Ice Storm: Freezing Rain and Sleet

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From a bit of weather overload, I decided to take a break from this blog for a while, but the interest and importance of what is happening now is bringing me back online (and UW classes are cancelled so I don't have to be in class for my real job).

Over the past day and particularly this morning, something relatively unusual and, quite frankly, dangerous is going on outside--a freezing rain/sleet storm--and let me talk about it a bit.

I was woken up this morning by the sound of sleet (ice pellets) banging on my window.  Walking outside, there were ice pellets falling and the snow had a thin crust on it.  Yesterday, after the main snow, there was light freezing drizzle (and for some very small ice pellets) over much of the area.

This is a highly unusual situation here in western Washington:  outside of the famous ice storms of the western Columbia Gorge (also known as the silver thaw), western Washington does not get freezing rain very often.  The last big event was over the southern Sound in December 1996, when Sea Tac airport closed.  The airport is closed right now

Why is this happening?

Right now a warm front is to our south, with temperatures in the 50s in the Willamette Valley, while north of Olympia temperatures are below freezing:
Yesterday, after the main action was over, we were left with a saturated lower atmosphere in which there was supercooled clouds---water droplet clouds that were below freezing through their entire depth.  Yes, believe it or not liquid water can exist below freezing (typically in clean air from 0 to roughly -15C).  Some of this cloud water was converted into freezing rain.   Usual for us to see that.

But there is another way to get freezing rain (and sleet too):  when warm air pushes in above a sub-freezing air mass near the surface.  And that is happening right now.

Here is the temperature variation above Seattle now from the profiler located at the National Weather Service office at Sand Point.
 Cool air at the surface, but then a rapid warming (an inversion) above 400 meters (temps in C--really something called virtual temperature--subtract about 1 for air temperature.  The warm air (and southwesterly winds) have moved in aloft--a typical warm frontal structure (see graphic):

The precipitation area is reaching roughly to Everett based on the weather radar  (see radar below).

Over the northern part of the domain the southwesterly air is high enough so that it is below freezing, so there is snow (Everett is snowing).   But farther south, we have above-freezing air aloft and rain.  Rain that is falling into below-freezing air.  Some of this rain is freezing on the way down into ice pellets or sleet (what woke me up), but a lot of it is staying liquid and hitting the ground as freezing rain...which freezes on contact with the cold surface.  The current freezing rain is a mixture of the two icing mechanisms (freezing drizzle from shallow clouds and freezing rain from aloft)

Dangerous stuff, that freezing rain.  Not good for driving.  I just noticed my colleagues at the National Weather Service have an ice storm warning out.  Of course, this freezing rain/drizzle is on top of roadways that melted yesterday and then refroze. And here is the weather icon the NWS used for our weather this morning.  You see an icon like that and you worry.

Anyway, today there will be wintry mix--freezing rain, ice pellets/sleet, snow, rain until the air warms up enough aloft to turn it all to rain (probably by later this afternoon)

For those of you who like to geek out on cool technology, here is the correlation map from the dual pol capability of the Camano Island weather radar.  The pink indicates relatively uniform conditions an farther (and higher) out that indicates snow.  Closer in (lower) you see reds and yellow.  That is indicating melting snow and a mixture of snow, melting snow, and rain.

NORTHWEST WEATHER WORKSHOP:  The Pacific Northwest Weather Workshop, the main annual gathering for those interested in Northwest weather and climate, will be held on March 2-3, 2012 at the NOAA Sand Point facility in Seattle. For more information and to register please check the meeting web site:
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