for a short while, Enumclaw could generate a lot of wind power. It happens up north as well. Up by Bellingham, the Fraser Outflow can bring wind and moisture causing major hassles for Whatcom County with ice and snow storms.

Oh yes, and out over the water there's wind. Even on relatively calm days, the Sound may be whitecapped as systems flow in from the south with nothing to stop them. Check the ferry weather site for winds recorded as the boats make their runs.

That great mass of water called Puget Sound (part of the Salish Sea) has a great effect because it changes so little. Its temperature remains about 50 degrees; so, in the summer the water is chilly enough (relatively) to only be appealing on the warmest days, and in the winter the water is warm enough (relatively) for kayakers and divers to warmer by the water than out of it. That stability also means snow has a rougher time settling down to sea level, and the hottest days are inland, not by the beach.

Of course, that's a lot to take in. And then there's marine layers, inversions, freaky little waterspouts, and that stuff called blue sky. It does appear.

There's an easier way to understand a neighborhood's weather and it has little to do with data and charts. Look at the gardens. Are they growing palm trees or moss-covered big leaf maples? Are the tomatoes fat and red and in the fresh air, or are they inside moss-covered greenhouses? Are the lawns xeriscaped deserts, lush and green blades, or just moss? Come to think of it, just check for the moss.


Photo by Tom Trimbath

· Western Regional Climate Center [WRCC]
· Cliff Mass & PSCZ [CM]
· Olympic Rain Shadow [NOAA]
· Temperate Rainforest [wikipedia]
· WA Ferry Weather [WSF]
· Salish Sea [wikipedia]
Written by Tom Trimbath.