THE RPTR AT FL390 EXPERIENCED INCREASING AIRSPD TO MACH BUFFET AND MODERATE TURB AND AN 800 FT CLB ALL WITH PWR REDUCED WHILE ATTEMPTING TO CROSS A BREAK IN A LINE OF TSTMS. THEN THE FLT STARTED LOSING AIRSPD TO 200 KTS WITH THE PWR AT MAX CONTINUOUS AND ACFT DSNDING.

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WE FLY FROM AEX DIRECT MSP; NAVING OVER AND AROUND SOME WX IN ARKANSAS. WX IS DEVELOPING IN NE KANSAS THAT WE CAN SEE ON RADAR AS WE CROSS THE MISSOURI BORDER. PROCEEDING N AT FL390 WE ARE WELL ABOVE THE TROPOPAUSE AND IN SMOOTH AIR. WX IN N MISSOURI IS STARTING TO DEVELOP INTO A LINE OF TSTMS. THE TOPS APPEAR TO BE AROUND FL350 AND THERE IS A GAP IN THE LINE APPROX 40 MI WIDE STRAIGHT AHEAD. I CHK THE GAP FOR WX BY TILTING THE RADAR UP AND DOWN; AND SEE NO RETURNS EXCEPT FOR GND CLUTTER. I AM EXPECTING A SMOOTH RIDE AS WE CROSS THIS LINE BECAUSE OF WHAT THE RADAR SHOWS AND OUR ALT; BUT I TURN THE SEAT BELT SIGN ON ANYWAY. AS I CROSS THE LINE THE AIRSPD INCREASES RAPIDLY AND THE MACH WARNING COMES ON AS MODERATE TURB STARTS. I CLICK OFF THE AUTOPLT AND AUTOTHROTTLES AND PULL BACK THE PWR. THE SPD IS STILL INCREASING AND I CAN FEEL MACH BUFFET; AND BEGIN TO CLB TO DECREASE AIRSPD. I AM NOW IN THIN CLOUDS GLOWING WITH ST ELMO'S FIRE AND INTERMITTENT LIGHT HAIL. STILL CLBING; IN AND OUT OF MACH BUFFET. ABOUT FL398 THE MACH BUFFET STOPS; THE TURB STOPS; THE AIRSPD IS BACK TO NORMAL (260 KTS) AND I START TO LEVEL OFF AND RETURN TO CRUISE PWR. THE CABIN AIR SMELLS OF OZONE. I AM NOW LOSING AIRSPD RAPIDLY; AND SEEM TO BE IN A DOWNDRAFT. WITH AIRSPD NOW BACK TO 200 KTS; I PUSH THE PWR TO MAX CTL AND PITCH DOWN TO GAIN AIRSPD. EVERYTHING IN THE CABIN IS BECOMING WEIGHTLESS. SOME OF THE FLT ATTENDANTS WHO WERE STANDING IN THE AISLES IN THE AFT OF THE CABIN ARE NOW FLOATING TO THE CEILING. I GET AIRSPD BACK AND RAISE THE PITCH VERY GENTLY TO BRING THE FLT ATTENDANTS BACK DOWN GENTLY; LEVELING OFF AT FL388. THE FO RPTS SEVERE TURB TO ATC. ATC ASKS IF WE WANT TO CHANGE ALT; AND WE REQUEST FL410. AS WE START TO CLB; THE AIR IS AGAIN CLR AND SMOOTH. SEVERAL FLT ATTENDANTS COME UP TO ASK WHAT HAPPENED. I TELL THEM SEVERE TURB; AND ASK IF ANYONE WAS HURT. THEY SAY EVERYBODY IS ALL RIGHT. THE SO GOES BACK TO CHK ON THE REST OF THEM AND RPTS NO INJURIES; ALTHOUGH SOME FLT ATTENDANTS IN THE BACK OF THE ACFT FLEW SEVERAL ROWS. FLT ATTENDANTS IN THE MID-CABIN WERE WEIGHTLESS BUT DID NOT LEAVE THEIR SEATS. THOSE SEATED IN THE FORWARD CABIN FLOATED OUT OF THEIR SEATS.