December 29, 2010

Its Snowing! (But not everywhere)

I5 in North Lynnwood

During the past few hours it has been snowing in Everett and at Sea Tac but RAINING at Boeing Field and Renton. In fact, the snow was relatively hard around Everett to Lynnwood and is accumulating on the roads (see above). Why? There is an intense band of precipitation...very narrow... extending over Everett and environs...take a look at the radar to see:

The models were hinting at such a feature yesterday and it appears to be associated with a Puget Sound Convergence Zone. Temperatures are marginal for snow over the lowlands right now. Where precipitation is light in such get rain near sea level. But at higher lowland stations and where precip is heavy you can get snow. What is the freezing level right now? You can find that out using the Seattle profiler, a device that gives us wind and temperature aloft in real time. Here is the temperature plot (actually something called virtual temperature so subtract one degree C for regular temp):Looking at this, the freezing level is about 400 meters (roughly 1300 ft) and the snow level can drop to around 1000 ft below the freezing level (takes about 1000 ft to melt completely). Paine Field and Sea Tac are also relatively high (roughly 500 ft). All our precipitation starts as snow higher up in the atmosphere, but usually melts in the warmer atmosphere near the surface. Intensity is important because precipitation evaporates and melts as it falls, causing the freezing levels and snow levels to drop. Heavier precipitation...the freezing level drops more.

So today will be a difficult challenge. Our temperatures are marginal for snow at sea level. On the higher hills (above 300 ft) the chances of snow are increased and where precipitation is heavy the snow level could descend to sea level. So watch the radar. If the convergence zone revs up perhaps we might see a few inches near sea level in the central Sound. If not, this could be a big bust for snow lovers in Seattle. Again, this is NOT November 22! The ground temps are above freezing and air temps are above freezing. The road surfaces are not going to freeze, although slushy snow can be a bit slippery. And daytime and warmer temps are coming.

This convergence zone will probably slip south during the morning and probably weaken later...but the details are everything now.


  1. Cliff, you say "All our precipitation starts as snow higher up in the atmosphere, but usually melts in the warmer atmosphere near the surface. "

    Can you clarify or qualify that statement? It's been 20 years since I was in school, has CCN/collision/coalescence in non-supercooled situations been discounted by better science?


  2. There was hail in Port Townsend this morning, about 1/4 inch in diameter or larger, blanketing the ground.

  3. It's snowing hard where I'm at in Woodinville. We're about 400ft. I'd say there is about 1.5"-2" out there and it's still coming down hard. :)

  4. Duvall - we have 2" and climbing in the last 35 minutes. Blowing snow, the size of quarters and about 1/4 mile visibility 32 degrees. We're about 400' elevation.

  5. On Tolt Hill near Carnation, about an inch and coming down hard.

  6. one inch of snow here in north Tacoma, near UPS. I submitted to Cocorahs (am observer for them)

  7. San Juan Island here, just above sea level, snowing hard outside 12:30pm. About 1.5" already. Hope it stops!

  8. Finally stopped up here on San Juan though, looks like you were on the money. Thanks!


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