THE NON INSTRUMENT-RATED PILOT-IN-COMMAND'S DECISION TO CONTINUE FLIGHT INTO KNOWN INSTRUMENT METEOROLOGICAL CONDITIONS (THUNDERSTORM) AND FAILURE TO MAINTAIN AIRSPEED (VSO) WHILE MANEUVERING. CONTRIBUTING TO THE ACCIDENT WAS THE PILOT-IN-COMMAND'S FAILURE TO OBTAIN A WEATHER BRIEFING AND TO ENSURE THAT REQUIRED VFR DAY FLIGHT INSTRUMENTS WERE INSTALLED.