MD80 IN IMC RESPONDED TO A TCAS RA AND CLIMBED TO AVOID TFC. TFC WAS OPERATING UNDER VFR.

Narrative

VFR ACFT DID NOT MAINTAIN ADEQUATE SEPARATION FROM A DSNDING IFR ACFT ON A PUBLISHED ARR IN IMC CONDITIONS. DSNDING ON THE EAGUL ONE RNAV ARR APPROX 12-15 MI PRIOR TO HOMRR INTXN; CTR CALLED VFR TFC TO US TWICE WHILE WE WERE IN IMC CONDITIONS. BOTH TIMES WE REPLIED THAT WE WERE IMC. AFTER OUR SECOND REPLY; CTR REQUESTED WE LEVEL OFF. AS I WAS LEVELING THE ACFT; WE RECEIVED AN RA WARNING AND I INITIATED A CLB. AFTER 4-8 SECONDS; WE RECEIVED A CLR OF CONFLICT NOTIFICATION AND I RESUMED MY DSCNT. WE WERE IMC THE ENTIRE TIME AND I HAVE TROUBLE UNDERSTANDING HOW THE OTHER ACFT WAS MAINTAINING VFR CONDITIONS. PAX AND FLT ATTENDANTS WERE UNAWARE OF THE EVENT. ONCE WE RECEIVED THE 'CLR OF CONFLICT' MESSAGE I REESTABLISHED OUR DSCNT TO 12000 FT AND REPORTED THE RA TO ATC. I CAN'T SAY WHY THE OTHER ACFT WAS FLYING VFR IN MARGINAL WX CONDITIONS ON A BUSY ARR RTE TO A MAJOR ARPT. I FEEL ATC TRIED TO PROVIDE VERT SEPARATION AS SOON AS THEY BECAME AWARE THAT A POSSIBLE CONFLICT WAS PRESENT. WE WERE IMC UNTIL WELL AFTER WE REESTABLISHED OUR DSCNT ON THE ARR. TCAS PROBABLY SAVED THE DAY.