ENCOUNTERED STRONG UPDRAFT AND PENETRATED THE FLOOR OF THE TCA.

Narrative

I WAS FLYING FROM MINNEAPOLIS CRYSTAL ARPT TO D97 VIA STP. AFTER LEAVING CRYSTAL ATA I LEVELED OFF AT 2200' MSL AND RPTED MY POS TO STP AND REQUESTED TRANSITION TO FLEMING. TRANSITION WAS APPROVED AT 2200'; WHICH IS NORMAL. I THEN CHKED THE INS AND DISCOVERED THE ACFT WAS CLBING AT ALMOST 1000' FPM AND MY ALT WAS NEARING 2400' MSL. THE FLOOR OF THE TCA IS 2300' MSL. MY ACFT HAS A FIXED PROP AND THERE WAS NO RPM REDUCTION OR REDUCTION IN AIRSPD AS IN A NORMAL CLB. I PUSHED THE NOSE OVER AND REDUCED PWR; EVENTUALLY TO IDLE STOP; BUT I ONLY COULD GET A 300 FPM DSCNT AT CRUISE AIRSPD. I BELIEVED THAT I WAS CAUGHT IN AN UPDRAFT AND WAS AFRAID OF DIVING THE ACFT THAT CLOSE TO THE GND 1500' AGL. I MUST HAVE FLOWN OUT OF THE UPDRAFT AFTER A COUPLE OF MINS BECAUSE THE INS SUDDENLY INDICATED A VERY FAST DSCNT. I SLOWED THE DSCNT TO 500 FPM THEN LEVELED AT 2200'. I DID NOT RPT THIS INCIDENT TO STOP TWR. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR CONFIRMED CONDITIONS WERE VERY UNCOMFORTABLE DUE TO THE TURB AND XWINDS. EXCURSION WAS ONLY MOMENTARY AND RETURN TO ALT WAS IMMEDIATE. HAD FLOWN THIS RTE MANY TIMES AND KNEW IT WAS THE PREFERRED METHOD OF FLYING TO FLEMING. NO FOLLOW-UP BY THE FAA.