Monday, December 24, 2012

Strong Foothills Winds and Localized Snow

It's all about mountain slopes.   

So much of our interesting weather is associated with air moving up and down our substantial terrain and tomorrow will be a stellar example.  Huge weather contrasts will occur tomorrow over the lowlands.  In some locations, wind will gust to 60-80 mph--as strong as Hurricane Sandy--while 20-30 miles away you will barely feel a breeze.   In some lowland locations there will be not a single flake, while 20-30 miles away nearly a foot could snarl traffic.
December 25, 2012 Weather
 We are lucky to live in such a place.  Imagine being in IOWA or KANSAS where you could drive for hours and barely notice a chance in the weather.  If you live in Seattle, you can CHOOSE whether you want to experience boring rain, hurricane force winds, or a major snowstorm...all within an hour's drive or ferry ride! 

Boring Weather Comes With No Terrain
 Christmas Storm 2012 will strike tomorrow.  I know the TV stations will have something better:  The Santa Storm, Holiday Express, etc.  And the Weather Channel will give it some scary name:  Storm Boris or something.  At 10 AM tomorrow a low center will be parked off our coast, precipitation will be moving in over the region, and a very large pressure difference will develop over the Cascades.

With strong southeasterly winds over the Cascade crest and a big east-west pressure difference over the mountains, we have a very good set up for very strong easterly  downslope winds on the western Cascade slopes and adjacent lowlands of western Washington.  Enumclaw, Black Diamond, North Bend and many eastside communities will get hit hard, with wind gusts reaching 50-70 mph in a few places.  Trees will fall.  Power will fail.  And strong winds will occur in the Strait and over the offshore waters.  Forget your sailing lessons unless you want to end up in the mid-Pacific. 

Here are the predicted maximum gusts (in knots) at 1 PM tomorrow from the super-high resolution WRF 1.3km resolution model.  Wow.  Gusts reaching 60 knots over parts of the Strait and offshore of the central WA coast.  45 knot  (52 mph) gusts along the western Cascade foothills at that time (with some higher values).

What about snow?   The National Weather Service and I are generally on the same page (SE of the Olympics is the focus), but they are going for the possibility of a few inches over Seattle and vicinity.

Looking at the latest runs, I believe it will be too warm for snow in Seattle and that the powerful easterlies will dry things out enough to lessen the snow chances near sea level around the Seattle Metro area.  Here is the latest WRF model forecast.  First, the 24h ending 4 PM Christmas Day. Same story as yesterday: Kitsap and Mason counties get the brunt of the lowland snow. 

The next 24-h (ending 4 PM on Wednesday).  Eastern Washington's turn!

But although I think the above is the most probably scenario, the snow level could descend toward the high hills of the lowlands if the precipitation is heavier and the easterly flow weakens more than forecast.  This is a classic Seattle snow forecasting problem:  temperatures are marginal and everything depends on precipitation rate and the temperature/humidity structure of the air above us.  By the end of the afternoon it will be too warm for snow that is clear.  But I have learned not to be too confident about Seattle snow and freezing rain forecasts.

Roads are relatively warm right now.  Here is the latest air and road (boxes) temperatures from Seattle SNOWWATCH.  Elevated and non-elevated roadways are in the 40s.  That is good place to start.  If clouds come in soon enough tonight, temperatures will not fall too much and even if few flakes hit the road, they will melt quickly.

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