PA32 PILOT ATTEMPTS TO NEGOTIATE THE HUDSON RIVER VFR CORRIDOR AT 1500 FEET AND ENCOUNTERS IFR CONDITIONS. A CLIMB TO 2500 FEET AND REQUEST FOR IFR CLEARANCE IS INITIATED; MUCH TO THE CHAGRIN OF AN N90 CONTROLLER.

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I WAS FLYING FROM ZZZ TO ZZZ1 AT 3000 FT; DSNDING TO 1500 FT ALONG THE HUDSON RIVER CORRIDOR AFTER OBTAINING A CLASS B CLRNC. WX AT ZZZ WAS CLR BELOW 4000 FT. FORECAST FOR ZZZ1 WAS 3000 FT CEILINGS. XING NEW YORK HARBOR; VISIBILITY WAS 5 MI IN HAZE; AS FORECAST IN MY PREFLT BRIEFING. AFTER REACHING APPROX MIDPOINT OF HUDSON RIVER RTE (VERRAZANO BRIDGE TO TAPPAN ZEE BRIDGE); I FOUND THAT A LOW LAYER OF FOG HAD FORMED UNDER ME; AND THE CEILINGS ABOVE ME HAD DROPPED TO MY ALT. BECAUSE I COULD NO LONGER MAINTAIN VISUAL CONTACT WITH ENVIRONMENT; AND BECAUSE NARROWNESS OF CORRIDOR PRECLUDED A SAFE 180 DEG TURN; I INITIATED AN IMMEDIATE CLB TO 2500 FT AND INFORMED ATC; SAYING THAT I WAS UNABLE TO MAINTAIN VFR. ATC TOLD ME THAT MY RADAR SVCS WERE TERMINATED; TO DSND TO VFR CONDITIONS; AND TO SQUAWK VFR. BECAUSE OF UNCERTAINTY OF NEARBY TERRAIN (BUILDINGS AND BRIDGES) I SAID I WAS UNABLE. I ASKED FOR AN IFR CLRNC TO ZZZ1. ATC ASKED; 'WHY DID YOU TAKE OFF?' I REPLIED; 'WELL; I'M HERE NOW;' AND AGAIN REQUESTED AN IFR CLRNC TO ZZZ1. ATC SAID THEY WERE UNABLE TO GIVE CLRNC TO ZZZ1 BECAUSE IT WAS NOW BELOW MINIMUMS; SO I ASKED FOR AND RECEIVED A CLRNC TO ZZZ2. (LATER I CHANGED DEST TO ZZZ3 AND LANDED THERE WITHOUT DIFFICULTY.) FIRST; THE FLT CONDITIONS TURNED OUT TO BE WORSE THAN FORECAST. SECOND; I DID NOT APPRECIATE THE RAPIDITY WITH WHICH THE CONDITIONS WERE DETERIORATING. THIRD; ATC TRIED A COUPLE OF TIMES TO 'SLAM DUNK' ME OFF THEIR RADAR SCREENS WITHOUT APPRECIATING THE DEVELOPING DANGER. IN THE FUTURE; I WILL TRY TO MAKE BETTER USE OF INFLT WX ADVISORIES (ATIS'S ENRTE; FLT WATCH; ETC) AND TERMINATE FLT IF CONDITIONS ARE DETERIORATING RAPIDLY. SECOND; IF CONDITIONS ARE MARGINAL; I WILL TRY TO WORK OUT WITH ATC IN ADVANCE WHAT I SHOULD DO IF I LOSE VMC. THIRD; I WILL USE THE WORDS 'DECLARE AN EMER' IF I FEEL I NEED MORE IMMEDIATE HANDLING FOR SAFETY.