THE SCHEDULED PASSENGER FLIGHT DEPARTED WITHOUT A FLIGHT PLAN AND WAS LAST HEARD FROM WHILE DESCENDING IN ATTEMPT TO REMAIN IN VFR CONDITIONS. WITNESSES ON THE GROUND STATED THAT THERE WAS EXTREMELY HEAVY RAIN AND THUNDERSTORMS AT THE TIME OF THE ACCIDENT. THE WRECKAGE WAS LOCATED THE NEXT DAY AND EVIDENCE INDICATED THAT THE AIRPLANE HIT THE GROUND IN A 50 DEGREE NOSE DOWN ATTITUDE AT A HIGH RATE OF SPEED.