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Dec 19, 2008

What's Up With This Convergence Zone

Convergence Zones are caused by colliding winds -- a north wind
from the north and a south wind from the south. When those winds
collide, they get forced upward, where the air condenses into clouds
and storms form.

Zones historically are responsible for some of the area's greatest
snows, and this one was no different. Arlington had a snow report of
23" as of 10 p.m. Wednesday, and all that snow that fell into Skagit
and Snohomish County from about noon on has been at the hand of these
colliding winds.

The massive snow storm of Dec. 18, 1990, was also born from a
Convergence Zone that set up shop over Downtown Seattle, as was the
snow and freeze of Nov. 29, 2006 (sort of a mix of a convergence zone
and arctic front for that event).

This time, the zone had a stronger southerly component, keeping the
zone up north for most of the day Wednesday. Computer forecasting
models correctly predicted this zone's formation, leading to forecasts
of heavy snow in the Seattle area Wednesday afternoon, but
underestimated the storm's southerly wind component, keeping the zone
north of the city. It wasn't until Thursday morning that this southerly
component weakened, allowing the north wind to gain strength and push
the zone south.

Zones form quickly and can shift very quickly, and many times
there is not much time to react, as many motirists found Thursday
morning. As much as it snarled the Thursday morning commute, imagine
if this happened at 1 p.m. Wednesday when everyone would have been in