B737-800 captain reported encountering a severe mountain wave.

Attributes
ACN	1523664
Time
Date	201803
Local Time Of Day	0601-1200
Place
Locale Reference	ZDV.ARTCC
State Reference	CO
Environment
Light	Daylight
Aircraft 1
Make Model Name	B737-800
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Person 1
Function	Captain Pilot Not Flying
Qualification	Flight Crew Multiengine Flight Crew Instrument Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 380 Flight Crew Type 4000
Events
Anomaly	Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Inflight Event / Encounter Weather / Turbulence

Narrative:

We encountered severe mountain wave and were unable to maintain altitude. Loss of 600 ft.

Original NASA ASRS Text

Title: B737-800 Captain reported encountering a severe mountain wave.

Narrative: We encountered severe mountain wave and were unable to maintain altitude. Loss of 600 ft.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.