B767 CAPT DSNDED TO A LOWER ENRTE ALT OVER THE PACIFIC OCEAN IN AN ATTEMPT TO AVOID SEVERE TURB WITHOUT CLRNC.

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WHILE CRUISING AT FL310; ENRTE FROM ZZZ TO HNL; OUR FLT ENCOUNTERED TURB THAT WAS BORDERING ON SEVERE. I FELT THAT THIS POSED A THREAT TO THE SAFETY OF OUR PAX AND CREW. WE HAD A CLRNC TO CRUISE IN A BLOCK FROM FL280 TO FL310. EARLIER FORECASTS HAD INDICATED A POTENTIAL FOR MODERATE TO SEVERE TURB IN OUR LOCATION BTWN FL320 AND FL390. IN ADDITION; WE HAD GOTTEN A RPT FROM AN ACR FLT THAT HAD EXPERIENCED WHAT THEY DESCRIBED AT CODE 3-4 TURB (MODERATE TO SEVERE) AT FL350 XING 170 DEGS W LONGITUDE. WE WRE EBOUND IN THE VICINITY OF 176 W AT THE TIME. GIVEN THE FORECASTS AND RPT; AS WELL AS VISUAL INDICATIONS THAT CLBING TO A HIGHER ALT WOULD AGGRAVATE THE SIT WE BEGAN A DSCNT TO FL290; AND THEN CONTINUED DOWN TO FL280. THE TURB CONTINUED TO INCREASE IN INTENSITY. WE THEN MADE A REQUEST TO ZOA THROUGH SFO RADIO FOR A BLOCK ALT OF FL240-FL310. AFTER WAITING A CONSIDERABLE TIME FOR A RESPONSE; I DETERMINED TO DSND IN VISUAL CONDITIONS TO FL260 IN THE INTEREST OF SAFETY. WE ADVISED SFO RADIO OF OUR DSCNT AND WERE VIGILANT FOR TFC DURING THIS TIME BOTH MONITORING TCASII AND MAINTAINING A VISUAL SCAN OF THE HORIZON. THE RIDE WHILE NOT GOOD WAS SIGNIFICANTLY BETTER. SHORTLY AFTER LEVELOFF AT FL260 WE FINALLY RECEIVED A RESPONSE FROM SFO RADIO STATING OUR REQUEST HAD BEEN DEFERRED. THE RADIO OPERATOR NOTIFIED OAKLAND OF OUR DSCNT AND WE WERE ADVISED THAT THERE WAS CONFLICTING TFC FORECAST TO CROSS 170 W.