MD-11
Flight Crew Air Transport Pilot (ATP)
Distraction; Workload
Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Weather / Turbulence
Flight Crew Returned To Clearance
Weather
We were deviating right of course (to the west of course) to avoid a thunderstorm that [was] shown on our radar.  We entered a layer of high cirrus clouds and were IMC with light precipitation (patchy green return) showing on our radar.  With the autopilot engaged we encountered moderate to severe turbulence when suddenly the airspeed became uncontrollable and the aircraft entered the low speed stick shaker.  I disconnected the autopilot and lowered the nose in an attempt to regain airspeed.  The aircraft remained in the low speed stick shaker for 10-20 seconds as we attempted recovery.  When sufficient airspeed was regained we made a controlled climb back to assigned altitude.  The total altitude loss was approximately 700 feet.  ATC was notified of the altitude deviation and the circumstances involved at that time.  ATC told us that they had noticed the altitude deviation; that there were no other aircraft in the vicinity; and there was no loss of separation.  This was an inadvertent encounter with severe turbulence/thunderstorm that did not show on the radar.
A MD-11 encountered moderate to severe turbulence in light cirrus clouds and with a low speed stall warning descended 700 feet then returned to their cleared altitude. ATC was notified.