The Terrifying Turbulent Flight of United 1676: Why did it happen?

 There is the potential for lowland snow over the weekend...will have a blog dealing with this issue later in the afternoon.  A LOT of uncertainty about it...cliff


 --------------------------------------------------------------------------------------------------
Announcement:  The Northwest Weather Workshop will be on Feb 28th/March 1st in Seattle.

The NW Weather Workshop is the big annual gathering of those interested in the weather of the Pacific Northwest and everyone is welcome.  For more information, including the agenda and registration information, please check out: https://www.atmos.washington.edu/pnww/
______________________________________________________


United Flight 1676, en route from Denver to Billings Montana, were cruising at 34,000 ft above Wyoming around 3 PM Mountain Standard Time (MST) Monday when the severe turbulence hit the plane.

Passenger heard a loud boom, the  Boeing 737 aircraft tilted to the right and plummeted downward for several seconds.   Unbelted passengers and crew hit the ceiling, a baby was pulled form a parent's arm, and several passengers were seriously injured, one critically.  No warning was given prior to the encounter and the plane landed in Billings less than a half hour later.

Why did this incident occur?  Is their any way to avoid such serious events?  Such are the topics to be explored in this blog.

According to FlightAware, the aircraft left Denver at 12:12 PM MST and landed in Billings at 1:25 PM MST.  The route crossed Wyoming from the SE to NW and no thunderstorms were evident on the radar (see image).

This flight track took the aircraft over the terrain of the Bighorn National Forecast and east of the more substantial terrain surrounding Yellowstone National Park (see graphic).

The severe turbulence hit the plane around 1 PM  MST (3 PM EST) when the aircraft was over northern Wyoming.  Around this period the plane experienced a sudden loss of speed and a drop in altitude (see plot from FlightAware below).

Several aircraft in the area on Monday afternoon reported moderate to severe turbulence:  here are some sample pilot reports documenting this fact:

GCC UUA /OV CZI280042/TM 1957/FL340/TP B737/TB SEV/RM SVR MTW ZDV
GCC UUA /OV CZI245027/TM 2036/FL380/TP B737/TB SEV 380-340/ZLC

TB SEV means severe turbulence. Altitudes between 34,000 and 38,000 ft.

Since there were no thunderstorms in the area, the number one candidate for producing this turbulence was high amplitude breaking mountain waves.

In a stable atmosphere (where the air want to return to its original level when pushed up or down), mountain waves often form when air is forced upwards by a terrain feature (see image)

High amplitude mountain waves occur when strong winds approach mountain accompanied by the proper changes of stability (related to temperature) and wind with height.  Such waves can amplify to the point that they break, producing severe to extreme turbulence (see figure below).  The turbulence from breaking mountain waves can be highly localized in altitude and position, with change of only a few thousand feet or a few miles, taking an aircraft from extreme turbulence to a smooth ride.  Turbulence can also be highly transient, developing or disappearing over a few minutes or an hour.

Let's check model forecasts for this time to see whether they predicted such mountain waves.  Here is the UW WRF forecast (12-km domain, 7hr forecast) for noon MST showing the winds and vertical motions (colors) at 500 hPa (around 18,000 ft).  You see the strongly varying colors over Wyoming?,with  alternating bands (red to blue) --those are couplets of upward and downward motion associated with high amplitude mountain waves.

And take a look at the 1 hr forecast for maximum updraft velocity  in the vertical air column for the same time from the NOAA/NWS HRRR model (3 km resolution).  You see the light blue band over north central Wyoming.   The HRRR had the mountains waves as well.

And now another tool: high-resolution MODIS satellite water vapor image provided the University of Wisconsin (CIMSS).  Where the air is descending there is less water vapor (dark purple), rising--more water vapor (lighter colors).  You see the striations near the Bighorm Mountains....that is probably the signature of the mountain waves.

The National Transportation Safety Board, the FAA, and United Airlines will undoubtedly be investigating this turbulence event in more detail, but it sure looks like the result of the plane heading into a breaking high-amplitude mountain wave.

As our forecast models continue to our improve and satellite imagery gets more sophisticated, perhaps one-day aircraft will preemptively avoid such locations.  If you really don't want to risk getting into such situations, by watching the models and cancelling flights on days with strong mountain wave activity you would escape most of them.  The lee (eastern) side of the Rockies is notorious for mountain waves, so avoid airports like Denver and Billings if you really dislike mountain wave turbulence.