A WDB AIRCREW HAD A LOW SPD BUFFET THAT CAUSED THEM TO LOSE ALT. THE LOW SPD WAS CAUSED BY A DOWNDRAFT.

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FLT WAS LEVEL AT FL370 IN SMOOTH AIR. WX RADAR WAS ON WITH APPROX 2/3 DEG DOWN TILT. POS WAS APPROX 1 HR AND 35 MIN ENRTE BTWN CUTEM FIX AND N42 E 160. MAX ALT PER FMC WAS FL380. AT THAT TIME; WE ENCOUNTERED SEVERE UPDRAFT FOLLOWED IMMEDIATELY BY SEVERE DOWNDRAFT WITH ASSOCIATED STICKSHAKER AND BUFFET (15 KTS BELOW VS). L WING DROPPED AND AUTOPLT DISCONNECTED. IMMEDIATELY DSNDED TO 36500 WITH A L TURN OFF COURSE N TO PARALLEL COURSE BY 10 MI AT MACH .82. FLEW AT 36500/.82 FOR APPROX 10 MINS UNTIL RIDE SMOOTHED OUT. ATTEMPTED RADIO CONTACT ON HF PRIMARY AND SECONDARY WITH TOKYO RADIO. NO CONTACT. OBSERVATIONS: 1000 FT BELOW MAX ACFT ALT PER FMC COMPUTATIONS IS AN INADEQUATE BUFFER FOR SEVERE TURB. POTENTIAL FOR CLR AIR TURB EXISTS UNDER ALL CIRCUMSTANCES AND WDB SHOULD BE FLOWN AT OR BELOW OPTIMUM ACFT ALT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTING FO STATES THAT THIS INCIDENT FELT JUST LIKE A MOUNTAIN WAVE ENCOUNTER. THE AIRSPD INCREASED TO THE HIGH SPD WARNING; THEN DECREASED TO A LOW SPD BUFFET. SINCE THIS INCIDENT; THE AIRLINE COMPANY HAS LOWERED THE LOW SPD WARNING FROM 1.3 TO 1.4 MACH NUMBER. THIS PROC LOWERS THE MAX ALT THAT THE ACFT IS ALLOWED TO FLY AND INCREASES THE MARGIN BTWN THE HIGH AND LOW SPD WARNINGS AT MAX ALT. THERE WERE NO INJURIES OR DAMAGE. SUPPLEMENTAL INFO FROM ACN 224193: NOTHING ON RADAR -- PICKED UP CHOP THEN ST ELMO'S FIRE -- THEN TURB. STARTED TO GET AIRSPD EXCURSIONS; OVERSPD; THEN UNDERSPEED. SPD DROPPED TO 15 KTS BELOW YELLOW FT (1.3 VS) ON PFD. NOW THE COMPANY IS TELLING ITS PLTS TO NOT GO ABOVE OPTIMUM ALT.