Friday, November 10, 2017

A Turbulent Landing at Sea-Tac Airport on Wednesday Evening

On Wednesday night, I was returning home from giving talks at the National Weather Center in Norman, Oklahoma.   I was on American Airlines Flight 1295 from Dallas-Fort Worth and was descending through roughly 20,000 ft over the Cascades (around 5 PM) when the flight started getting rough (the flight path is shown below, courtesy of the FlightAware website).   We descended to a few thousand feet just east of Sea-Tac (SEA) and turned north, the plane still shaking a bit. But by the time we got north of Boeing Field the flight smoothed out and remained smooth as we made the turn southward towards the airport.


And then, south of downtown, the plane hit turbulence again, and as we approached  the airport, the winds were tipping us up and down, and as we landed I was a bit worried a wing might hit the runway or that we would be blown off the runway.  I was slammed into my neighbor in the next seat (who was pretty much inebriated after drinking double shots during much of the flight).

The pattern of the turbulence was familiar to me and probably to most seasoned travelers into Sea Tac, and is one associated with strong easterly flow moving through and downstream of a gap in the Cascades, known as the Stampede Gap.

We can start by looking at the winds at Sea-Tac that night, displayed as a time-height cross section, with time increasing to the left and height increasing in the y-direction (heights given in pressure, 850 is about 5000 ft).  09/00 is 4 PM on Wednesday.   Very strong easterly flow (sustained winds of 30 knots) is found at low levels, centered around 950 hPa (about 1500 ft).

Such strong easterly flow tends to be turbulent, with large vertical wind shear at low levels, which contributes further to the turbulent flow.

Next, let's look at the pattern at the surface at around 5 PM (below), where easterly flow from Seattle southward is evident.

The UW WRF model did a nice job in simulating this flow, with a small timing error.  Here is the WRF high-res forecast for 7 PM (started 15 h before)....you can see the current of strong southeasterly flow coming in south of Seattle.



Why the strong easterly flow?  Because as a low center approached offshore, a strong east-west pressure difference formed over the Cascades, which accelerated air to the east from high to low pressure.


The air  preferentially moves westward through the lower areas of the Cascades, the largest being Stampede Gap, between Mt. Rainier and Snoqualmie Pass (see topo map).   My plane descended into the strong easterly flow in Stampede Gap, which produces a lot of turbulence wave-like activity as it interacts with the terrain.



Finally, pilots often report interesting weather while in flight using PIREPS (pilot reports).    Turbulence extended to Boeing Field, where a pilot reported low-level wind shear (LLWS) from the surface to 900 ft at 5:26 PM

BFI UUA /OV BFI/TM 0126/FL009/TP C210/RM LLWS +/- 12 KT 009-SFC DURGD RWY 14R BFI

And turbulence (LGT/MOD CHOP) was experienced by a pilot landing at SEA TAC.

SEA UA /OV KSEA/TM 0030/FL180/TP B739/TB OCNL LGT CHOP 180-360/RM DURGD SEA GLASR1. LGT/MOD CHOP ON FINAL RWY 16C. ASA029. AWC-WEB/










5 comments:

Jim Terry said...

Cliff,

I live 3 miles NNE of Monroe - a place that gets battered by the gap winds, even when it's calm down in Monroe on the valley floor. When I came home, there were trees down and power outages in the fairly localized area North of Hwy 2 from just West of Sultan to roughly Chain Lake Road. I don't hear it mentioned often by TV meteorologists, but we do get some rough gap winds at times. Wednesday night was a perfect example. Apparently a lot of the worst winds came between 6 and 7:30 p.m., as that was when several outages happened, as per SnoPUD's new outage monitor website. Just thought you might find that interesting.

Doug Wise said...

Great report Cliff and comment Jim Terry!

Carl Dinse said...

About 9 years ago lived in the Big Bend community east of Gold Bar right between US2 and the Skykomish River (across from Zeke's Drive In). 2 miles or so east from Gold Bar city limits I think. East gap winds there were common whenever a warm front from a storm was approaching, gusts to 40mph were the norm in the winter. Never during the time I lived there were these winds mentioned by local weather reports, even National Weather Service forecasts for Gold Bar never mentioned these winds at the time. I have noticed in the past 2-3 years that local forecasting has started to include these winds with some forecasts. However I know they're not forecasting these as often as they occur in those areas.

C.P.O. said...

I also landed Wednesday evening and was wondering what was going on. Thanks for the post!

Janet.Peach said...

The turbulence over the Cascades can be really, really bad on approach to SeaTac. I’ve been flying all my life but I pretty much thought I was going to die on an Alaska flight from Atlanta GA last March.