SMA PLT IS UNABLE TO MAINTAIN CRUISE ALT DUE TO HVY TURB FROM TSTMS AND RAIN.

Narrative

AROUND XA00 I WAS FLYING NEAR CENTERVILLE; TN; IN IMC AT 7000 FT MSL. I KNEW THAT THERE WERE TSTMS IN THE AREA; BUT I HAD NO WX AVOIDANCE EQUIP. AT THE TIME I WAS TALKING TO ZME AND ASKED THE CTLR TO HELP PICK MY WAY AROUND ANY HVY PRECIP. THE CTLR THAT I WAS TALKING TO TOLD ME HE COULD NOT SEE ANY PRECIP IN MY AREA. SCANNING MY INSTS; I CALLED ZME AND TOLD THEM I COULD NOT MAINTAIN 7000 FT MSL AS I WAS CLBING 1000 FPM. THE CTLR TOLD ME HE WAS SEEING NOTHING ON HIS RADAR. NOW THE AIRPLANE IS ALL OVER THE SKY; IT WAS POURING DOWN RAIN AND I WAS IN THE WORST TURB I'VE EVER BEEN IN; IN MY 18 YRS OF FLYING. IT WAS EVERYTHING I COULD DO TO KEEP THE WINGS LEVEL. MY ALT WENT FROM 7000 FT MSL TO 8000 FT MSL AND DOWN TO 6000 FT MSL IN A MATTER OF SECONDS. ZME HANDED ME OFF TO BNA APCH; AND THE CTLR WAS ABLE TO SEE THE WX I WAS IN AND VECTORED ME AWAY FROM IT. PROB: 1) I DID NOT HAVE ANY WX AVOIDANCE EQUIP ON THE ACFT. 2) THE CTLR AT ZME COULD NOT SEE THE WX ON HIS RADAR. CORRECT THE SIT: 1) PUT WX AVOIDANCE EQUIP ON THE ACFT. 2) UPDATE EVERY ATC RADAR SO THAT CTLRS CAN PICK UP WX.