PLT OF C182 ON VFR PART 135 PAX FLT ENCOUNTERS DETERIORATING WX ENRTE. FORCED TO FILE IFR TO CONTINUE TO DEST SAFELY.

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A VFR CHARTER WAS PLANNED FROM ZZZ2 TO ZZZ3. AREA FORECAST AND ALL AVAILABLE WX OBSERVATIONS ALONG PROJECTED RTE OF FLT INDICATED UNIFORM COOLINGS BTWN 2800-3500 FT AGL THAT WERE FORECAST TO REMAIN FOR THE ENTIRE DURATION OF FLT. CUSTOMERS WERE ADVISED THAT THIS FLT MIGHT TAKE LONGER THAN SCHEDULED IF DEVS FOR WX BECAME NECESSARY. FLT DEPARTED ON SCHEDULE. WX AT TIME OF DEP WAS 9000 FT SCATTERED. FLT PROGRESSED AS PLANNED FOR APPROX 50 KM. THE CEILING ALONG THE RTE STARTED TO THICKEN AND LOWER. A DIVERSION TO THE NE WAS INITIATED. ONCE BACK IN GOOD VFR; A CLB WAS MADE TO 11500 FT MSL AND RE-ESTABLISHED A SE COURSE AROUND A DEVELOPING LINE OF WX. CONTACT WAS ESTABLISHED WITH CENTER FOR FLT FOLLOWING PURPOSES. CENTER ADVISED OF DEVELOPING WX AND SUGGESTED 2 OPTIONS: AN EASTERLY OR WESTERLY DIVERSION. THE SATELLITE VISUAL AND WATER VAPOR CHARTS THAT I VIEWED PRIOR TO DEP INDICATED THAT A DIVERSION E WOULD BE THE BEST COURSE OF ACTION. ATC ASKED IF I WAS INST RATED AND EQUIPPED. I STATED THAT I WAS. I WAS THEN CLRED PRESENT POS DIRECT ZZZ4 DIRECT ZZZ5. I WAS ABLE TO MAINTAIN VFR UNTIL ABOUT 15 MI W OF ZZZ. I FLEW IN HVY RAIN FOR ABOUT 15 MINS. MOST TIMES DURING THIS PERIOD; VISIBILITY WAS BELOW VFR MINIMA. INTUITION TOLD ME THAT THE FLT COULD NOT BE COMPLETED VISUALLY. I ONLY DEPARTED AFTER ALL AVAILABLE INFO INDICATED THAT IT WOULD BE OK. ALTHOUGH IT WAS AGAINST MY PART 135 AUTH; GOING IFR WAS THE ONLY SAFE ALTERNATIVE ONCE THE FLT WAS UNDERWAY. WE NEED MORE WX INFO FOR PLTS. IF AN ARPT HAS AN IAP; IT SHOULD ALSO HAVE AN ACCURATE MEANS FOR OBTAINING THE ACTUAL WX CONDITIONS AND THEN DISSEMINATING THOSE RPTS NATIONALLY.