December 01, 2013

Sunday Update: Amazing Rain Shadow and Snow Outlook

If you have lost your sun glasses, as many Northwesteners do during the winter, you better find them.  Bright sunshine and cold, crisp conditions are only 1-2 days away.  Cold air will be streaming over our region tomorrow, with only a few pesky snow showers in portions of western Washington.  I don't think there will be major problems with the Seahawks game on Monday night, unless the models are very wrong.

The big weather story today  is heavy rain and wind...but many of you are experiencing neither!  How is that possible?

Here is the precipitation over the past 24h (ending 11 AM Sunday) at a collection of NW stations.  Some folks to the east of the Olympics and Vancouver Island have gotten only a few hundredths of an inch, while the western slopes of the Cascades have received 2-3 inches (and will get more).

 Take a look at blow up of the 24 h totals.   ONE HUNDREDTH of an inch on Kitsap, while a few inches fell on the western Cascade foothills.

 Why this profound rain shadow in central Puget Sound?  The reason is that we have a strong trough embedded in northwesterly flow and the flow at crest level is westerly....producing a rain shadow to the lee (east) of the Olympics.

Here is the flow at 850 hPa (around 5000 ft and a good level to see what is approaching the mountains at crest level) at 8 AM.  VERY strong winds from the west to northwest (the winds are proportional to the gradient of heights shown by the solid lines.  The heights are like pressures on a surface chart.  Winds are parallel to the lines).  The air over us this morning is quite warm (denoted by red/orange colors in that chart)

 Warm enough that rain is falling in the passes (as shown below)...heavy rain at that.
 With a large north-south pressure difference and strong winds aloft, some locations, particularly in the south Sound and the Strait, have been buffeted by winds gusting about 40 mph (see graphic)

 Anyway, no issues for travel across the Cascades other than water ponding on the roadways and poor visibility.

      As I mentioned before, the cold air is coming on Monday.   The models are NOT producing major snowfall in the lowlands, but some of you will see some light white stuff.   As the cold air pushes out of the Fraser River gap on Monday AM, there could be some light snow at its leading edge over NW Washington (see map of 3-h snow ending 10 AM).

 As the cold air pushes south and west, some of it will be forced to rise by the Olympics, perhaps producing some light snow over Port Angeles and southern Sequim (see map for 3-h snow ending 1 PM)

During the next few hours Monday afternoon the cold air pushes south through Puget Sound.  The various models I have looked at are not producing much (see map below)...perhaps a few snow flurries.  If our models are wrong, this is when any snow would fall...Monday afternoon.

 Why are we not getting a big snowstorm in the lowland?  The set up is not correct.  The upper trough...and the upward motion associated with sliding to the east too quickly, so there won't be good upward motion when the cold air pushes in.  Furthermore, the position of the trough is not good for setting up a strong convergence zone over Puget Sound when the cold air approaches.  The subtleties are important in snow forecasting in our area...everything needs to be just right to overcome the ocean's warmth.

The total predicted snowfall for the next 48-h is shown below.  The mountains should do some business on Monday, but the lowlands are mainly snow free.  Disappointing to kids.

Anyway, get those sunglasses and protect your hoses, pipes, plants, and animals from unaccustomed cold.


Are you in venture capital/private equity and have an office in the Bay Area, or knows someone who does?
If so, I have a question for you. Please send me an email (cmass at ....thanks, cliff


  1. Thanks for the update.

    At some point you should probably explain to us why the winds are parallel to the isobars.

    I mean, the first-order naive physicist in me thinks the flow is parallel to the pressure gradient, not perpendicular to it. The second order physicist keeps whispering something about Coriolis forces and long fetch. But shouldn't we see at least some component to the flow vector that is parallel to the gradient?

  2. Cliff,

    I've noticed that the WRF-GFS and WRS- GFS Extended pages on the Pacific Northwest Environmental Forecasts and Observations website have two models for the Western Wa 3, 24, 48, and 72 hour snowfall.

    It appears at least somewhat different because the 12/1/2013 12UTC run shows different snowfall amounts at 18 UTC for the Bellingham area (as an example).

    Can you tell me/us what the difference is? Thank you!


  3. Is this event at all similar the snow that occurred early November 19th 2010 in Bellingham? Obviously it looks like we won't get much snow tomorrow. I was just curious if the scenario is similar at all.

  4. As I look at today's CPC 6-10 and 8-14 day outlooks, it looks to me like temps only begin to really moderate well into next week . . . all the while the precip chances appear to be ratcheting up, BEFORE the temps appear to moderate much. Is there a significant "overrunning" moisture/snow event possibly in the offing for next week? . . .


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