WeatherWatcher: winter weather advisory for the convergent zone

Wednesday, December 29, 2010

Shoreline 9:09 am.  A very narrow Puget Sound Convergence zone shower has developed and is shifting slowly south from Everett and has now reached Shoreline. I've been watching the radar this morning and it seems to be "wiggling" around in the area. It is expected to get stronger in the early afternoon and appears to have established its spot over or near our area.

Winter Weather Advisory issued:
At 8:09 AM the National Weather service of Seattle issued a winter weather advisory for the Everett-area, including Lynnwood, and we appear to be effected as well. Accumulations 1-3 inches expected with possibly 5 inches in some spots. I suspect this may change soon to cover North King County as well.

What exactly is a Puget Sound Convergence zone?
A Puget sound convergence zone is when you have winds blowing out of the north, north of Everett, and winds blowing out of the south, south of Seattle. When the two winds collide in the middle some where, (Between Seattle and Everett typically, Shoreline-Lynnwood being the hot spot) the wind goes up towards the sky. This cools the air rapidly causing water vapor to condense into clouds, and eventually precipitation if the winds are strong enough. Fluctuations in the north winds, or south winds can cause the convergence zone to drift north or south, which is what makes these types of events very difficult to predict an effected location. Also, if the precipitation is intense enough, it can bring down colder atmospheric air, bringing surface temperatures below freezing, resulting in snowfall and freezing roads, even though 10 miles away it could be 45°F.

Typical causes of a Puget Sound Convergence zone:
Normally a strong westerly wind that is split by the Olympic Mountain range, causing equal north and south winds as they round the mountain range and get trapped between the Olympics and the Cascades. These rarely produce any accumulating snow showers.

The snow producing ones typically are caused when a low pressure traveling south of the state is pushing moist air up north through the Puget Sound, while also dragging cold air down from British Columbia. This was what caused our Thanksgiving week snowstorm.

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