B727 FLT CREW DEVIATES FROM ASSIGNED CLRNC TO AVOID SEVERE WX ON APCH TO BWI.

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APCHING THE VORTAC CSN (THE BEGINNING OF THE OTT5 ARR INTO BWI) CAPT ASKED ZDC FOR PERMISSION TO DEVIATE N (L) OFF COURSE FOR WHAT APPEARED TO BE A WX CELL OVER CSN; BOTH VISUALLY AND ON THE RADAR. I WAS FLYING THE ACFT AND CHOSE A COURSE THAT WOULD TAKE THE ACFT DOWN A CORRIDOR BTWN A SMALL LINE OF TSTMS ON OUR L AND THE TSTM OVER CSN ON OUR R. WE WERE ON A HEADING OF ABOUT 050 DEGS; WHICH WAS FLYING US TOWARD THE R SIDE OF THE CORRIDOR; CLOSER TO THE CSN WX CELL. AS WE APCHED WITHIN 15 MI OF THE CSN TSTM; THE RADAR DISPLAYED IT AS LARGELY GREEN WITH A WELL DEFINED YELLOW CTR SPECKLED WITH RED RETURNS. THE TSTMS TO OUR L WERE MOSTLY YELLOW WITH WELL-DEFINED RED CTRS. AS THE W SIDE OF THE CSN TSTM PASSED OUR 3 O'CLOCK POS; CTR SWITCHED US TO WASHINGTON APCH. WASHINGTON APCH ASSIGNED US A R TURN TO A HEADING OF 140 DEGS. I NOTED THAT THE TURN WOULD TAKE US INTO THE CSN TSTM. THE CAPT ADVISED THE CTLR THAT WE HAD WX ON OUR R SIDE AND THAT WE WOULD BE UNABLE TO COMPLY WITH HIS INSTRUCTIONS. THE CTLR INSISTED THAT THE WX WAS ON OUR L SIDE AND THAT WE MUST COMPLY WITH HIS INSTRUCTIONS IMMEDIATELY. THE CAPT REITERATED HIS REFUSAL TO TURN INTO WHAT WE BELIEVED WAS A TSTM; REGARDLESS OF WHAT APCH'S RADAR INDICATED ABOUT THE CELL. THE DEBATE BTWN THE CAPT AND THE CTLR CONTINUED FOR SEVERAL MINS; WHILE I CONTINUED TO FLY THE ACFT TOWARD THE NE END OF THE CSN TSTM. IN THE COCKPIT THE FLT CREW DISCUSSED THE SIT AND WE ALL AGREED THAT IT WOULD BE UNDESIRABLE; UNWISE; AND RISKY TO MAKE A TURN TO THE SE INTO THE CSN WX CELL. WE FELT WE COULD POSSIBLY ACCOMMODATE WASHINGTON APCH IF WE RETURNED TO THE W END OF THE CSN TSTM AND FLEW AROUND IT TO THE OTHER SIDE. IT APPEARED WE COULD MAKE A L 180 DEG TURN IN THE CORRIDOR WITHOUT ENTERING INTO THE TSTMS ON OUR L. HOWEVER; WE ALL AGREED THE MOST DESIRABLE COURSE OF ACTION WAS TO CONTINUE UNTIL WE HAD CLRED THE CSN TSTM AND THEN PROCEED TO BWI. EVENTUALLY; THE CTLR GAVE US A DSCNT TO 7000 FT OR 8000 FT. AS WE APCHED 10000 FT IT BECAME OBVIOUS THAT WE WERE AT A PLACE WHERE WE COULD FLY UNDER THE TSTM AND PROCEED TO BWI. CAPT ADVISED WASHINGTON APCH; AND THE CTLR GAVE US A HEADING TOWARD BWI AND SWITCHED US TO BALTIMORE APCH. THE FLT CONTINUED TO BWI WITHOUT FURTHER INCIDENT.