DRG A WX BRIEFING, FLT PRCTNS FOR OCNL MTN OBSCN & IMC WERE RELAYED TO THE PLT, BUT HE WAS NOT PROVIDED A FORECAST FOR THUNDERSTORMS, NOR ADVISED VFR FLT WAS NOT RCMDD. THE ROUTE OF FLT WAS PORTRAYED AS MARGINAL VFR, AS WAS THE DESTN AT MORGANTOWN (MGM). ALSO, THE PLT WAS ADVISED THE MGM WX WAS PARTIALLY OBSCURED, ESTD CEILING 1700' OVERCAST, VIS 1 MI WITH MODERATE RAIN & FOG. THE BRIEFER TERMED THESE CONDS AS MARGINAL VFR & STATED 'LOOKS LIKE THEY'RE FORECASTING TO CONTINUE TO DETERIORATE THROUGH TONIGHT & BECOME IFR BY MORNING.' THE NON-INSTRUMENT RATED PLT TOOK OFF ON THE FLT (TO TRANSPORT A PAX TO A HIGH SCHOOL REUNION). THEY WERE TO BE MET AT THE DESTN BY FAMILY MEMBERS. SUBSEQUENTLY, THE ACFT STRUCK THE 2900' LVL OF AN OBSCURED 3368' MTN BTN MANASSAS & MGM. THE WRECKAGE PATH WAS ALONG A SOUTHEASTERLY DIRECTION TOWARD MANASSAS. THE PLT HAD RECENTLY FLOWN ABOUT 1 HR OF DUAL INSTRUCTION IN ACTUAL IMC, BUT HIS TOTAL INSTRUMENT TIME WAS NOT DETERMINED. NO PREIMPACT PART FAILURE OR MALFUNCTION WAS FND.