AN OVERWATER B757 DEVIATES FOR WX AVOIDANCE. HAD REQUESTED HIGHER TO FL350 BUT FL310 THE MAX PER TFC. COORDINATED WITH PHONE PATCH VIA NEW YORK RADIO TO ZNY AND DISPATCH.

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FREQ: 5550 HF. ALT: FL310. ATC FACILITY: NEW YORK RADIO. LOCATION: R763; 70 M S OF NANCE. WHILE FLYING NBOUND ON R763 AT FL280; TALKING TO NY RADIO ON HF; WE HAD TO DECLARE AN EMER TO DEVIATE AROUND WX. SHORTLY AFTER MAKING A POS RPT AT LETON; WE SAW WX AHEAD. WE CALLED NY AND ASKED FOR A CLB TO FL350 DUE TO WX AND WERE TOLD TO STAND BY. AFTER 5 MINS NY CALLED BACK TO ASK FOR OUR CURRENT FLT LEVEL. I RPTED FL280 AND AT THE CAPT'S REQUEST SAID THAT WE NEED TO CLB NOW. WE WERE AGAIN TOLD TO STAND BY. A COUPLE MINS LATER NY CALLED BACK WITH A CLRNC TO FL310 AND SAID FL350 WAS UNAVAILABLE DUE TO TFC. WE STARTED THE CLB TO FL310; AND AS INSTRUCTED TO DO BY THE CAPT; DECLARED AN EMER AT XX40Z PM; INFORMING THEM WE WOULD BE DEVIATING W AROUND WX. WE KEPT NY INFORMED OF THE AMOUNT OF DEV WHICH WAS UP TO 40 MI. NY PHONE PATCHED US TO BOTH ACR DISPATCH AND LATER ZNY. ZNY ADVISED US THAT THERE WAS NO OTHER TFC IN OUR AREA AT OUR ALT. WHILE TALKING TO CTR WE TERMINATED THE EMER AT XX10Z RETURNING TO ON COURSE. THE WX WAS FROM 25 NM S OF NANCE TO 80 NM S OF BURTT WITH TOPS ESTIMATED AT FL330. OCCASIONAL MODERATE TURB WAS ENCOUNTERED WHILE DEVIATING AROUND THE WX.