PLT OF EXPERIMENTAL AMPHIBIOUS FLOAT PLANE; AT IDLE TAXI ON HIGH ALT LAKE; IS OVERTURNED BY GUST FRONT ASSOCIATED WITH DISTANT CONVECTIVE ACTIVITY.

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I LEFT ZZZ ABOUT XA00 ON SEP/WED/04. THERE WERE SOME CUMULOUS CLOUDS TO THE NW; NOT LARGE; NOR WAS I AWARE OF ANY THUNDER OR LIGHTNING. I MADE 3 LNDGS AND TKOFS ON THE LAKE. THEN I LANDED; TURNED OFF THE ENG; RELAXED AND ATE A SANDWICH. AFTER 15 MINS; I STARTED THE ENG; SET IT AT IDLE; AND STARTED THROUGH MY CHKLIST; WHEN A TREMENDOUS BLAST OF WIND HIT ME WITH NO WARNING. I WATCHED IN AMAZEMENT AS THE L FLOAT WAS PUSHED DOWN UNDERWATER AND R WING WENT UP AND UP. THE ACFT TURNED SLOWLY COMPLETELY OVER. I OPENED THE DOOR AFTER UNBLOCKING MY HARNESS AND SWAM OUT. 2 MEN ONSHORE SAW THAT THE ACFT HAD DISAPPEARED. THEY CALLED 911 AND THEN CAME OUT IN A BOAT. SOON AFTER THE SHERIFF'S BOAT AND COAST GUARD ARRIVED AND TOOK ME ON BOARD. THE ACFT DID NOT SINK; BUT REMAINED UNDER WATER SUSPENDED BY THE FLOATS. THE PROB AROSE FROM BEING ON THE WATER IN THE VICINITY OF A TSTM. THE 2 MEN WHO RESCUED ME ESTIMATED WINDS AT 70 MPH. THE RIGHT DECISION WOULD HAVE BEEN TO NOT LAND ON THE LAKE AT ALL WITH CUMULOUS CLOUDS ANYWHERE IN THE VICINITY.