HVY LOOKING ANVIL TYPE TSTM CAUSED FLC TO DEVIATE FROM COURSE. UNABLE TO OBTAIN IMMEDIATE CLRNC FROM ZNY CTLR.

Narrative

FLT HAD BEEN CLRED TO ELIOT INTXN AT 17000. WE WERE PROCEEDING VIA OMEGA NAV. THE METEOROLOGICAL CONDITIONS WERE IN AND OUT OF CLOUDS AND HVY RAIN WITH SCATTERED AREAS OF TSTMS OF UNKNOWN INTENSITY. OUR HDG WAS APPROX 275 DEGS MAGNETIC. APPROX 3 OR 4 MI E OF ELIOT INTXN WE BROKE OUT OF THE CLOUDS AND RAIN AND DIRECTLY AHEAD AT APPROX 5 MI WAS WHAT LOOKED LIKE (VISUALLY) A DARK ANVIL HEAD OF A DISSIPATING AREA TSTM. WX RADAR WAS ON AND INDICATED HVY PRECIPITATION. I IMMEDIATELY REQUESTED A 35 DEG DEV TO THE L OF COURSE FOR SEVERE WX AVOIDANCE. I CONSIDERED PROCEEDING STRAIGHT AHEAD; A RISK TO THE SAFETY OF THE FLT. ATC ASKED IF WE COULD TURN R OF COURSE WHICH WE WERE UNABLE TO DO SINCE THE MAIN BODY OF THE STORM WAS IN THAT DIRECTION. I PROCEEDED TO TURN L TOWARD A HDG OF 210 DEGS AND MY FO INFORMED ATC THAT WE NEEDED TO DEVIATE IMMEDIATELY FOR WX AVOIDANCE. ATC DID NOT ANSWER US IMMEDIATELY AND WE CALLED AGAIN ABOUT OUR DEV AT WHICH TIME MY FO AND I HEARD ATC CALL FOR TURN TO 180 DEGS. WE WERE AT 210 DEGS AT THE TIME AND TOLD ATC WE COULD PROCEED DIRECT ELIOT MOMENTARILY. AGAIN; ATC DID NOT ANSWER IMMEDIATELY BUT FINALLY TOLD US TO PROCEED TO ELIOT WHEN ABLE. WE WERE ON THE DEV HDG FOR APPROX 45 SECONDS BEFORE PROCEEDING DIRECT TO ELIOT.