PLT OF AN SMA SEL ON SHORT FLT ENCOUNTERS UNFORECASTED WX AND TURNS BACK AND RETURNS TO DEP ARPT.

Narrative

I DEPARTED ILG FOR A FLT TO N92 SUMMIT. WHEN I LEFT ILG THE CEILING WAS 4200 FT OVCST; VISIBILITY 10 PLUS NM. SUMMIT IS ABOUT 10 MI SW OF ILG. AS I GOT CLOSER TO N92 A THIN SCATTERED LAYER AT 2000 FT AGL GOT THICKER AND LOWER. THE FORECAST WAS FOR VFR AND SUMMIT UNICOM DID NOT MAKE NOTE OF ANY BAD WX WHEN I CALLED THEM. ABOUT 4 OR 5 MI OUT FROM N92 I SAW THEIR BEACON ON AND LOW CLOUDS OVER THE ARPT EXTENDING TO THE S. I IMMEDIATELY TURNED BACK FOR ILG. I WAS ABLE TO CONTINUE TO ILG VFR WITHOUT INCIDENT. WHILE ON THE FLT I NEVER BECAME IMC OR VIOLATED CLOUD CLRNC REQUIREMENTS; AND WAS ABLE TO MAINTAIN ADEQUATE TERRAIN CLRNC. I AM WRITING BECAUSE I WAS AMAZED HOW QUICKLY THE WX DETERIORATED. I WOULD NOT HAVE HESITATED TO SEND A STUDENT PLT ON THE SAME FLT WITH THE INFO I HAD ABOUT THE WX. I HAD HAD A BRIEFING BY DUAT ONLY MINS BEFORE THE FLT. A STUDENT PLT IN THE SAME SIT MAY HAVE PRESSED ON IN THE SAME SIT; WITH DISASTROUS RESULTS. I HAVE LEARNED TO BE VERY SKEPTICAL OF THE WX INFO IN THIS AREA WHEN THE CONDITIONS ARE SIMILAR TO THESE. CLOSE TEMP DEWPOINT. I THINK STRESSING TEMP/DEWPOINT SPREAD IN TRAINING WILL HELP REDUCE CONTINUED VFR INTO IMC ACCIDENTS BY RAISING AWARENESS IN THE STUDENT PLT POPULATION OF THIS HAZARD.