B737 Next Generation Undifferentiated
Flight Crew Air Transport Pilot (ATP)
Flight Crew Type 1733
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Flight Crew Requested ATC Assistance / Clarification; Flight Crew Took Evasive Action
Procedure; Weather
Enroute [while over the Pacific Ocean] I was notified by ATC and Dispatch of a SIGMET that had a report of Observed Severe Turbulence on our route and altitude. I attempted for one hour to get a new route north of the area or lower altitude below the reported/forecast area of turbulence. Approaching fix GITLE after ATC was only willing to let me offset 50 NM south into the forecast area to get a lower altitude and Dispatch was unable to get ATC to change my route; I [exercised my Captain's authority]. We turned 45 degrees north and offset 15 NM then descended to FL270. Then we returned to our route. FL270 was below the SIGMET area. I prepped the cabin for turbulence while we passed the area just in case. The flight was normal and ATC and Dispatch were advised.
B737 Captain reported deviating around an area of reported severe turbulence on a trans-Pacific flight without ATC approval.