March 23, 2018

Cold, Unstable Air Brings Snow Down to Higher Hills Around the Puget Sound Region

Unusually cold air has moved over the northeast Pacific and the Pacific Northwest, creating unstable conditions and snow descending to the higher hills.

During the past hour (10 AM), snow was reported at Paine Field in Everett (604 ft) and at Sea-Tac Airport (452 ft).  At Peter Benda's house in Bellevue (1300 ft), snow was accumulating in a winter-like scene.

Using the capabilities of the City of Seattle's SNOWWATCH app, we can see that the freezing level this morning has dropped to about 1300 ft (see plot of temperatures above SEA-TAC airport below), which means the snow level would be roughly 300 ft (below that level there would be only rain).

Cold air has moved in aloft, associated with an upper-level low pressure center that is centered west of Vancouver Island (see 500 hPa--around 18,000 ft-- weather map below for 5 AM Friday morning).

Here is a temperature map at roughly 10,000 ft (700 hPa)--you can see the cold air aloft (purple colors) associated with the low.

With cold air aloft and warming temperatures at the surface (it IS late March), there is a very large change of temperature with height and that promotes instability:  having the lower atmosphere break up into upward moving thermals and sinking air around them.   Just like in your hot cereal pot when you turn on the burner.    Upward motion results in cumulus, cumulus congestus, and cumulonimbus clouds, resulting in intermittent heavy showers.  Maybe some soft hail or graupel if you are lucky.

You can see the instability in the visible satellite image this morning.  The small elements are the anvils from convective clouds.

Perhaps a more impressive view is from the infrared satellite image (the shading shows the temperature of the cloud tops, with white being the highest).  Cold, deep convection over and off our coast.

The problem for lowland snow lovers is that low-level air is coming off the warm ocean (which is about 50F), making it too warm for snow, and particularly accumulating snow, to reach sea level.

The threat of light lowland snow over the hills is not over yet.  Tomorrow morning, the upper level low and its surface reflection will approach and move south of us (see surface weather map at 8AM Saturday), with cool air over western Washington.  Very weak onshore flow.  There will be enough upward motion to promote some showers of snow and rain.

The latest UW WRF model run shows some light snow getting close to sea level for the 24h ending 5 PM Saturday.  Enjoy the snowflakes...this could be your last view of them until next winter.


  1. We received a brief round of hail yesterday evening in Portland, but no snow as of yet. Off - topic but potentially significant, another possible solution to our carbon atmospheric woes:

    I wonder why potentially huge breakthroughs like this and others on the nuclear energy front are consistently downplayed or utterly ignored by the MSM. Actually, I don't wonder about it at all anymore.

  2. I work downtown Seattle, it snowed for about an hour here.

  3. Replies
    1. CO2 isn't plant food, but it is necessary for the photosynthesis process that allows green plants to make carbohydrate foodstuff. As is the case with O2 intake by humans and other animals, it is possible for plants to get too little OR too much CO2. Much research is being conducted about what happens when plants get too much CO2. Read:
      Or I am happy to summarize the key points.

  4. I was pleasantly surprised with snow driving to from Edmonds to Northgate this morning around 11 am. But I did have to triple check, and it didn't help my incredulity that it was too hot for it to stick anywhere.

  5. We had grauple last eve ( at 4'ish ) here in Downtown Greenbank... and I'm bettin' on some snow over nite !

  6. Cliff?

    Does SeaTac launch weather balloons/radiosondes, or how *do* they arrive at the "in situ obs" dots shown on your "estimated freezing level" chart?


  7. Winter is back around us big time. Snowing this AM at dawn, but not surviving warm ground temps ~ just wetting surfaces (leaves, grass, paving, etc). But now !!! may be 2 inches deep and continuing to accumulate.

    We are about 550 ft elevation, mid way between Everett and Mount Vernon, tucked into the foothills that partially shield us from the warming influence of Puget Sound.

    Pretty but problematic.

  8. Do the weather models take into account the change in albedo following snowfall? Earlier this morning, we got five inches of unpredicted snow where we live (outside of Boise). I'm sure that drastically reduces the heating from insolation, making the previous predictions all off.


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