A LOSS OF AIRSPD CREATES A PARTIAL LOSS OF ACFT CTL AND SUBSEQUENT LOSS OF ALT DURING AN INFLT ENCOUNTER WITH WX ENRTE AT FL330 NEAR FMN; NM.

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THE CAPT AND I DEPARTED FOR DFW. ABOUT 1 HR INTO THE FLT; WHILE IN AND OUT OF CLOUDS AND DEVIATING AROUND WX; A LOUD RUMBLING SOUND STARTED UP; SEEMINGLY OUT OF NOWHERE. WE HAD JUST RECENTLY RETURNED TO VFR CONDITIONS AND WERE WAITING FOR THE TAIL DEICE TO CYCLE BEFORE TURNING OFF THE DEICING AND ANTI-ICING FOR THE AIRFRAME AND ENGS. THE RUMBLING SOUND SEEMED TO START AT THE BACK OF THE PLANE AND MOVE FORWARD; ENVELOPING THE PLANE LIKE A GIANT WAVE BREAKING OVER A SMALL BOAT. AS THE SOUND REACHED THE COCKPIT; THE AIRPLANE BEGAN TO BUFFET; LIGHTLY AT FIRST; THEN STRONGER. AT FIRST THE CAPT AND I THOUGHT WE HAD A STRUCTURAL PROB; BUT THEN WE NOTICED THE AIRPLANE WAS STRUGGLING TO MAINTAIN ALT. I DISENGAGED THE AUTOPLT AND AUTOTHROTTLES AND LOWERED THE NOSE WHILE THE CAPT DECLARED AN EMER WITH ATC AND ASKED FOR THE CLOSEST SUITABLE ARPT. IT WAS NOT UNTIL WE WERE IN THE DSCNT THAT I REALIZED WE MAY HAD NEARLY APCHED A STALL SIT. DURING THE DSCNT FROM FL330 TO FL290; THE AIRPLANE WENT THROUGH A SERIES OF BUFFETS FROM LIGHT TO STRONGER WITH BREAKS IN BTWN; EACH BREAK LEADING ME TO BELIEVE THE PROB WAS SOLVED. AT ABOUT FL300; THE CAPT ASKED TO TAKE CTL OF THE PLANE. I RELINQUISHED CTL AND HE REQUESTED TO LEVEL OFF AT FL290. AFTER LEVELOFF; THE CAPT RETURNED CTL OF THE AIRPLANE TO ME AND AFTER CHKING THE TRIM; I RE-ENGAGED THE AUTOTHROTTLES AND AUTOPLT. THE AIRPLANE FLEW NORMALLY. AT THE TIME OF THE EVENT; THE AIRPLANE WT INDICATED APPROX 129000 LBS AND PERFORMANCE MODE ENGAGED. I HAD INITIALLY TRIMMED THE AIRPLANE UPON REACHING FL330 AT .77 MACH. ALL WAS NORMAL. AT THE TIME PRIOR TO THE EVENT; WE WERE INVOLVED IN WX AVOIDANCE. HOWEVER; I DO NOT BELIEVE WE WERE SO FIXATED ON THE WX THAT WE MISSED ANY LARGE CLUES THAT SET THE STAGE FOR THIS EVENT. WHATEVER CAUSED THE AIRPLANE TO LOSE AIRSPD; SUCH AS MOUNTAIN WAVE OR A SEVERE DOWNDRAFT; CAME ON IN AN INSIDIOUS MANNER. NEITHER THE CAPT NOR I BECAME AWARE OF ANY TRIM OR THROTTLE MOVEMENT THAT WOULD HAVE DIRECTED OUR ATTN TO THE AIRSPD INDICATOR. AS AN EX-ACR Y PLT; I HAD TO PERFORM ALL TYPES OF STALLS AT THE BEGINNING OF EVERY RECURRENT SIMULATOR TRAINING SESSION. WHAT I EXPERIENCED IN THE AIRPLANE WAS NOTHING LIKE THE STALL TRAINING EXPERIENCES I HAD IN THE SIMULATOR. I BELIEVE THAT IS THE REASON I DID NOT IMMEDIATELY REACT AS IF THIS WAS A STALL RECOVERY SIT.