Thursday, December 3, 2015

Very Strong Winds Today, Particularly on the Coast

Hurricane-force winds, with gusts above 70 mph, will strike our coast today, with winds over the Puget Sound lowlands reaching 40-60 mph.  Expect power outages and falling trees.

The winds along the coast will be particularly severe.  The origin of the winds will be a very sharp trough/low pressure area.   To illustrate, here is the 12-h forecast of sea level pressure (valid 4 PM).  The solid lines are isobars, lines of constant pressure.  Note the huge gradients of pressure (change of pressure with distance) over southwestern Washington, with the low/trough over Vancouver Island.  Big changes in pressure produce strong winds.

The forecast gusts by the UW WRF high-resolution model at that time are amazing along the coast.  Gusts above 70 knots (white color).  Gusts to 50-55 knots over the waters of central Puget Sound

This is a fast moving system. One hour later, the strong winds have pushed into NW Washington including the western exit of the Strait of Juan de Fuca. Still strong over Puget Sound.

One hour later (6 PM), as the low moves past, winds around Seattle and along the coast have weakened, but huge westerly winds are pushing through the eastern Strait of Juan de Fuca.

Remember the above are max gusts, sustained winds will be substantially (perhaps 50%) less.  There is substantial uncertainty with this event, particularly over Puget Sound.  A small shift of the low offshore would drastically reduce Puget Sound winds.  Also, one of our short-term forecasting systems (HRRR), which is updated hourly, is pulling back on Puget Sound winds.

If you are along the coast, this is a serious event, but a short one.  Keep away from trees.  Don't drive along tree-lined roads.  Every storm like this, someone gets killed.  The chances are slight for any one person, but why be the one smashed by a fallen fir?

And while you are thinking about the winds, don't forget to express your feelings about the proposed termination of KPLU, in the ill-advised sale to UW.  More information here and here.  This sale is marked by secrecy and disinformation by PLU and UW administrators.

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