November 08, 2014

Will our mountains protect the Northwest from frigid air this week?

Weather forecast models are predicting that a massive intrusion of very cold air will move southward into the interior of North America.

But what will happen here in the Northwest?

As I have talked about in my blog and in my book, our mountains substantially block the westward movement of cold air.  We have a double defense with the Rockies holding back cold air in Montana and Alberta.  Then the Cascades blocks cold air from eastern Washington from getting into western Washington and Oregon.  If only the Washington Huskies had similar defensive depth.

But these barriers are not perfect.  First, if the cold air is really deep some of it can pass OVER  these mountains.   And there are gap in the mountain ranges, such as the Columbia Gorge and the Fraser River Valley that allows some cold air to sneak through.  So will our defense hold?  And if not, where will the frigid air break through?

This is going to be a very interesting week...some of you will get strong winds, some well-below freezing temperatures, and all of will experience much colder air than we are used to.

Let me show you the surface temperature prediction from the UW WRF from the coarser larger domain.   The legend is on the is warmer, blue and purple are colder.

Today at 4 PM.   Close to normal temps for us and cold air is over northern Canada.

By Monday at 4 PM the situation is very different.  Cold air has pushed southward east of the continental divide, but the Rockies are holding back the cold stuff from eastern Washington.

At 4 PM on Tuesday has cooled further in Montana (white is air below -8F!!) and some cold air has moved into eastern Washington and the Cascade crest.

Fast forward to Thursday at 4 PM...same story except Montana warms a bit, while western Washington cools (but still warmer than eastern WA).

The invasion of cold air will produce some HUGE pressure gradients (differences) across our mountains and very strong winds along the western slope of the Cascades, downstream of the Fraser River Valley (north of Bellingham) and in the Columbia Gorge.

Let me illustrate.  Here is the predicted sea level pressure (solid lines), lower atmosphere temperature (at about 3000 ft above the surface, colors) and surface winds for 4 AM Wednesday.  Wow.  Cold air in eastern Washington and an amazing difference in pressure across the Cascades as high pressure builds over the interior   Winds in the western portion of the Gorge (e.g., Portand to Stevenson) will be ferocious:  60-70 mph gusts easily at exposed locations.  Strong easterly winds will descend over the western slopes of the Cascades and strong NE winds will push through the Fraser Gap to hit communities north of Bellingham.

How cold will it be in Seattle?  For a change, lets check, which does a very decent job with their predictions.  High temperatures drop to the lower 40s by Thursday.  That will be a shock, with lows getting to near freezing.

But just cross the Cascades and you will hit the real frigid stuff.  Temperature at Spokane remaining below freezing all day by mid-week with lows in the teens
But cross the Rockies and Arctic-like conditions await (here is Miles City)

So if you have cold-sensitive plants, you might add a little mulch, but west of the Cascades temps won't break many records.  But it will be a shock to all us, considering we have come off the warmest summer/early fall in NW history.


  1. Sunday afternoon - SeaTac Radar Composite Reflectivity shows a "Victoria Express" hitting us here with heavy rains

  2. I love that reverse convergence zone flapping around west of the Olympics all day Tuesday.

  3. I picked all of my peppers on Friday and we had light frost here in Kingston Saturday AM. The one that I am overwintering is doing great in our attached sunroom.

    Will any of this weather be enough to freeze pipes here?

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