AN MD88 CAPT WAS CONCERNED THAT TSTMS PREVENTED HIM FROM FOLLOWING PCT'S INSTRUCTIONS; INITIALLY; BUT THAT HE WAS ABLE TO FOLLOW THEM AFTER DISCOVERING A BREAK IN THE CLOUDS ONLY TO PERFORM A MISSED APCH DUE TO AN UNSTABILIZED APCH TO DCA.

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UPON ARR TO POTOMAC APCH CTL (FREQ 119.85); WE ENCOUNTERED A STRONG LINE OF LEVEL 3-4 TSTMS; 3-5 MI WIDE NEARLY SOLID RED LINE AT LEAST 20 MI ACROSS; WITH LIGHTNING CLOUD TO CLOUD (AS A MINIMUM); WHICH HAD JUST PASSED OVER DCA TO THE SE. OUR ARR WAS FROM THE S. THE INITIAL BRIEFED APCH WAS THE ILS RWY 1. S FLOW THEN OCCURRED FOR WINDS AND WE BRIEFED THE RIVER VISUAL RWY 19. WE WERE BEING VECTORED INTO THIS LINE FOR AN INTENTIONAL PENETRATION OF THE LINE OF STORMS; WHICH NOW GREW INTO A DOUBLE LINE AT ONE AREA. WE ASKED FOR A MORE WESTERLY HDG OF 340 DEGS AND WERE GRANTED THAT WITH CTLR TREPIDATION. MUCH RADIO CHATTER WAS PRESENT AND WE NEEDED A FURTHER WESTERLY VECTOR (ABOUT 300 DEGS) TO CLR THE WX. THE APCH WAS NOW AGAIN CHANGED TO THE ROSYLIN LDA TO RWY 19. THE CTLR ADVISED US WE WERE LEAVING HIS AIRSPACE AND AN IMMEDIATE TURN TO 360 DEGS WAS NECESSARY. THIS TURN WOULD HAVE FLOWN US THROUGH A LEVEL 3-4 CELL AND WE ADVISED UNABLE. HE AGAIN DEMANDED A TURN TO 360 DEGS SAYING HE COULD NOT GUARANTEE SEPARATION. WE ASKED FOR A HDOF AS WE WOULD NOT TURN TO 360 DEGS. ULTIMATELY I ADVISED HIM THAT I WOULD DECLARE AN EMER IF I HAD TO; TO AVOID THE WX. HE SEEMED RELIEVED AND TOLD ME HE ACKNOWLEDGED MY EMER STATUS. NOTE: I AS THE CAPT NEVER SAID I AM AN EMER ACFT OR THAT I REQUIRED SPECIAL HANDLING. I DIDN'T KNOW A CTLR COULD UNILATERALLY LABEL YOU AN EMER ACFT. LITERALLY MOMENTS AFTER THIS EVENT; WE FOUND A BREAK IN THE CLOUDS ALLOWING US TO TURN TO 360 DEGS WHICH WE DID; AND IT WAS AT THAT POINT WE WERE ABLE TO FOLLOW ALL SUBSEQUENT DIRECTIONS FROM APCH CTL. HE DSNDED US TO 7000 FT BUT THEN LEFT US AT 180 KIAS; 7000 FT 15 NM FROM DCA. WE ATTEMPTED TO MAKE THE APCH; BUT QUICKLY DECIDED IT WOULD BE WELL OUTSIDE ACR'S PARAMETERS FOR A STABILIZED APCH AND TOLD THE CTLR WE NEEDED TO BREAK IT OFF AND REATTEMPT. HE SEEMED VERY UPSET WITH US AND VECTORED US FOR A L REENTRY OF THE LOC FINAL. THE REMAINDER OF THE FLT WAS UNEVENTFUL.