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.
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.
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