THE CERTIFIED FLIGHT INSTRUCTORS DECISION TO FLY INTO KNOWN FORECAST WEATHER CONDITIONS SPECIFICALLY PROHIBITED BY THE AIRPLANE'S OWNER'S MANUAL AND DECISION TO ALLOW AN UNQUALIFIED PILOT WITH LIMITED INSTRUMENT FLIGHT EXPERIENCE TO OCCUPY THE LEFT SEAT BEHIND THE FLIGHT INSTRUMENTS WHICH SUBSEQUENTLY RESULTED IN A INFLIGHT LOSS OF CONTROL. CONTRIBUTING TO THE ACCIDENT WAS THE CERTIFIED FLIGHT INSTRUCTOR'S INADEQUATE SUPERVISION OF THE UNQUALIFIED PILOT.