THE PLT (WITH A RECENT INSTRUMENT RATING) RCVD A WX BRIEFING FM CEDAR CITY (CDC) FSS & FILED AN IFR FLT PLAN FM PROVO (PVU) TO ST GEORGE (SGU). CURRENT SGU WX WASN'T AVAILABLE, BUT WX FOR CDC (ABT 45 MI NNE) WAS PROVIDED. THE PLT WAS ADZDOF MOIST UNSTABLE AIR, RAIN, OBSCURED MTNS & CUMULUS CLOUDS ALONG THE ROUTE. ALSO, HE WAS ADZD OF A PIREP CONCERNING AN ENCTR WITH CLR ICG NR CDC AT 13,000'. APCHG MILFORD AT 11,000', THE PILOT CALLED CDC FSS & RCVD A WX UPDATE. CURRENT SGU WX WAS STILL NOT AVAILABLE, BUT HE WAS ADZD OF THUNDERSTORMS EN ROUTE. HE RETURNED TO ARTCC FREQ, GOT CLNC TO DESCEND TO 10,000', THEN TO CLIMB TO 12,000'. SOON THEREAFTER, RADIO AND RADAR CONTACT WERE LOST. SUBSEQUENTLY, THE ACFT BROKE UP IN FLT (WINGS & STABILIZERS SEPD) & WRECKAGE WAS SCATTERED OVER A 1-1/4 MI AREA. NO PRE-ACCIDENT CRACKS OR FATIGUE WAS FOUND. AN AIRCRAFT WAS SEEN FLYING INTO A WX CELL. GROUND WITNESSES THEN HEARD AN EXPLOSIVE NOISE AND SAW PARTS OF THE AIRCRAFT FALLING FM THE WX CELL. SOON THEREAFTER, PEA SIZE HAIL FELL FM THE CLOUD.