PA34 PLT IS UNABLE TO LEVEL AT ASSIGNED ALT DUE TO MODERATE TURB AND VARIABLE UP- AND DOWNDRAFTS. CTR CTLR GIVES HIM A CALL AND ISSUES TFC AND HIS PHONE NUMBER FOR A CALL AT DEST ARPT.

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I CHKED IN WITH ZME CLBING THROUGH 3200 FT. I EXPERIENCED LIGHT TURB THROUGHOUT MOST OF THE CLB. PASSING 3500 FT FOR 4000 FT; I STARTED TO ENCOUNTER LIGHT TO MODERATE TURB WITH HEFTY UP- AND DOWNDRAFTS. THERE WAS A LEVEL 3-4 CELL TO MY 9-10 O'CLOCK POS AND 1/4 - 1/2 MI. I WAS BEGINNING TO LEVEL AT 4000 FT BUT DUE TO THE ADVERSE CONDITIONS AFFECTING THE AIRPLANE IT WAS DIFFICULT TO MAINTAIN MY ASSIGNED ALT. ZME CALLED AND SAID TO MAINTAIN 4000 FT. I IMMEDIATELY NOTICED THAT I WAS 200 FT HIGHER THAN MY ASSIGNED ALT AND INFORMED THE CTLR THAT I WAS DSNDING BACK TO 4000 FT. AFTER LEVELING AT 4000 FT AND SHORTLY THEREAFTER; ZME CONTACTED ME AND TOLD ME THAT I SHOULD CALL ZME UPON REACHING MY DEST. I CALLED THE SUPVR AT ZME AND HE INFORMED ME THAT THERE MAY HAVE BEEN A POSSIBLE PLTDEV FROM AN ASSIGNED ALT. HE SAID THERE WAS TFC; AN ACR; AT 2.6 MI AND OUR VERT SEPARATION HAD CLOSED TO 500 FT. HE TOLD ME THAT THERE HAD BEEN A VIOLATION OF SEPARATION REQUIREMENTS BTWN MYSELF AND THE TFC. THE REQUIREMENTS FOR SPACING AT THE TIME WERE 1000 FT VERT AND 5 MI HORIZ SEPARATION. I BELIEVE THERE WERE A COMBINATION OF FACTORS THAT LED TO THIS SIT. FIRST; THE FACT THAT THERE WAS A COLD FRONT EXTENDING FROM NE OKLAHOMA; NORTHERN ARKANSAS; WESTERN KENTUCKY; AND CENTRAL OHIO. THIS DIFFERENCE IN AIR MASS WAS CAUSING QUITE A DISPARITY IN ALTIMETER SETTINGS ALONG MY DEP RTE. SECOND; THE WX IN WHICH I WAS TRYING TO CIRCUMNAV (TSTMS AND TURB). AND LAST; ATC'S ASSIGNMENT OF ROUTING AND/OR ALT. GIVEN THE WX CONDITIONS IT MAY HAVE BEEN WISE TO HAVE PROVIDED A GREATER MARGIN FOR SPACING SO AS NOT TO INADVERTENTLY VIOLATE FAA SEPARATION REQUIREMENTS. THE FACT THAT I WAS 200 FT HIGHER THAN MY ASSIGNED ALT OF 4000 FT; THE DIFFERENCE IN ALTIMETER SETTINGS ALONG THE RTE; AND THE POSSIBILITY OF MODE C ERROR; MAY HAVE CAUSED THIS SIT.