As the sun set in Seattle yesterday, the National Weather Service was forecasting an evening mix of rain, sleet or snow, turning to all rain by morning. While freezing rain paralyzed...

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As the sun set in Seattle yesterday, the National Weather Service was forecasting an evening mix of rain, sleet or snow, turning to all rain by morning.

While freezing rain paralyzed the Portland area yesterday, the Washington State Patrol reported ice-free Seattle-area highways through the afternoon.

Whether Seattle turns icy or white depends on the temperature both close to the ground and up in the clouds.

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Last evening, the ground-level temperature hovered around 32 degrees. But at cloud level, pockets of both relatively warm and cool air passed over the region, said Johnny Burg, a meteorologist with the National Weather Service in Seattle.

So, far up in the sky, precipitation alternated between rain and snow.

As that precipitation fell to the ground, it passed through a low-lying blanket of cold air. The coldness and thickness of that blanket, along with temperatures higher up, determined what hit the ground.

Warmer temperatures at cloud level combined with a thin layer of cold air near the ground can produce sleet or freezing rain like what hit the Portland area yesterday.



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For regularly updated local weather information and warnings from the National Weather Service, www.wrh.noaa.gov/sew/



Forecasting which form precipitation will take when the temperature is near freezing is tricky because so much depends on small local variations in the air temperature above ground level, Burg said.

Meteorologists are handicapped by a lack of data on temperature conditions above the surface.

The nearest National Weather Service weather balloon station is in Quillayute, near Forks, Clallam County, on the Olympic Peninsula.

So, instead, forecasters use data collected by airplanes taking off and landing at Seattle-Tacoma International Airport. As planes pass through the atmosphere, they collect weather data at different elevations and transmit it to the weather service.

Jim Downing: 206-515-5627