B747-400
Flight Crew Air Transport Pilot (ATP)
Deviation - Speed All Types; Inflight Event / Encounter Weather / Turbulence
Flight Crew Took Evasive Action
Weather
Conditions - Night time conditions; clear on top. Minimal radar returns; widely scattered level 1 approximately 50-100 nm ahead of route. Otherwise VMC on top at FL390. Event - [The] aircraft unexpectedly went into light IFR conditions with a noticeable jolt of moderate turbulence. My pilot monitoring and I turned on the external lights to confirm we were entering IFR conditions. St. Elmo's fire began to quickly build on the aircraft; and I began a 45 degree turn (not bank; but heading) to the left where clear air could be seen visually with starlight to the left of the aircraft. The turn was initiated approximately 10 seconds after first encounter with IFR conditions; and continuous ignition was armed. There were no radar or visual cues present of impending convective activity ahead. My pilot monitoring simultaneously called Yangon to notify them of our course deviation. The Captain; who had just vacated the flight deck for rest approximately 3-5 minutes prior; returned to the flight deck to observe. Turbulence worsened for the next 20 seconds with considerably updrafts that gave almost instantaneous increases in performance; with several increases in airspeed upwards of 15 knots. Power was reduced twice to keep the aircraft from exceeding Mmo limit.  The airspeed was never observed to go below the bugged speed of .84 Mach; and we were attempting to input a speed of .82 Mach to slow to maneuvering speed by the time we cleared the turbulence. On one occasion; after a considerable updraft; a substantial and almost instantaneous downdraft followed; causing the red stall margin indicator to shoot right up to the bugged speed; causing a stick shaker; lasting approximately 1/2 second. By the time we made an effort to add power; the stall margin indicator had shot back down to its normal range; approximately 30 plus knots below our bugged (and flying) speed. The total duration of the encounter was about 45 seconds from the first indication of turbulence; with the worst lasting about 15 seconds; which included moderate turbulence with very heavy updraft and downdraft activity. The aircraft's altitude; however remained constant; with negligible deviations. Weather was clear ahead; with no moon; but various TS activity along the route. The flight proceeded normally after without further event. Perhaps having more radar products available for that part of Asia prior to departure [would be useful]. Also; during monsoon weather season performing periodic radar scans at lower than normal angles to search for potential building cells; not easily seen with a normal look ahead scan.
B747-400 First Officer reported large speed deviations at FL390 associated with storm cells in the vicinity of Tibet.