A PVT SMA TWIN PLT HAD TO RETURN LAND DUE TO SEVERE TURB.

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THE LINE WAS APPARENTLY INTENSIFYING BECAUSE EVERYONE FROM CHEROKEES TO COMMUTERS WERE DEVIATING AND RPTING MODERATE TO OCCASIONAL SEVERE TURB. I HAD ENCOUNTERED ONLY OCCASIONAL MODERATE TURB. I COULD NOT RELY ON THE WX RADAR BECAUSE; WITH THE ALMOST CONSTANT CHANGES IN PITCH ATTITUDE; THE RADAR HAD A TENDENCY TO PAINT GND CLUTTER. I HAD THROTTLED BACK TO A PWR SETTING I KNEW WOULD YIELD AN AIRSPD THAT WAS LESS THAN THE PLACARDED MANEUVERING SPD JUST IN CASE I HIT A NASTY BUMP. ALL OF A SUDDEN; WHAM! EVERYTHING THAT WASN'T TIED DOWN IN THE COCKPIT AND CABIN WENT AIRBORNE. THE AIRPLANE WAS COMPLETELY OUT OF MY CTL. I MANAGED TO DECLARE AN EMER. THE WHOLE CABIN HAD FILLED WITH A WHITE; SMOKY LOOKING SUBSTANCE. I ASSUMED IT MAY HAVE BEEN SOME SORT OF SMOKE; EVEN THOUGH IT HAD NO ODOR. SO WHEN DAYTON APCH RESPONDED TO MY EMER CALL I TOLD THEM I WISHED TO DSND. I WAS CLRED TO 3000 FT MSL. BY THE TIME I REACHED 3000 FT MSL; I WAS OUT OF THE CLOUDS AND ABLE TO SEE THE CHURNING; BLACK MASS OF CLOUDS I HAD JUST BEEN IN. THERE WAS NO FIRE. THE WHITE CLOUD CAME FROM THE FIRE EXTINGUISHER ITSELF. THE PORTABLE FIRE EXTINGUISHER WAS TOSSED ABOUT AND DISCHARGED CAUSING THE CLOUD. I CONSIDER MYSELF LUCKY TO BE ALIVE. I RETURNED TO 3I7 AND CALLED IT A DAY. NOT 30 MINS AFTER I LANDED; SEVERE TSTM WARNINGS WERE ISSUED FOR MY AREA; AND A TORNADO TOUCHDOWN WAS RPTED NOT 15 MI FROM 3I7.