Convergence Zone snow

There are a number of moving pieces that have been occurring during the past six hours, but the most significant was the convergence zone that set up between Lynnwood and Everett, resulting in 2-4 inches of wet snow.

And then there is the cold air (the Arctic front) that is now pushing southward, with strong flow coming out of the Fraser River Valley.  As shown by this radar image a few hours ago, there has been a band of often heavy convergence zone precipitation over the North Sound, which was stronger than the models predicted. The convergence was produced by the  confluence of air moving eastward in the Strait of Juan de Fuca and air from the southwest coming around the Olympics.


The air aloft had started cooling after around 1 PM as the low center aloft moved through, and by later in the afternoon the combination of this cooling air aloft and the cooling effects of heavy precipitation (melting and evaporation) were enough to bring wet snow down to the surface in the convergence zone.    In addition, we are now getting snow showers over NW Washington associated with the invading cold air from the north.

Seattle to Tacoma have gotten little snow so far.   If you want to see the cold air coming in, here are the observations from 8 PM.  Cold, gusty winds are blowing from the northeast out of the Fraser towards Bellingham and the San Juans.


This cold air will push southward during the next few hours.   The convergence zone in the north Sound will probably weaken during the next few hours, and snow showers will move southward over Seattle and Tacoma.  It should be over by 3 AM or so.

Comments

Post a Comment

Popular posts from this blog

The Time of Year You Can See the Air Move

Image
  Most of the time, you can not see the air move.   You can feel it, but the complex, turbulent motions are invisible to you, except when it interacts with leaves and branches on trees and plants. But for a few weeks of the year, the sky is full of small, cotton-like tuffs that like a natural MRI machine reveals the complex three-dimensional motions of the atmosphere.   Cottonwood season.  And we right in the middle of it.   Being cottonwood aware is of particular value this year.  It reveals the active dispersion and movement of air that makes the outdoors essentially COVID safe, and thus should be of some comfort for those nervous about transmission outside. Cottonwood seeds are embedded in small cotton-like fibers.   A million seeds weigh about 3 pounds.  Yes, each seed and tuff weights about .000003 pounds.   And that fact, plus the cottonwood tuff, means that these seeds fall VERY, VERY, VERY slowly.   So slowly that they are excellent markers for the three-dimensional flow of the
Read more

Why so little lightning in the Pacific Northwest? And a very nice weekend ahead.

Image
During the past 24-hours, there have been a few lightning strokes over the region (see below) but nothing to write home about (see below). It turns out that the Pacific Northwest and the remainder of the West Coast have some of the lowest frequencies of thunderstorms in the entire country (see below). But why?    My podcast provides the answer. And I also give the weekend forecast....and I think most will like what I have to say. To listen to my podcast, use the link below or access it through your favorite podcast service. Some major podcast servers:     Like the podcast? Support on Patreon 
Read more

Strong Atmosphere River Heads into British Columbia and Southeast Alaska

Image
While the Northwest has been drier than normal, it has been drenching wet a few hundred miles to the north. An unusually strong atmospheric river ....a long current of moisture stretching southward to the subtropics...has been in place for days, bringing large amounts of water vapor to Alaska.   And the atmospheric river will be in place for several more days, slowly moving southward. To illustrate, the scale and beauty of the atmospheric river, here is a water vapor satellite image for 11 AM today. The lighter areas have more water vapor.  You can see the current of moisture extending from near Hawaii into central BC.  This current is the result of low pressure over the Gulf of Alaska and high pressure offshore of California.   A forecast of total water vapor content in the atmosphere for 11 PM tonight shows a healthy atmospheric river (white and blue colors indicate the higher amounts of moisture). As this atmospheric river slams into the mountainous West Coast, the air is forced to
Read more