Wednesday, March 9, 2016

Huge Gas Explosion Creates Pressure Wave over the Northwest PLUS a Powerful Storm Hits Our Coast

Around 1:45 AM today, a huge gas explosion destroyed a large building in the Greenwood neighborhood of north Seattle (see picture, courtesy of KOMO TV)

Folks throughout north Seattle and Shoreline was awakened from their sleep, some checking their homes for damage.  Heavy damage hit buildings near the explosion, including broken windows.

Large explosions produce very load sounds, which in reality are powerful pressures waves in the atmosphere.  Sound, as you learned in middle school, is associated with propagating pressure waves, with alternating areas of compression and rarefaction (see image).  A large explosion can produce an intense, rapidly increase zone of pressure called a shock wave that propagates away from the explosion.

After I heard about the explosion, I check the local barometers (microbarographs) used by meteorologists.   A microbarograph records pressure variations over time.   Here is the record from my department (the explosion happened about 0945 UTC).   Look at the first panel that says SLP.   Nothing, but that is not surprising considering the small amplitude of the expected signal and the long averaging time of most weather-related pressure sensors.

But then I got a message from Professor Steve Malone of UW's Earth and Space Sciences Department.  Steve is an expert in seismology and it turns out that many seismographs are paired with very accurate barometers.   Eureka!   A number of his microbarographs recorded the pressure signal, even ones 10s of kilometers away.   Here is a sample from a unit about 50 km distant from the explosion.  A pulse of roughly .17 Pascals (Pa) around 0948 UTC (1:48 AM), with some aftershocks/vibrations for about a minute after.
.17 Pascals (Pa) is a very small signal.    Average sea level pressure is about 1013 Hectopascal (hPa).   Hecto mean 100.   So average sea level pressure is about 101300 Pa.  Normal pressure variations are on the order of several hPa over a few hours...thus, the signal above would be in the noise level.    Of course, the signal would have been greater in neighborhood of the explosion.

You want a big pressure signal?  A very deep low center is now moving up our coast and will cross Vancouver Is. tomorrow morning.

At 10 PM this evening the WRF model shows a very deep low (972 hPa) off northern Oregon with a HUGE pressure gradient on he south and west sides.

 By 7 AM (1500 UTC), the low crosses northern Vancouver Island, with a very strong pressure gradient over the northern WA coast and northwest WA. Winds will gust to 70-90 mph along the coast and 30-60 mph over northwest WA. Expect power outages.

As of 9 PM, winds are gusting to 90 mph along exposed hills along the WA and southwestern WA coast, with 104 mph gusts at Naselle Ridge near Astoria, with a 97 mph nearby (see map  of max gusts over the past 24 hr)

And yes, lots of rain tonight.

Announcement: Public Talk: Weather Forecasting: From Superstition to Supercomputers

I will be giving a talk on March 16th at 7:30 PM in Kane Hall on the UW campus on the history, science, and technology of weather forecasting as a fundraiser for KPLU. I will give you an insider's view of the amazing story of of weather forecasting's evolution from folk wisdom to a quantitative science using supercomputers. General admission tickets are $25.00, with higher priced reserved seating and VIP tickets (including dinner) available. If you are interested in purchasing tickets, you can sign up here


seussnelson said...

I think it's interesting to note the shock wave produced by an explosion such as this actually travels supersonically. If it seemed like you felt the explosion last night before you heard it that was not your imagination.

The Drennans said...

Indeed. Shockwave. My bed shook, waking me up at 1:45, after which, I heard the boom. I suspected a small earthquake. I quickly checked USGS for earthquake reports. Seeing none, I was a little confused. I knew something happened. But forgot about it until tonight, when I saw this on Cliff's blog. Thanks for the info-- it all makes sense now.

The Drennans said...

oh yea, woke me up at 5.5 miles north, in north central Shoreline.

audioskeptic said...

It is interesting to relate this to sound pressure levels, for which 0dB SPL is defined as 0.00002 pascals.

That means that your .17 pascals is about 20*log10(.17/.00002) or `79dB SPL, which is average speech level if it were in the same frequency range.

Given that it's very low frequency, it is still, at that point, very nearly at the absolute threshold of hearing. has a set of converters from SPL to pascals, etc.

This goes to show a couple of things, one of them the sensitivity of the undamaged human ear, which is 0.00001 pascals, give or take at the ear canal resonance, and also how small in terms of SPL a very loud sound is.

I'll stop lecturing now. Hope this was interesting.

Sabrina said...

It shook the picture on my wall and sounded like thunder or an earthquake up in Lynnwood as well. I also checked USGS for seismic activity. Just wanted to chime in to say it was heard north of Shoreline, even (others heard it in my area as well).

schipknit said...

Western Skagit county is really windy this morning. Fences and power down. An RV trailer blew over on westbound Hwy 20 just off I-5, blocking traffic.

Christina Wilsdon said...

Some folks up here in Woodinville say they felt something too, such as a front door suddenly making a noise as if opening, a dog suddenly barking its head off at that time, and the like.

Chris Curtin said...

Cliff, Can you post a link to a graph or table showing the sea level change Vs atmospheric pressure? I was surprised this morning to see the high tide much higher than the predicted graph for our area. Thanks, Chris

Zathras said...

Chris, here are three links to get you started

Brian Blackmore said...

Cliff, now's as good a time as any to ask: How do the models calculate wind gusts, and how were wind gusts historically predicted? Is it "simply" a matter of including turbulence in the models to predict statistical energy factors multiplied against the 'constant' wind speed from the pressure/temperature gradients, or is there something more basic (or complicated) in use? Did they have gust predictions in 1940 and, if so, was it anything more than "Oh the last time it looked like this the anemometer blew away"?

Happy to ready any previous blog entries if this has already been answered. I tried simple Internet searching, but must admit I haven't spent hours looking for an answer.

iamlucky13 said...

"I think it's interesting to note the shock wave produced by an explosion such as this actually travels supersonically. If it seemed like you felt the explosion last night before you heard it that was not your imagination. "

It's rare, to the best of my knowledge, for natural-gas + air mixtures to actually detonate - that is, propagate supersonically. Even when such a mixture does, the resulting shock wave usually slows to subsonic in a fairly short distance, so the possible time lag is very short.

Perhaps you're right, but I suspect the more likely reason some felt the explosion before they heard it is that lower frequencies, especially sub-audibles, are generally less attenuated by interactions with objects they encounter near the ground (like buildings and trees) - just like the bass from loud music is easier to hear in another room than the higher frequencies. In such case, the lower frequencies might make it to you by the shortest path possible and arrive first, but the higher frequencies might only make it to you by longer paths, such as by echoing off of thermoclines in the atmosphere.