FLC OF AN ACR MLG LOST CTL DURING MOUNTAIN WAVE TURB RESULTING IN AN ALT EXCURSION FROM ASSIGNED.

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I WAS THE FO AND THE PNF OF THE FLT MENTIONED ABOVE. WE WERE AT THE BEGINNING OF THE POWDR 2 ARR GATE FOR DEN ARPT; ASSIGNED FL250. AT APPROX 130 NM FROM THE DVV VOR (114.70) WE ENCOUNTERED SEVERE DOWN- AND UPDRAFTS THAT MADE IT IMPOSSIBLE TO MAINTAIN THE ASSIGNED ALT. WE ENDED UP DEVIATING AS LOW AS FL240 AND AS HIGH AS FL255. VERT MOVEMENT WAS BOTH UNEXPECTED AND RAPID. AS WE APCHED THE AREA DESCRIBED ABOVE; WE INFORMED THE PAX THAT WE WOULD BE ARRIVING IN DENVER SOON AND ASKED THAT THEY RETURN TO THEIR SEATS AND BUCKLE UP FOR THE LNDG. WE ALSO INFORMED OUR FLT ATTENDANTS TO BE SEATED EARLY; AS THIS IS FREQUENTLY THE CASE FOR ARRS OVER THE ROCKY MOUNTAINS. DURING THE ARR TO DENVER; WE USED THE RADAR TO ASCERTAIN IF THERE WERE ANY SIGNIFICANT CLOUD DEVELOPMENTS THAT WOULD BE OF CONCERN. THE SCREEN SHOWED NO INDICATION OF ANY SEVERE RAIN ACTIVITY THAT WOULD ALSO INDICATE VERT DRAFTS. BY 103 NM FROM DVV WE WERE ABLE TO MAINTAIN ALT SUCCESSFULLY. THE APCH CTLR MENTIONED THE DEV; TO WHICH WE RPTED THE VERT MOVEMENT ON THE ARR GATE. HE EXPRESSED HIS THANKS; AND THAT HE WOULD PASS THIS INFO ALONG TO OTHER ACFT ARRIVING ON THE GATE. HE ALSO EXPRESSED CONCERN FOR POSSIBLE ONCOMING TFC; BUT STATED THAT THEY WERE NOT A FACTOR.