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Originally published January 16, 2012 at 8:57 PM | Page modified January 16, 2012 at 9:10 PM

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Understanding 'convergence zone,' its effect

Geography influences Western Washington's weather more than people realize, experts say.

Seattle Times education reporter

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On behalf of all citizens in Kenmore......we get it. MORE

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

Snow? Here? This weekend?

While northwest Seattle has seen precious few flurries in recent days, other parts of the city and region have seen lots of snow.

The explanation goes back to a term that seems to pop up whenever 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 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 winds from the west that run into the Olympic Mountains and splits into parts that go around the mountains to the north and 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.

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

On Monday, snow fell heavily to the north, south, east and west of Seattle but largely spared the city because it was protected by the mountains, said Ted Buehner, a National Weather Service meteorologist.

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," Mass said, "it'd be pretty boring. 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 10 miles away."

Brian M. Rosenthal: 206-464-3195

or brosenthal@seattletimes.com

On Twitter @brianmrosenthal

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