Narrative:

We were in cruise at FL370 when we heard [another aircraft] report sudden severe turbulence from mountain wave just ahead and below us at FL310. As a precaution we immediately turned on the seat belt sign and advised the flight attendants to take their jumpseats. We slowed our speed and questioned ATC about rides at FL370. ATC told us no other reports in the area and no one at FL370 ahead of us. We called the lead flight attended and verified everyone was seated and explained there was a chance of sudden turbulence. The captain and I discussed using the spoilers in the event of an over speed. Suddenly the speed increased more than 35 knots; spoilers fully deployed at first sign of speed increase as it was evident they would be needed. We had at least a momentary 10 knot over speed and 200 ft climb with the autopilot on. I checked the TCAS and did not see any traffic above or below us. As the speed dissipated I stowed the spoilers and told the captain to advise ATC we would are vacating FL370. I turned off the autopilot as the speed rapidly dropped; spoilers stowed as soon as we were out of the overspeed. The speed dropped suddenly requiring me to descend to FL360 to maintain a safe flying airspeed with full throttle and spoilers stowed. ATC replied with 'do whatever you need to'. We kept ATC advised and requested a block altitude of FL350-FL370.no loss of separation occurred as far as I know. No one was injured; in fact it was mostly smooth throughout the event but with sudden pitch; power; spoiler changes I'm certain it was uncomfortable in the back. The flight attendants were appreciative of the warning we gave them. This event fit the fom (flight operations manual) description of severe turbulence based on the sudden airspeed fluctuation and altitude deviation even though it wasn't 'turbulent' or 'bumpy' during the event.

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Original NASA ASRS Text

Title: B777 flight crew reported encountering severe mountain wave turbulence.

Narrative: We were in cruise at FL370 when we heard [another aircraft] report sudden severe turbulence from mountain wave just ahead and below us at FL310. As a precaution we immediately turned on the seat belt sign and advised the flight attendants to take their jumpseats. We slowed our speed and questioned ATC about rides at FL370. ATC told us no other reports in the area and no one at FL370 ahead of us. We called the lead flight attended and verified everyone was seated and explained there was a chance of sudden turbulence. The Captain and I discussed using the spoilers in the event of an over speed. Suddenly the speed increased more than 35 knots; spoilers fully deployed at first sign of speed increase as it was evident they would be needed. We had at least a momentary 10 knot over speed and 200 ft climb with the autopilot on. I checked the TCAS and did not see any traffic above or below us. As the speed dissipated I stowed the spoilers and told the Captain to advise ATC we would are vacating FL370. I turned off the autopilot as the speed rapidly dropped; spoilers stowed as soon as we were out of the overspeed. The speed dropped suddenly requiring me to descend to FL360 to maintain a safe flying airspeed with full throttle and spoilers stowed. ATC replied with 'do whatever you need to'. We kept ATC advised and requested a block altitude of FL350-FL370.No loss of separation occurred as far as I know. No one was injured; in fact it was mostly smooth throughout the event but with sudden pitch; power; spoiler changes I'm certain it was uncomfortable in the back. The flight attendants were appreciative of the warning we gave them. This event fit the FOM (Flight Operations Manual) description of severe turbulence based on the sudden airspeed fluctuation and altitude deviation even though it wasn't 'turbulent' or 'bumpy' during the event.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.