WHILE EN ROUTE, THE PLT ENCOUNTERED DETERIORATING WX. HE WAS AN INSTRUMENT PLT, BUT HAD CHOSEN NOT TO FILE & FLY UNDER IFR DUE TO TERRAIN, WX, LOW PERFORMANCE OF THE ACFT & LACK OF PITOT HEAT. THE PLT NOTED A CEILING AT 5500 FT MSL WHEN HE CROSSED BANNING PASS. HE SAW DARK CLOUDS & HEAVY RAIN IN THE PALM SPRINGS & BERMUDA DUNES AREA. THE MOUNTAINS NORTH OF DESERT HOT SPRINGS WERE COVERED WITH CLOUDS, SO HE TRIED TO GET THRU YUCCA VALLEY WHERE THE WX LOOKED BETTER. HOWEVER, LOW CEILINGS & RAIN WERE ENCOUNTERED, SO THE PLT MADE A 180 DEG TURN TO RETURN TO RIVERSIDE, CA. HE SOON ENCOUNTERED 'A FAST WESTERLY MOVING MASS OF CLOUDS THAT WENT ALL THE WAY TO THE GROUND.' DUE TO THE SPEED OF THE STORM, HE WAS FORCED TO LAND QUICKLY & ELECTED TO LAND DOWNWIND ON A ROAD. HOWEVER, OVERFLEW THE ROAD & LANDED IN A ROCKY WASH AREA, EXTENSIVELY DAMAGING THE ACFT.