Narrative:

Level at filed cruise altitude of FL400 in constant light to intermittent moderate turbulence. There was a large storm system over south florida; the keys; and the eastern portions of the gulf of mexico. Miami center requested we climb to FL410 for a smoother ride; we agreed as we were right in the tops and the crz page in the FMC indicated a max altitude of FL410. We accepted a climb to FL410. Now with havana center; we had a build up along our route just to the north that we estimated topped FL430-FL450. Westerly winds exceeding 100 KTS at altitude. We requested a southwesterly heading from havana center to keep a safe distance from this cell. Approximately 20-30 miles east of this cell (and directly downwind) we experienced very abrupt moderate chop. Immediately the speed trend vector decreased very rapidly into the min safe range/lower fishhook. As the stick shaker activated the captain stated 'I have controls'. The airplane banked right approximately 30 degrees and back to the left approximately 20-30 degrees. I observed altitude deviation of up to +300 and -200 ft. As the now pilot not flying; I requested immediate descent to FL400 from havana center. Descent clearance was approved. We descended to FL400 with no further incident. The event occurred because we did not maintain an adequate buffet margin at altitude with the present weather conditions. When the captain took control he added thrust and hand flew the airplane attempting to maintain assigned alt and heading. When operating in the weather conditions that we were; a greater margin of safely operable speed range is required. I.e. [We should have been at] a lower altitude. We also had the option to go the long way around the storm cell to the north which would have been on the upwind side yet required a deviation of a great distance. This option would have been prudent.

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Original NASA ASRS Text

Title: B737-800 flight crew at FL410 reported encountering turbulence downwind of a thunderstorm causing the aircraft to roll sharply right and left and activating the stick shaker. The Captain assumed control; adding thrust; and the First Officer requested descent to FL400.

Narrative: Level at filed cruise altitude of FL400 in constant light to intermittent moderate turbulence. There was a large storm system over South Florida; the Keys; and the eastern portions of the Gulf of Mexico. Miami Center requested we climb to FL410 for a smoother ride; we agreed as we were right in the tops and the CRZ page in the FMC indicated a max altitude of FL410. We accepted a climb to FL410. Now with Havana Center; we had a build up along our route just to the north that we estimated topped FL430-FL450. Westerly winds exceeding 100 KTS at altitude. We requested a southwesterly heading from Havana Center to keep a safe distance from this cell. Approximately 20-30 miles east of this cell (and directly downwind) we experienced very abrupt moderate chop. Immediately the speed trend vector decreased very rapidly into the min safe range/lower fishhook. As the stick shaker activated the Captain stated 'I have controls'. The airplane banked right approximately 30 degrees and back to the left approximately 20-30 degrees. I observed altitude deviation of up to +300 and -200 FT. As the now pilot not flying; I requested immediate descent to FL400 from Havana Center. Descent clearance was approved. We descended to FL400 with no further incident. The event occurred because we did not maintain an adequate buffet margin at altitude with the present weather conditions. When the Captain took control he added thrust and hand flew the airplane attempting to maintain assigned alt and heading. When operating in the weather conditions that we were; a greater margin of safely operable speed range is required. I.e. [we should have been at] a lower altitude. We also had the option to go the long way around the storm cell to the north which would have been on the upwind side yet required a deviation of a great distance. This option would have been prudent.

Data retrieved from NASA's ASRS site as of July 2013 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.