The Real Story Behind Tuesday's Windstorm and Massive Power Outage

 If you read media reports, such as in the Seattle Times, one is told that the localized winds on Tuesday evening were the result of an unusually deep "bomb cyclone".   

Some of the more irresponsible outlets stated that this event was the result of climate change.

As described here, the truth is far more complex and nuanced.  

Reality Check

A major issue is that the intense cyclone was well offshore, with only modest local impacts from the storm's strong winds.

 Consider the highly accurate regional forecast of sea level pressure and near-surface winds (color shading) at 7PM, near the height of the wind event over western Washington.  The storm center was about 380 miles offshore!

The powerful winds directly connected to the low center were hundreds of miles out in the Pacific, with a band of strong winds associated with a front over the coastal waters of the region.

You will notice a strong pressure difference (gradient) across the mountains, with higher pressure to the east.  THAT pressure difference is very important since it helped drive the easterly (from the east) winds over the western slopes of the Cascades.

The low pressure offshore contributed to the pressure gradient across the Cascades, but so did unusually high pressure west of the Cascade crest, something shown by the pressure anomaly (difference from normal) map below.

                                       

This inland high pressure was associated with COLDER than normal air east of the Cascade crest (see temperature map at around 5000 ft below).

In short, the critical pressure gradient across the Cascades had two contributors:  low pressure offshore and high pressure inland.    Yes, the powerful offshore low contributed, but a much weaker low, closer to the coast, could have done the same thing.

The winds approaching the crest level of the Cascades were strong from the east/southeast, as shown by the weather map at 850 hPa (around 5500 ft).  The wind barb shows 35 kt from the east-southeast.  At this level, you can see the cold air (blue colors) banked up against the eastern slopes of the Cascades. 

Finally, we can view the simulated vertical structure of the air approaching the Cascades that night near Stampede Pass (see model vertical sounding below).  The x-axis is temperature and the y-axis is height.  On the right are the winds.   The red line is temperature and the blue line is dewpoint. 

Strong southeasterly winds were approaching the crest, with a stable layer (temperatures not falling much with height) near crest level.  And there is a reversal of the component of the wind towards the mountains aloft.   These are very favorable conditions to produce a lee side (in this case westside) windstorm.

And that is EXACTLY what happened.  The area west of Puget Sound experienced strong winds not directly associated with the "bomb" cyclone but from something else: a downslope wind event.

Below is an east-west vertical cross-section across the Olympics and Cascades from a very high-resolution model forecast at 7 PM, with winds shown by colors (purple is the strongest, followed by red).  There are strong winds along the coast, but a separate area of high wind speeds is found on the western side of the Cascades.  That is the downslope wind event that caused all the problems.    Look closely and you will notice a waveness in the wind strength associated with atmospheric gravity waves.

Examining the surface wind speed and direction at the same time for western Washington, you can see the strong winds (kt), indicated by orange and red colors.  Look closely and you can see some waviness and non-uniformity in the winds.

There was a lot of structure in the winds on Tuesday evening, with locations only a few miles away experiencing very different wind strengths.  The winds were also very gusty in nature.  At my home, there were gusts to around 60 mph one second and nearly calm the next.  These are characteristics of downslope wind events.

Below are the maximum wind gusts on Tuesday.  HUGE variation both locally and regionally.  Winds were less over the South Sound since this area was in the wind shadow of Mt. Rainier.   Lesser winds west of Puget Sound.

Yesterday I surveyed the damage around NE Seattle and was stunned by the contrasts.

At Mathew Beach Park, lines of huge trees were downed and a power pole was snapped off.

A half mile away, there was little damage.   A mile away at Magnuson Park, some areas were untouched, while nearby areas looked like a clear cut (see below).

This was an extreme local event and a lot of meteorological pieces had to come together to make it happen.  Much more than an intense low center almost 400 miles off our coast.

I worry that the media has not been accurate in describing the origin of this event and its great predictability.  "Bomb" headlines and other hype have replaced careful journalism.  Importantly, saving lives in the future relies on the truth about such events being accurately communicated by the media and other information sources.