February 09, 2018

La Nina-Like Cool Period with Some Limited Lowland Snow

(Sherlock Holmes and the Olympic tree fall will return on Sunday---and the answer may be in hand!)

Relatively cool air is now over the Northwest and should in place over the weekend.  And a few folks over the lowlands might seem some flakes before the weekend is over.

With high pressure offshore, northerly winds have developed in the lower atmosphere, as seen in the time-height plot above Sea-Tac airport (below, red is temperature, heights in pressure--850 is about 5000 ft, time increasing to the left).  Over the last 24 h, temperature at 850 hPa (again about 5000 ft) has dropped from 3C to -4C.

The cool air will continue to spread southward today and, as shown in the plot below, will reach the Oregon border by 7 AM (1500 UTC) tomorrow morning.  Note that there is a large pressure change (gradient) in northern CA associated with the leading edge of the cool air.  This makes sense, since cooler air is more dense than warm air, thus a gradient in temperature produces a pressure gradient.

Expect frosty temperatures on Saturday AM with cold air aloft and clearing skies (which allows good radiational cooling to space).

On Sunday morning an upper level trough will approach our region, bringing clouds and some precipitation.  I would be talking about the potential for lowland snow, except the trough is going too far offshore and south of western Washington....not quite the right set up for Puget Sound.

The 24-h precipitation total ending 4 PM Sunday shows plenty of precipitation along the coast, but little over Puget Sound (due to rain shadowing from the Olympics and Mountains of Vancouver Island under NW flow).

The forecast snow total for the same period is disappointing.  Not much on the coast because it is too warm there and nothing over Puget Sound.  Only over far NW Washington and southern BC, will there be sufficiently low temps and enough moisture to get a dusting.  And not much good for the mountains.

During the past few days and this weekend, we have had an area of high pressure offshore, with lower pressure inland, similar to the typical La Nina pattern.  But only similar... strong La Nina's have the high pressure farther offshore.  Here is the typical 500 hPa (upper level) height anomalies (difference from normal heights) for La Nina years, with red indicating above normal heights (pressures), and blue/purple the opposite.

Compare that pattern with the forecast pattern for 4 AM Sunday (below).  Similar, but the features are displaced eastward.  And it looks like La Nina's days are numbered....the latest model forecasts show a transition to neutral (or La Nada) conditions by summer.

In any case, there should be lots of sun on Saturday.

(Again, the third and final blog on the Olympic Mountain mystery tree fall will be released on Sunday....)


  1. Seems like you are REALLY trying hard to prove their is correlation between colder ocean temps in the fall and a colder/wetter winter in the PNW. Last year was not a La Nina year and it was the coldest and snowiest winter I have ever seen. This year is a La Nina year and every time we see a little cold (usually preceded and followed by a pineapple), you are quick to remind us of La Nina. If this is a La Nina year, this skier likes weak El Ninos better...

    1. But: Last year, according to NOAA, we had weak, short-lived La Nina conditions.


  2. Super Bowl of Winter Weather final score:
    RRR: 42
    La Nina: 3

    As in 6 touchdowns(with extra points)verses one field goal. Fair to say the ridge was the MVP.

    La Nina really only gets a participation trophy this winter. A winter that probably turned the corner 2-3 weeks ago. However there were some notable moments. Particular plays were the White Christmas Fake and Inverse Rain Shadow Conversion. There still might be some controversial tape to review in retrospect if a flag gets thrown on a March snow or wind event.

    Otherwise, time to get busy on overdue spring training chores and shop for a split ductless AC system for the stadium.

    Summer is the next big game!

  3. Pretty incredible no show winter where I live. Yakima is only running 17.5 degrees above average for the first 9 days in February setting an all time daily record on the 8th. That accumulated severe winter weather index has us ranked #1 mild non winter ever recorded. Which I swear must break the winter of 3 yrs ago. The leeward slopes are really suffering from lack of snow with brown hills up over 5000 ft.

    While Seattle and northern Washington has had the grace of precip and some snow due to a northern storm track, it quickly becomes dire further south. Water for irrigation won't be a problem but we'll suffer a lack of spring runoff for green hillsides and wildflowers.

  4. Still waiting for the Sunday post for the thrilling epic conclusion...

  5. It looks like the winter currently has its grip on the Northwest! It sounds like the perfect time for me to curl up next to the fire, and listen to my favorite radio broadcast show, with a hot chocolate. For anyone who loves the cold and snow, this is our time to enjoy it while it lasts!


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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