First Major Blow of the Winter

I have gotten a number of complaints from Seattle weather lovers, expressing their frustration at the lack of big winds this fall.  What is a Seattle fall without a big windstorm and power outages?  Well FINALLY, we are going to get some wind, although not the kind of tree feller that cripples the region.

The forecast sea level pressure map for 10 PM tonight shows a fairly strong low pressure system offshore, with a lot of pressure variation (pressure gradient) over the coastal waters--which means strong winds there.

By tomorrow at 1 PM, the low moves into coastal British Columbia, while a strong trough of low pressure, projecting to the south of the low, is about to make landfall on the coast.  At this point, big pressure variations and strong winds are on the Oregon coast.

By 4 PM (0000 UTC) things are really cranking over southwest Washington and southern Puget Sound, as the trough moves to the northeast. Strong winds will be spreading over Puget Sound around that time.

Three hours later (7PM), a large north-south pressure gradient is over western Washington and winds will be sustained 15-25 mph, with higher gusts (30-50 mph).

Here is the forecast wind gusts from the City of Seattle WindWatch (which we developed here at the UW) at 4 PM.  Big gusts over the water (50-60 mph) and 40-50 mph over some land locations.

So can we trust the forecast shown above?  As discussed many times in this blog, to get insights into forecast uncertainty, we examine ensembles of MANY forecasts.  My group at the UW is running one of the largest and highest resolution ensembles in the U.S., and below is a plot of the highest predicted wind gusts over Seattle tomorrow from the high-resolution single forecast shown above, and a collection of ensemble members (these runs were started 4 PM last night).  

Some, but not all, of the simulations show a big acceleration of the winds after 7AM, with a peak around 10 AM.  Others have the big peaks later in the day.   So there is a lot of confidence about big winds tomorrow, but they might be delayed until  later into the afternoon.    The new ensemble will be available this evening.  The National Weather Service wind forecast last night was much less, but they updated their prediction today to include high wind warnings and watches (see graphic below).

Because the trees have not been tested by strong winds this season, expect branches to fly and some power outages.   You might make sure your flashlights have good batteries and your car isn't sitting under vulnerable tree.

from Cliff Mass Weather and Climate Blog https://ift.tt/2zZ2Eiq