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West Coast Ice Storms
Wed Feb 18, 2004
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Hi, I'm Bryan Yeaton for The Weather Notebook. Freezing rain and ice storms are a scourge of
winter weather, particularly when such weather downs power lines and disrupts transportation.
The East suffers through major ice events a few times in an average winter. Less well-known,
though, are the damaging freezing rain events in the Northwestern United States.
Freezing rain conditions and ice accumulations occur most frequently east of the Rockies, in
the Great Lakes, the Northeast, and the Mid-Atlantic States from October to April - 20 or more
events per year on average. Meteorologists and other weather watchers still talk about the
great January 1998 Ice Storm that devastated New England and Quebec.
However, climate analyses show a second freezing rain maximum occurs in the Pacific Northwest
- centered over eastern Oregon, eastern Washington, and Idaho - with ten to fifteen events per
year.
These freezing rain events generally arise when storm systems entering the area push onto the
coast, following the break up of a strong cold spell. The cold air, being much heavier than
the warmer, moist air of the coming rain system, tenaciously guards its hold in pockets of low
terrain among the mountain ranges. When rain falls from these storms, some will traverse these
cold air pockets and freeze at the surface. If the cold layer is too thick, the rain will
freeze before it hits the ground, and that's sleet.
At higher elevations, freezing rain may also occur when the rain falling from overriding warm
air aloft enters the cold air lying near mountain summits. Such freezing rain can cause
considerable disruption to power lines crossing mountain ridges and transportation links
through high mountain passes.
Thanks to our contributing writer, meteorologist Keith Heidorn. The Weather Notebook is
produced by the Mount Washington Observatory, and funded by Subaru of America and the National
Science Foundation. We can be found online at www.weathernotebook.org.
Today's Links
Freezing Rain Climatology
http://cimms.ou.edu/~cortinas/preprints/waf15/climo.html
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