Tuesday, November 17, 2015

A Very Stormy Tuesday!

Tuesday, November 17, 2015
10:34 am

Credit: Seattle National Weather Service Forecast Office

Things are really ramping up now throughout Western Washington. The all-important PDX-BLI gradient, which was forecast to max out at 14-18 hPa, is already at 18 hPa, and continues to slowly rise. There is a chance that the gradient could reach 19 hPa at some point today. Regardless of what it reaches, today will be the windiest day of the year so far for many people in Western Washington.

However, I want to make one thing clear: this windstorm will not be on par with the Columbus Day, Inauguration Day, or Hanukkah Eve storms, which are generally regarded as the three most intense storms to hit the Puget Sound region in the last 60 years. It will be stronger in most places than the August 29th windstorm that knocked out power to 500,000 people, but because the deciduous trees have lost many of their leaves, it will not be nearly as destructive. On the other hand, soils are saturated from the recent heavy rain, particularly soils in the Cascades and Olympics, making trees more prone to toppling under high winds. Either way, at least 100,000 people will lose power, but definitely not 1,000,000.

First, let's talk about rain. Moderate rain is falling over the lowlands, and the heavy rain is falling in the mountains. Just like the event this past weekend, there is relatively little rain over the Pacific, but once that moist air encounters some terrain, rain starts pouring down. You can also see a thin line of precipitation off the coast, which marks the location of a very strong cold front. More on that later.

Credit: WeatherOn

Here are some 24-hour totals from around the region. The area between central Whidbey Island and Seattle has been relatively shadowed from the rainfall over the past 24 hours, but as the radar image shows, Seattle is now mostly out of the rain shadow, which is mainly situated over Everett. The mountains, coast, and lowlands from Seattle south are getting the heaviest rain right now, with one gauge near Lake Quinalt on the Olympics picking up 6.40 inches! This is the same gauge that received 12 inches in 24 hours this past weekend, so it's no coincidence that it has received the heaviest rainfall thus far.

Credit: NWS

Due to all this rain, saturated soils, and the rivers already running high from last weekend, we currently have 8 flood warnings across the area. I've listed them below.

Major Flooding:
Skokomish River near Potlatch affecting Mason County

Moderate Flooding:
Snoqualmie River near Carnation affecting King County
Snoqualmie River near the Falls affecting King County
Skykomish River near Gold Bar affecting Snohomish County

Minor Flooding:
Stillaguamish River at Arlington affecting Snohomish County
Chehalis River at Porter affecting Grays Harbor County
Elwha River at McDonald Bridge affecting Clallam County
Grays River near Rosburg affecting Wahkiakum County

Now that we've got that covered, let's talk about the main story for today: wind.

It's now 11:26 at my house, and winds are really starting to pick up.

As you can see, winds are pretty gusty around Washington... we recently had a 55 mph gust at West Point in Magnolia... but nothing out of the ordinary. Most peak gusts in the lowlands thus far have been 30-50 mph. Sea-Tac has hit 40, and Everett (Paine Field) has hit 53.

Credit: NWS
Credit: NWS

I expect winds to really start ramping up by noon as the cold front associated with this storm draws closer. There will be higher pressure gradients and the atmosphere will be less stable, allowing for some of those gusty winds aloft to make it down to the surface.

520 Bridge is still open, but I think they will close soon... they are getting sustained winds of 39 mph with gusts to 47 as of 11:40. They need 15 minutes of frequent 50+ mph gusts to close.

Credit: WSDOT 
Credit: WSDOT

Winds will probably peak in the Puget Sound region around 1 pm as the front passes, but high winds will continue well into the evening for those along the Strait of Juan de Fuca, particularly near the eastern entrance by Fort Ebey on Whidbey Island. A storm warning is in effect for that region calling for 40-50 knot sustained winds coming from the west. Some of these winds will make it over to Mukilteo and Everett, so they will remain windy well after Seattle had quieted down, and these westerly winds could be stronger for some areas than the southerly winds being experienced now, before the frontal passage. The Seattle City Light "WINDWATCH" (created by the University of Washington) clearly shows a strong surge of 50-60 mph gusts behind the front coming through.

Credit: Seattle City Light

Expect these winds to be in full force around 4 pm and die out by 7 pm... earlier for less-exposed places.

Credit: Seattle City Light

One more point before I leave... the cold front with this storm is intense and has a large temperature and pressure gradient. We usually don't get intense fronts of any kind because of the moderating effects of the Pacific, but this front is worthy of special recognition. UW's ultra-high resolution 1.3 km WRF-GFS shows this well... note how the pressure drops immediately ahead of the front and sharply rises afterward. There should be strong winds and heavy rain with this feature for many areas. 

Valid 01:00 pm PST, Tue 17 Nov 2015 - 9hr Fcst
Credit: UW Atmospheric Sciences

It is even more visible on the radar! This is the part of the storm I am most excited to see.

Credit: WeatherOn

Stay safe, avoid large trees, and keep charging those devices, because many of you could be powerless by tonight. 

Enjoy the storm!

1 comment:

  1. Great post!!Thanks for sharing it with us..While searching I had come across one more site that also provides services of weather forecasts.It provides timely and accurate weather conditions & forecasts for airport at the best prices.Just have a look http://www.weather4cast.com/