B757-200 CREW WAS REQUIRED TO DECLARE AN EMER IN ORDER TO BE AVOID BEING RTED INTO A TSTM.

Narrative

DURING DUVAL ARR; WE WERE CLRED DIRECT FROM MAXIM INTXN TO WEVER TO CROSS AT 14000 FT AND 250 KTS. WE HAD JUST BEEN HANDED OFF TO A NEW SECTOR SHORTLY AFTER PASSING OVER KEY WEST. WX RADAR AND VISUAL EXAM INDICATED 2 SIGNIFICANT CELLS BTWN OUR POS AND WEVER. I WAS UNABLE TO DETERMINE THE HTS OF THE CELLS; BUT THEY WERE CLRLY OVER FL300 WITH CONTINUING VERT DEVELOPMENT. THE ONLY SAFE RTE AROUND THE WX WAS A R DEV OF ABOUT 15 DEGS. THE FO REQUESTED THE DEV. CTLR SAID WE COULD ONLY GO L WITH A HDG OF 360 DEGS. I TOLD THE CTLR THAT WAS NOT ACCEPTABLE BECAUSE IT WOULD TAKE US DIRECTLY THROUGH THE LARGER OF THE 2 CELLS. HE SAID THAT MIA APCH WOULD NOT LET HIM APPROVE E DEVS AND THE ONLY WAY WE COULD GO THAT WAY WOULD BE TO DECLARE AN EMER AND SQUAWK 7700. I RESPONDED THAT WAS OUR ONLY OPTION AND THAT WE WOULD SQUAWK 7700 AND BEGIN THE DEV. WE WERE DIRECTED THAT AFTER THE DEV TO RETURN TO PREVIOUSLY ASSIGNED SQUAWK. I WOULD ESTIMATE THAT THE DEV WAS LESS THAN 10 MI E OF COURSE. OUR TCASII SCAN OF 40 MI DID NOT INDICATE ANY OTHER ACFT IN OUR VICINITY.