January 09, 2013

Convergence Zone Snow and Colder Weather Ahead

We are now about to enter the most sustained period of cold air of the winter so far.  Temperatures aloft have progressively cooled today, with freezing levels right now at around 1600 ft and thus snow levels around 600 ft.  But in heavy precipitation, melting and evaporation can cool the atmosphere further, driving the snow level to the surface.  And the atmosphere should continue to cool over the next few days.

This is not a vague threat.....moderate precipitation in a Puget Sound convergence zone has started to bring wet snow to the ground from north Seattle up to Everett tonight.

If you take a look at Seattle SnowWatch, supported by the City of Seattle, you can view the temperatures above the city based on data from aircraft arriving and leaving from Sea-Tac.  Here is the latest (the dashed horizontal line shows the freezing level).  The green dots are the aircraft temperatures.  The blue dots are temperatures from another source:  the Sand Point profiler over north Seattle.  Temperatures are cooler to the north (freezing level about 500 ft) because of the cooler effects of the precipitation in the convergence zone.

The low center and front that brought the strong winds and heavy rains of the past day have moved through and the winds on the coast are now westerly.  The result is not only cooler air, but the formation of our old friend, the Puget Sound Convergence Zone.  Here is a recent radar image...the convergence zone is very obvious!

Paine Field in Everett switched to snow around 6 PM and the Seattle Forecast Office in north Seattle is reporting mixed rain and snow.  Here is the proof from a cam in Mountlake Terrace:
 As the air continues to cool, other locations will see some light snow showers in the scattered showers coming off the Pacific.  No big deal though...road surface are relative warm still and the showers should lessen during Thursday.  By Friday, we will be entrenched into the cooler air but it will be dry, with sun and temperatures only rising into the 30s.  An East Coast winter day!


  1. Yes, the sudden stratospheric warming (major) is happening and the AO/NAO index is going to plummet.

  2. Got about an inch here 3 miles NNE of Monroe! Nice looking outside this morning, unless you have to drive!

  3. Cliff ~ Joseph C. mentioned this major, sudden, stratospheric warming...I've heard a bunch of others mention it also. An article I read on Accuweather.com said that it would effect areas mainly from the Rockies east, but I've seen comments on other sites where people are saying it's going to effect us too? Exactly what is it, and how will it effect us?

  4. Its snowing right now on Orcas Island, east side, facing Bellingham. Big flakes, but no sticking at this time.

    Thanks for your work Cliff!

  5. I'll take it. Commuting by bike this week has been a test of will. And makes me question my sanity.

  6. Another cold dry stretch is fine by me. Compensation for the persistently rainy weather in November and December! Love how the frost sparkles in the morning sunlight.


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