AN IFR C210 PVT PLT GETS EMBROILED WITH A TSTM WHILE IMC AT 5000 FT; LOSING CTL AND ALT TO 4000 FT BEFORE EXITING THE CELL.

Narrative

I WAS FLYING IFR FROM LONGVIEW; TX; TO ONEIL; NE. I HAD CHKED WITH FLT SVC SHORTLY BEFORE AND WAS TOLD THERE WERE NO CONVECTIVE SIGMETS ALONG MY RTE AND NO CONVECTIVE ACTIVITY DETECTED. I HAD ASKED FOR A DEV W OF THE RTE TO SKIRT SOME BUILDUPS; THAT I COULD SEE. UPON TURNING BACK TOWARD MY COURSE I WENT BACK INTO IMC. APPROX 10 MINS LATER I ENCOUNTERED WHAT APPEARED TO BE A STORM CELL WITH HVY TURB AND RAIN. I ASKED FOR A VECTOR AROUND THE WORST OF IT AND CTR RESPONDED THEY COULDN'T HELP AS THEY COULDN'T SEE ANY PRECIPITATION. I THEN ASKED TO LEAVE THE FREQ AND CHK WITH FLT SVC. THEY SAID TO FLY STRAIGHT N AND I SHOULD FLY OUT OF IT IN A FEW MI. DURING THIS TIME I COULD NOT MAINTAIN ALT. I SLOWED THE ACFT TO MANEUVERING SPD AND LOST ALMOST 1000 FT. MY ASSIGNED ALT WAS 5000 FT. AS SOON AS I WENT BACK TO CTR I RPTED THIS TO THEM. SHORTLY AFTER THIS I CAME OUT THE N SIDE OF THE CELL INTO SPORADIC VFR CONDITIONS AND CLBED BACK UP TO 5000 FT. I WAS THEN HANDED OFF TO ANOTHER CTLR AND HE GAVE ME A NEW ALTIMETER SETTING OF 29.90 WHICH I REPEATED BACK TO HIM AS IT WAS MUCH DIFFERENT FROM MY PREVIOUS SETTING. SHORTLY AFTER THIS HE RPTED ME TO BE 600 FT LOW. I AGAIN REPEATED MY ALTIMETER SETTING OF 29.90 AND HE SAID IT SHOULD BE 29.41. I RESET THE ALTIMETER AND CLBED BACK TO THE PROPER ALT OF 5000 FT. MY RECOMMENDATION TO HELP PREVENT A RECURRENCE IS TO EQUIP CTR WITH WX RADAR. THIS WOULD HELP RATHER THAN TRYING TO GO BACK AND FORTH BTWN CTR AND FLT SVC WHEN WX CONDITIONS ARE CHANGING RAPIDLY. ALSO ABOUT 1 MIN BEFORE I ENCOUNTERED THE STORM CELL; CTR ISSUED A CONVECTIVE SIGMET THAT INCLUDED THE AREA I WAS FLYING IN BUT IT WAS ISSUED TOO LATE TO BE OF ANY HELP TO ME. IF THERE IS ANY WAY TO SPD UP THE PROCESS OF ISSUING THESE CONVECTIVE SIGMETS WHEN THE WX IS CHANGING THAT MAY ALSO HELP. ALSO I BELIEVE THAT WHEN I READ BACK THE NEW ALTIMETER SETTING OF 29.90 THAT IF THIS WERE NOT THE CORRECT SETTING THE CTR CTLR SHOULD HAVE REPEATED THE PROPER ALTIMETER SETTING BACK TO ME.