Wednesday, January 18, 2017

Major Freezing Rain Event Across the Region

Interstate 90 is now closed in both directions and will be closed at least until tomorrow.

Interstate I-84 is now closed both ways in the Gorge.

The reason?  A major regional ice storm in the Cascades Passes, the Columbia Gorge and in portions of the Columbia Basin.   

Freezing rain has resulted in several inches of ice on roadways, trees, and powerlines.  Some pictures  below show the story.  First, several around I-90 provided by the Washington State Department of Transportation.


 Scenes in the Columbia Gorge were equally dramatic (here is one taken by Patrick Oldright outside the Vista House in the Columbia Gorge)


The meteorological set-up was classic.

We started with a layer of subfreezing air east of the Cascade crest and an offshore directed pressure gradient (higher pressure to the east).  This pressure difference pushed cold air into the Cascade passes and through the Columbia Gorge.

Then a warm, wet Pacific frontal system approached, producing an above freezing layer aloft and rain that fell into the cold layer near the surface (see schematic).  Even when cooled below freezing the falling rain (comprised of supercooled water) did not freeze until it hit the ground.

Freezing rain also fell in the Columbia Basin today, as rain fell into the cold air that was trapped in the lower elevations of the Basin.

The Columbia Gorge and Portland are well known for freezing rain, which is sometimes called the "Silver Thaw".   Ditto for the Columbia Basin.
A map of the climatology of freezing rain over the U.S. below shows that eastern WA and the passes to its west experience freezing rain more frequently than any other region in the western U.S. (2-4 events a year on average).  Freezing rain is

even more frequent over the central and NE U.S., but the origin is very different: generally associated with warm fronts, with warm rain above the front  falling into cold air below.

With the atmosphere warming above and the reduction in the offshore flow as the cold air is mixed out to our east, the threat of freezing rain is rapidly diminishing in the passes.

5 comments:

lhsouthern said...

WSU even cancelled classes because of the weather! They never did the 6 years I attended!

Stuart Jennings said...

Why does the "classic warm front" freezing rain that occurs over the midwest so regularly, not occur on the Pacific Coast?

jno62 said...

That shot from the the Vista House is awesome. Must have been a fun drive to get there!

CC said...

Fascinating. Would have never thought that there was a middle layer of warm air that melted the ice particles into rain. Thanks.

Bobby said...

I live in Hyak on the Pass and what is not being reported and is very odd is; we received at least 5" of real snow on top of the 5+" of frozen graupel here in Hyak. the weather stations all refuse to actually report what took place here, but we got 0" yes 0" of rain. even during periods of heavy snow you could turn on a weather station or website to see it say raining and reporting temps at least 10 degrees warmer than what multiple thermometers were saying. as a side note after driving over to Alpental yesterday it is safe to say Hyak received double the amount of accumulation during the supposed "ice storm"