Thursday, August 18, 2011

Strong Sound Breeze and Tragedy


Last night's 11 PM news and today's Seattle Times related a story about the tragic death of a young man near Seward Park who had been using a Yamaha personal water craft.  The ST article is found here.  In this article there was a brief mention of a sudden increase in wind and choppiness of Lake Washington.

Was there a meteorological origin to this terrible incident?  If so, could it have been predicted?

I think I caught a piece of whatever happened while bicycling home around 6:30 PM from the UW-- not far from NOAA Sand Point I was impressed by the strength of  the head wind out of the north.  It was really strong.  Now on warm sunny days a northerly wind often develops north of Seattle and is called the Sound Breeze.  But this time it came up fast and hard.

Probably the best observation for the boating incident was from the wind sensors at the central part of the Evergreen Floating Bridge.  As shown in the map below  the bridge was upwind of the accident site (X for the incident location and O for the wind sensor).  The winds were from the north so the Evergreen Point wind sensor was in the perfect location.


Here is the wind speed record on the Evergreen Bridge.  Wow.  Around 6 PM there was a very sudden ramp up of wind speed to over 20 mph from roughly 3-5 mph.  That wind, plus a good water fetch north of Seward Park meant rapid growth of waves.

Strong winds also developed down Puget Sound.

The high resolution computer models had a good idea that strong winds would push into the Strait of Juan of Fuca and down into Puget Sound.  Here is a forecast for 8 PM of surface winds from the UW's ultra high resolution model prediction (1.3 km grid spacing!):

Even the UW system did not have enough resolution to handle Lake Washington, which is too narrow to properly resolve.  But it was clear that strong northerlies would hit around dinner time (you can see all these graphics for every hour on my department web site).

But why were the winds so strong and why did they come up so suddenly on Wednesday?  I suspect the answer was the passage of an upper level disturbance that afternoon which really revved up the pressure differences between the coast and inland yesterday. Here is an upper level map showing you the heights of the 500hPa pressure surface at 5 PM--can you see the trough over us?  I think that is the culprit.

So we start with the normal Sound Breeze--essentially a large regional sea breeze.  As the western Washington interior heat up relative to the Pacific and Strait, westerly winds develop in the Strait each afternoon and then push southward into Puget Sound.  But this time the Sound Breeze was supercharged as the passing trough causes an enhanced east-west pressure difference that drove even stronger flow through the Strait and then down into Puget Sound.

My hope is that in a few years we can combine our observations and improved models with modern communications (e.g., smartphones and the like) to allow us to provide highly useful and timely warnings of such events.  We won't stop all such tragedies, but hopefully many injuries and deaths can be avoided.


5 comments:

Chris said...

In the case of this accident it's a good reminder to wear your pfd when on the water.

Robbo said...

Good call Chris.
I participated in the S&R efforts for this unfortunate individual and he indeed was not wearing a PFD nor did he have the "kill switch" lanyard attached between himself and the jet ski.
The winds that the good professor mentioned were indeed a factor but in this case, basic safety precautions could have saved a life.

Rob Jellinghaus said...

I was at work at about 6 pm on this day, and we actually had some power flickers (that took out our build lab), probably due to these unexpectedly large gusts flapping some power lines! So it was indeed a remarkable event. My sincere condolences to this man's unfortunate family.

Unknown said...

Robbo has an important reminder about a "kill switch" lanyard. The circling PWC could have killed someone else as well. We don't need Yamaha drone torpedoes zooming randomly among boats and swimmers.

SpruceGoose said...

Having seen the behavior of most of those who ride these "personal watercraft" over the years, I find it difficult to feel sorry for someone on one of them. If he hadn't been out there razzing around making noise and polluting, there would have been no problem.