THE PLT WAS GIVEN A PRE-FLT WX BRIEFING BY A FSS BRIEFER. HE RCVD A FULL & THOROUGH BRIEFING, EXCEPT THE BRIEFER DID NOT HAVE AN OPNL WX RADAR SET AVAILABLE. THE PLT WAS ADZD OF A LINE OF THUNDERSTORMS (TSTMS) TO THE SOUTHWEST, MOVING EAST AT 20 KTS. HE WAS ALSO GIVEN A PIREP CONCERNING A TSTM 20 MI SOUTH OF WICHITA FALLS (SW OF HENRIETTA), RPRTDLY MOVING NORTH. WITNESSES SAID THE ACFT TOOK OFF DOWNWIND, HAD DIFFICULTY GETTING AIRBORNE, THEN MADE AN ABRUPT RGT TURN INTO A LOW CEILING. SHORTLY THEREAFTER, A WITNESS SAW IT COME OUT OF THE CLOUDS & CRASH APRX 1-1/2 MI SOUTHEAST OF THE ARPT. IMPACT OCCURRED IN A STEEP LEFT WING DOWN, SLIGHTLY NOSE LOW, ATTITUDE. NO PREIMPACT PART FAILURE/MALFUNCTION WAS FND. A WITNESS SAID THAT JUST BFR TAKEOFF, THERE WAS A HVY DOWNPOUR OF RAIN WITH SMALL HAIL, LIGHTNING & THUNDER. THERE WERE INDCNS THE PLT WAS HURRYING TO DEPART BFR A CLNC VOID TIME OF 0900 CST. HE TOOK OFF ABT 8 MIN BFR THE VOID TIME. ACCORDING TO SPECS, THE ATTITUDE INDICATOR WOULD HAVE NEEDED A 3 MIN SPIN-UP TIME TO ACHIEVE ITS RATED PERFORMANCE.