Deluge

Western Washington was hit very hard by thunderstorms on Friday, with some locations getting as much as 1.5-2.5 inches, while a few Cascade foothills locations were doused with 4-5 inches.  Much of Puget Sound enjoyed over an inch on Friday.  As we will see, even more amazing than the daily totals were the amounts falling in 15 minutes to a half hour.   Even though the ground was very dry, there still was some localized urban flooding, particularly near impenetrable surface like concrete and asphalt (see the picture below near Seattle's University Village).

On a more personal note, I almost met my maker yesterday.   I waited at the UW until the rain started to slow (around 6 PM) and began riding home along the tree-covered Burke Gilman trail.  All of a sudden there was deafening sound behind me (I was about 400 feet south of the location of the picture).   I stopped and could not believe what I saw---a large branch had fallen inches behind me.  If it had hit me, you would not be reading this blog right now.  A person, behind me by about 50 ft, rushed up and told me I missed being hit by the branch by less than a second.  Wow.  Why did it happen?   Loading of the leaves by rain?   The wind had picked up a bit.    Happy to be alive.

Back to precipitation.

On Friday AM a series of thunderstorms started moving in around 3 AM, first on the coast, forced by an approaching upper level trough.  As the trough moved over Puget Sound during the morning, steady and very heavy precipitation moved over most of western Washington.

Here is the NWS radar-based precipitation analysis (shading) with observed values shown as well, for yesterday.  The heaviest rainfall was in a swatch from Olympia to the north Cascades.

Yesterday's lightning strikes followed a similar path.

Yesterday's rainfall totals were stunning.   Here are a few samples:

..KING COUNTY...
1 W WHITE CENTER             2.10 IN   0757 AM 08/15   278 FT               
1 W WHITE CENTER             1.92 IN   0809 AM 08/15   131 FT               
LYNN LAKE                    1.80 IN   0600 AM 08/15   3901 FT              
2 SW ISSAQUAH                1.71 IN   0810 AM 08/15   1378 FT              
3 NE SEATTLE                 1.42 IN   0810 AM 08/15   275 FT               
WHITE CENTER                 1.41 IN   0808 AM 08/15   403 FT               
MEADOWS PASS                 1.40 IN   0600 AM 08/15   3500 FT              
REX RIVER                    1.40 IN   0600 AM 08/15   4000 FT              
6 SSE PALMER                 1.36 IN   0803 AM 08/15   4100 FT              
COUGAR MOUNTAIN              1.30 IN   0600 AM 08/15   3200 FT              
1 NE SEATTLE                 1.29 IN   0804 AM 08/15   26 FT                
3 NW WHITE CENTER            1.24 IN   0810 AM 08/15   305 FT               
2 NE FAIRWOOD                1.24 IN   0801 AM 08/15   584 FT               
1 SSW SHERIDAN BEACH         1.23 IN   0800 AM 08/15   213 FT               
SEATTLE TACOMA  ARPT         1.20 IN   0453 AM 08/15   370 FT               
4 SE VASHON ISLAND           1.18 IN   0700 AM 08/15   0 FT                 
SEATTLE 3.0 WNW              1.18 IN   0700 AM 08/15   0 FT            

..THURSTON COUNTY...
5 S LACEY                    3.37 IN   0809 AM 08/15   229 FT               
OLYMPIA 2.3 ESE              1.36 IN   0600 AM 08/15   0 FT                 
OLYMPIA 1.3 S                1.16 IN   0445 AM 08/15   0 FT                 
2 WSW TUMWATER               1.08 IN   0807 AM 08/15   177 FT 

..WHATCOM COUNTY...
21A07 - EASY PASS AM         4.70 IN   0600 AM 08/15   5269 FT              
WELLS CREEK                  4.60 IN   0600 AM 08/15   4200 FT              
ELBOW LAKE                   2.80 IN   0600 AM 08/15   3200 FT              
BROWN TOP                    2.20 IN   0600 AM 08/15   5830 FT              
MARTEN RIDGE                 2.00 IN   0600 AM 08/15   3560 FT              
BEAVER PASS                  1.90 IN   0600 AM 08/15   3620 FT  

At the UW we had some amazing short-term precipitation amounts:
 
Peak 5 min amount: 0.23 inches ending 13:58 PDT 
Peak 10 min amount: 0.38 inches ending 14:02 PDT 
Peak 15 min amount: 0.51 inches ending 14:07 PDT 

A half-inch in 15 minutes is very heavy rain around here. The most extreme report was at South Lake Union (Weather Underground KWASEATT4190

This station received 3.24 inches over the day and had 1.43 inches during the 15 minutes ending 14:16 PDT. Here is the picture of their site:

Could they have gotten water splashing off a nearby roof? Was there localized flooding around south Lake Union?

Most areas around Puget Sound, based on one day's rainfall, have exceeded their normal August rainfall.  For a number of sites, this one event has erased HALF of the precipitation deficit of the entire summer.   There was roughly an inch in the western Cascade foothills, not enough to have a major impact on the level of our reservoirs---so we all still need to conserve.  But water demands for irrigation should be very small during the next week, which helps a great deal.

Predictability

An extraordinary aspect of this event was its long-term predictability.  Numerical forecast models (like the UW WRF) were skillfully predicting this event nearly a week ahead of time (a fact noted in my earlier blogs).   Want proof?  Here is the observed 500 hPa height at 5 AM Friday and the forecast for 12 hr later. Strong trough moving through...that is what caused the thunderstorms.

Here is the forecast 156 hr earlier (from Saturday afternoon) for Friday 5 AM..  A nearly perfect forecast, except for being 12h too fast.  Amazing.

And the the 156h precipitation forecast showed lots of rain on Friday!  A really stunning example of how far we have come.

A reminder that for folks on the San Juan's I will be giving a talk on Lopez on August 19th and on Orcas on August 20th.

First, Lopez Island at 7 PM, August 19th on "The Future of Weather Prediction"  Details here.

And on Orcas Island at 5:30 PM August 20th at the Orcas Community Church in Eastsound on "Why is the Northwest so Warm?"