You won’t want to land at Seattle-Tacoma Airport between 2 and 4 a.m. Tuesday. (observation shown below)
At 1:53 a.m., the wind blew from the south at 9 knots (10 mph).
At 2:42 a.m. the wind turned north-northeast with gusts of 37 knots
Arriving 9 minutes late (2 hours 51 minutes) the wind turned south again, gusting up to 37 knots.
At 3:43 a.m. the wind turned northeast at 27 knots
At a terminal near the airport, winds gusted up to 63 mph.
An aircraft landing or taking off can experience dangerous headwinds and loss of lift. Fortunately, this is the quietest part of the day at the airport.
Strong winds caused power outages for hundreds of electricity customers and some trees toppled.
Street view on Tuesday morning near SeaTac, provided by Michael Snyder
These gusts were highly localized near SeaTac (see Tuesday morning’s strongest gusts below)
The winds are not only spatially localized but are also short-lived, as indicated by the wind graph near SeaTac below (second panel). Sudden sustained wind speeds (blue line) up to about 40 mph and gusts (orange) up to about 60 mph. The strong winds only lasted for about 15 minutes.
But that’s not the end of the weather weirdness. Look at the pressure! Suddenly jump up and then jump down! (see below)
Local weather enthusiast Michael Snyder lives near the airport and his barometer shows high amplitude fluctuations (about 3 hPa up and down) (see below)
One last observation. Local weather radar indicates a weak stream of showers, coming to an end during this time (radar image at 2:09 a.m. shown below). Not impressive but shows an ongoing upward movement.
The origin of this mysterious feature
This event is a good example of a high amplitude atmospheric gravity waves.
Waves that form on the interface between the air have two different densities.
You are all gravitational wave experts…. just look at the waves on the ocean’s surface (see below)
Oceanic gravitational waves form at the interface of a dense liquid (water) and a less dense liquid (air). When some water is displaced upwards, gravity pulls it down, resulting in wave-like motion.
On the morning of Tuesday, May 2, the atmosphere above Puget Sound developed a “two-fluid” structure. With cool dense air below and warmer, less dense air above.
How did this happen? A low center moves west to our south, resulting in an easterly (from the east) current at high altitude carrying a stream of warm air over Puget Sound (see map below). At the same time, colder, denser air appears at the low end.
Between the two of you there was a reverse….a stable layer in which the temperature warms with altitude. A situation, unlike the interface between water and air above.
Altitude 850 hPa (about 5000 ft), wind and temperature
(shaded, warmer red) at 2 a.m. PDT Tuesday morning
The low moving to the west created an impulse to create a wave on the interface between the cold air at the lower level and the warmer air above. It may have done this by generating some of the stronger thunderstorms east of the Cascade Peak that interacted with the reversal to create strong waves. In fact, there was lightning and relatively high peaks associated with thunderstorms approaching the Cascades around 1 a.m. (see lightning bolts below from 12:30-1 a.m.)
The wave is not large in scale (about 100 miles) and is moving northwest. Large differences in pressure with waves lead to narrow regions of strong winds (winds produced by differences in pressure).
To illustrate, here’s a map of the pressure and wind observed around 3 a.m. (most observations in this map were made around 2:50 a.m.). You can see the bottom (or low pressure part) of the wave (the pressure is about 1007 hPa), which I have shown with a red dashed line.
The north wind is low pressure in the north, the south wind in the north is low pressure.
The best thing about this whole event is the event PRODUCTION capabilities of the high resolution UW model. I still can’t believe it.
The predicted pressure model for 3 a.m. clearly shows the wave
And the forecast wind (bottom panel) and pressure (bottom panel, orange line) at Seattle-Tacoma Airport clearly predict this event. We’ve come a very, very long way in high-resolution prediction.
Finally, let me note that this event, although rare, is not unique. For example, a similar event occurred in September 1990 and was reported in an article by a national weather forecaster (Bruce Bauck). This week’s event is one of the strongest in spring.
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The program and information of the Northwest Weather Conference is online. This meeting, which will take place May 12-13 in Seattle, is the key weather meeting of the year in the Northwest. We have a diverse and exciting agenda. The meeting is open to everyone and if you want to attend you must register (on the website).
We’ll also be hosting a party/chat at Ivar’s Salmon House on Friday, May 12. It’s a fun and hybrid (live and zoom) meetup.
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