What Knocked Down 24 City Light Power Poles Near Boeing Field?

Something extraordinary happened Friday night just before 4 PM in south Seattle near  the Museum of Flight at Boeing Field.

A line of TWENTY FOUR power poles fell on East Marginal Way near the museum, closing the road, taking out power to tens of thousands of people, and striking a car (miraculously no one was killed).  Initial reports were that a tree had hit a power pole--in fact, the opposite occurred.

The map below shows the location of the museum and fallen power poles----keep the exact location in mind.

So what happened yesterday afternoon--why a catastrophic failures of two dozen poles in a row?  

Let's do the meteorological detective work.   

A well-defined front was moving through the region at the time, which is clear from a weather satellite image at 2 PM (see below).  Rain and blustery winds were in the forecast.

2 PM Satellite Image

Embedded within the front there was a narrow band of convection--a line of cumulonimbus clouds with heavier rain in which the winds were accelerated.  

To illustrate this, here are two radar images form the National Weather Service Camano Island radar at 3:45 PM and 3:54 PM on Friday afternoon.  The oranges and reds indicate heavier precipitation.  This line went through exactly when the power poles started to fall.  I suspect this is not a coincidence.

There is an extremely good weather observation location at the north side of Boeing Field, about 1.5 miles from the incident site.  The observations there indicate the heavy rain from the line and wind gusts to 29 knots---which is 33 mph.   Winds were southerly.



There are no weather observing sites immediately near the Museum of Flight, but there are some amateur sites with a few miles.  They also show winds gusts to around the same level as this line went through.  They also show sudden temperature declines and a sharp spikes in pressure (see below--look around 4 pm), consistent with a narrow cold frontal band in which frontal characteristics are concentrated.

The coincidence of the passage of this band with the failure of the power poles can hardly be an accident.   But the winds at nearby sites don't seem strong enough to take down properly maintained poles.   To me there are two possibilities:

1.   There was one pole (or several) that were rotted or weakened.  It failed in the moderate winds and took down the adjacent lines though the interconnected power lines.

2.   That the winds were stronger in the location of the Museum of Flight.

How could (2) happen? 

 On possibility is some kind of localized microburst, but that does not seem probable considering the modest heights of the convection and lack of dry air near the surface (which helps rev up the descending air from evaporative cooling).

Perhaps a more promising mechanism would be interaction with terrain. Boeing Field is in a valley with a sharp rise up to West Seattle to the south-southwest of East Marginal Way (see below).   Could the interaction of the strong outflow winds from the convective line with the topographic feature accelerated the winds?  It would take some serious numerical simulation to test this hypothesis.

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NORTHWEST WEATHER WORKSHOP COMING SOON

Reminder:  The big local meeting for those interested in Northwest Weather is coming up:  the Northwest Weather Workshop will take place in Seattle on May 3-4th.   Everyone is invited but you have to register.   More information, registration details, and the draft agenda are found at https://atmos.washington.edu/pnww/.  A few presentation slots are open, so if anyone in the meteorological, fire weather, or associated communities would like to give a talk, let me know immediately.



from Cliff Mass Weather and Climate Blog http://bit.ly/2WSeiEW