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January 16, 2012 at 2:42 PM

That weirdly uneven snowfall? There can only be one explanation

If you live in Ballard, you’re probably wondering what the big fuss is all about.

Snow? Here? This weekend?

While Northwest Seattle has seen precious few flurries over the past few days, other parts of the city and region have seen lots of snow during the same time period.

The explanation goes back to a term that always seems to pop up when there’s a major weather event in Western Washington, even though few people understand it: the convergence zone.

Ah yes, the infamous convergence zone — a  phenomenon often cited as the reason for higher rain totals in the northern part of Seattle and beyond than to the south.

How does it work?

The zone is created when two currents of wind collide at a low elevation, pushing air up higher in the atmosphere, weather expert Cliff Mass explained. The air takes on moisture as it rises, eventually collecting so much that is has to offload it in the form of rain or, in this case, snow.

The normal convergence zone occurs about 25 times a year, Mass said. It results from strong wind from the west that runs into the Olympic Mountains, causing it to split off into parts that go around the mountains to the north and to the south. When the winds come together again on the other side of the mountains, usually over Lynnwood or Everett, a zone of heavy rain is created.

The zones creating the odd snow patterns we’re seeing now are different, said Mass, who wrote about the topic on his blog. They’re resulting from several currents of wind going in multiple directions, navigating the mountains and bodies of water.

On Sunday, two major currents collided over central Seattle, creating a band of heavy snow that stayed in place for several hours. That thin band dumped several inches on the Central District and West Seattle, leaving much of northwest and southeast Seattle dry for much of the day.

Today, snow is falling heavily to the north, south, east and west of Seattle but largely sparing the big city itself because it is being protected by the mountains, said Ted Buehner, a meteorologist at the National Weather Service.

Geography (mostly mountain ranges and bodies of water) influences Western Washington’s weather more than people realize, Mass and Buehner said. That’s very different than in other parts of the country, they said.

“If you were forecasting in Iowa, it’d be pretty boring,” Mass said. “What makes the weather here so extraordinary is that we have these extreme local variations in which the weather can be radically different five or ten miles away.”

Comments | More in Weather Beat | Topics: Cliff Mas, convergence zone, National Weather Service


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