EMER AUTH EXERCISED WHEN ARTCC WOULD NOT COOPERATE WITH WX AVOIDANCE MANEUVERING.

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AT APPROX 150 NM DISTANCE; WE BEGAN TO 'PAINT' CONVECTIVE ACTIVITY AHEAD. INITIALLY IT WAS THOUGHT TO BE SUFFICIENTLY SCATTERED TO NOT PRESENT ANY PROB NAVING THROUGH. WE SENT A MESSAGE TO DISPATCH. THEY RETURNED A MESSAGE WITH A NEW RE-ROUTE. ATC (ZME) WOULD NOT ALLOW ANY RE-ROUTE. MEANTIME AS WE CLOSED IN ON WX IT BECAME OBVIOUS THAT WX WAS CLOSING UP AND COULD NOT BE SAFELY NEGOTIATED. I ASKED FOR DEV CLRNC; ATC ONLY OFFERED LIMITED TURNS. ATC WANTED ME BACK ON COURSE ASAP. I NEEDED MORE TURNS THAN WERE BEING PERMITTED. NO REASON FOR LACK OF ASSISTANCE WAS GIVEN; IE; TFC CONFLICT; PROHIBITED AIRSPACE; ETC. EVENTUALLY I DECIDED I HAD TO SAFEGUARD THE FLT; AND DECLARED AN EMER IN ORDER TO USE MY CAPT'S AUTH. I ANNOUNCED MY INTENTIONS; AND TURNED L TO CIRCUMNAV THE ECHOES ON MY RADAR. I WAS USING PUBLISHED COMPANY AVOIDANCE GUIDELINES THROUGHOUT. ONCE CLR OF CONVECTIVE WX I DE-DECLARED EMER STATUS; AND ACCEPTED RE-ROUTING TO IAD. HOW COME NOT THE SLIGHTEST LEVEL OF COOPERATION? THIS IS NOT WHAT WE HAVE COME TO EXPECT.