B737-700 FLT CREW AT FL390 ENCOUNTERS SEVERE UP AND DOWN DRAFTS WHILE DEVIATING AROUND THUNDERSTORMS; AND DESCENDS TO FL370 WITH STICK SHAKER ACTIVATED.

Narrative

WHILE IN CLOUDS AND NAVING AROUND A TSTM; WITH THE AID OF THE WX RADAR AND STAYING OUTSIDE THE GREEN AREA; WE EXPERIENCED A LARGE INCREASE IN MACH NUMBER; FROM 0.76 TO 0.82 IN A MATTER OF SECONDS. I PULLED THE PWR BACK AND WATCHED THE WINDS ON THE MFD. AT THE SAME TIME; THE VERT SPD INCREASED FROM ZERO TO ABOUT 3000 FPM CLB. I PUSHED FORWARD ON THE YOKE TO MAINTAIN ALT. ABRUPTLY; THE AIRSPD DECREASED TO 0.72 MACH AND THE STICK SHAKER ACTUATED. I INCREASED PWR AND MAINTAINED FORWARD PRESSURE ON THE YOKE. THE ACFT STARTED DSNDING BUT WAS STILL BELOW AN AIRSPD WHERE I WAS ABLE TO APPLY BACK PRESSURE TO MAINTAIN ALT AND NOT STALL THE ACFT. THE CAPT REQUESTED FL370 SO WE COULD CONTINUE A DSCNT AND REGAIN A SAFE AIRSPD. ARTCC ASKED IF WE WERE DECLARING AN EMER AND WE REPLIED 'YES.' BY THIS TIME WE WERE IN VMC AND ARTCC CALLED OUT TFC AT 12 O'CLOCK GOING R TO L. WE ACKNOWLEDGED SEEING IT AND LEVELED THE ACFT AT FL370 AT A SAFE AIRSPACE. ALL THIS HAPPENED WITHIN A VERY SHORT TIME SPAN OF ABOUT 10 SECONDS. NAV FARTHER AWAY FROM TSTMS BY A MINIMUM OF 5 NM.