Air carrier flight crew reported while climbing through FL260-280; they encountered severe mountain wave turbulence which they reported to ATC.

Narrative

Climbing out of DEN; ATC advised us of isolated moderate to severe turbulence between FL260 and 280 in our vicinity. No severe turbulence had been forecast nor expected until our check on. Our previous leg from the same direction we were now flying had been surprisingly smooth despite forecasts for moderate turbulence. While climbing through FL 260; we appeared to hit a clear air rotor associated with mountain wave action. Indicated airspeed was 280 knots at the encounter. Seconds after; we were indicating 235. CA disengaged the AP (Autopilot. I called out the airspeed deviation. CA replied he was trying to recover our speed. Aircraft was in significant sink enough that tracking we set for climb out was just enough to maintain altitude at 235 knots. Turbulence at that point was easily severe; with a washing machine kind of feel to the ride; choppy without much G loading or significant rolling; yet violent. Climbing through FL 265 things settled down and our climb performance now recovered and then some. Upside of the rotor gave us 4;000 fpm climb rate. After that we smoothed out to a normal climb profile. I reported the turbulence to ATC as an 'URGENT PIREP'. We then checked in with our Cabin Crew; asked if any injuries or damage. None reported; I then made an announcement to the pax explaining the situation and that we were working with ATC to avoid any further turbulence of that intensity. We then monitored the frequency and at least 5 aircraft behind us reported same ride in the same area. After that; ATC closed that departure gate.