A B777 FLT CREW TURNS TO AVOID A WX CELL WITHOUT A CLRNC; WHILE ON RADAR VECTORS WITH PCT TRACON.

Narrative

WE WERE IN A LINE OF AIRPLANES BEING VECTORED OFF OF THE JASEN ARR TO RWYS 1L AND 1R AT IAD. OUR RADAR INDICATED A RED CELL WITH STEEP GRADIENT AT 25 NM RIGHT IN THE MIDDLE OF THE FINAL APCH COURSE. THE ACFT IN FRONT OF US WERE ALL BEING VECTORED OFF A L BASE ON A DOGLEG TO FINAL WHICH TURNED THEM INSIDE OF THE CELL. WE WERE 90 DEGS OFF OF FINAL HDG (HDG 090 DEGS) AT 2 NM FROM THE FINAL APCH COURSE AND THE CELL AND WERE SWITCHED TO THE FINAL CTLR'S FREQ. I ADVOCATED TO THE CAPT THAT WE SHOULD GET A TURN AS THE OTHER AIRPLANES IN FRONT OF US HAD TO AVOID THE CELL. HE TRIED TO CALL THEM TWICE WITH NO REPLY; SO WE DECIDED TO TURN WITHOUT THEIR APPROVAL TO AVOID THE CELL. WE WERE LOOKING AT TCAS AND STILL HAD GOOD SEPARATION FROM THE AIRPLANE IN FRONT OF US (5 NM) EVEN AFTER THE TURN. THE CTLR THEN CALLED US AND ASKED US WHAT HDG WE WERE FLYING. THE CAPT TOLD HIM WE HAD TURNED TO AVOID A 'STORM.' HE RESPONDED WITH 'YOU MUST GET APPROVAL TO TURN; YOU JUST CAN'T TURN ON YOUR OWN; THERE'S NO STORM OUT THERE.' OUR FOM UNDER ENRTE STORM AREA AVOIDANCE; STATES THAT MOST ATC RADARS ARE L-BAND AND CAN'T DETECT STORMS BUT ONLY PRECIPITATION. THAT WAS ALL THAT WAS SAID AND WE NEVER LOST SEPARATION AND LANDED UNEVENTFULLY.