PVT PLT IN C210 GETS INTO LOW CLOUDS PRIOR TO REACHING CPR; WY. HE WAS DIVERTING DUE TO WX ALONG HIS PLANNED RTE TO HUGO; CO. COMPLAINS OF ATIS NOT GIVING ACTUAL WX CONDITIONS 25 MI FROM ARPTS.

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RECEIVED FLT FOLLOWING ALONG RTE FROM PRINEVILLE; OR (S39); TOWARDS PVT STRIP NEAR HUGO VOR; CO (HGO). AT BURLEY; ID; RECEIVED WX UPDATE FROM BOISE RADIO OF VFR NOT RECOMMENDED ALONG RTE. DIVERTED AT THAT TIME TO CASPER; WY; WHICH WAS RPTING VFR WELL BEYOND MY ETA. APPROX 40 MI W OF CPR; RECEIVED ATIS FOR ARPT RPTED 10 MI VISIBILITY WITH A 6000 FT CEILING. AT 25 MI W I CONTACTED CPR APCH FOR RADAR ASSISTANCE AND CONFIRMATION OF ATIS RPT. WX APPEARED MARGINAL; HOWEVER; CPR APCH ASSURED ME IT WAS PATCHING TO THE W AND CONTINUED TO BE VFR. I PROCEEDED; REQUESTING CONTINUED RADAR ASSISTANCE. 18 MI W OF ARPT; I WAS ENTERING MARGINAL TO IMC CONDITIONS AT 7700 FT MSL INDICATED. AT 5 MI FROM ARPT; I BECAME VFR AND CONTINUED TO LNDG. AT NO TIME WAS I IN AN EMER SIT NOR WAS THE ACFT IN AN UNUSUAL ATTITUDE. 2 SOULS WERE ON BOARD. APPROX 30 MINS AFTER LNDG CPR BECAME IFR WITH BLOWING SNOW AND LOW CEILINGS. POSSIBLE SOLUTION: EXPAND ATIS TO SUMMARIZE SURROUNDING AREA (10-25 MI) CONDITIONS FOR ENRTE TFC.