SMT FLC RELYING ON RADAR TO CIRCUMNAVIGATE LINE OF TSTMS EXPERIENCES HVY RAIN; SOME HARD BUMPS AND ALTDEVS.

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WE WERE RETURNING TO AZO FROM 2I4. I HAD BEEN BRIEFED BY DAYTON FSS REGARDING TSTMS BTWN FWA AND AZO ACROSS OUR RTE OF FLT. EXACT LOCATIONS WERE NOT KNOWN DUE TO RAPID MOVEMENT AND DEVELOPMENT. I DECIDED TO DEPART BOLTON AND MAKE AN AIRBORNE DECISION AS WE APCHED FWA. I WOULD DO ONE OF THE FOLLOWING; 1) IF THE WX LOOKED OK; WE WOULD PROCEED AS PLANNED TO AZO FROM FWA DIRECT TO AZO. 2) IF THE WX PATTERN ALLOWED; WE WOULD DEV AROUND THE STORMS TO CIRCUMVENT THE SYS; OR 3) WE WOULD LAND AT FWA TO RECHK THE WX AND/OR TO WAIT UNTIL THE SYS PASSED BY OUR RTE OR DISSIPATED. AS WE APCHED FWA OUR AIRBORNE RADAR SHOWED A LINE OF TSTMS APPROX 20 NM PASSED FWA. WE WERE TOLD BY INDIANAPOLIS FLT WATCH THAT THE LINE WAS 15 MI WIDE. THIS IS ALSO WHAT OUR RADAR INDICATED. WE DECIDED TO DEV TO THE W TO CIRCUMVENT THE WX. THIS LOOKED GOOD TO US. ACCORDING TO ORU RADAR IT LOOKED OK TO PROCEED DUE W UNTIL DIRECTLY S OF OXI VORTAC AND THEN PROCEED TO OXI; THEN DIRECT TO SBN VORTAC; THEN DIRECT TO AZO. WE ASKED FOR THIS ROUTING AND FWA RAPCON APPROVED THE CLRNC. AFTER BEING HANDED OFF TO CHICAGO CTR; AND STILL HDG W; WE NOTICED ON OUR RADAR THAT THE LINE N OF US SEEMED TO BE BREAKING UP WITH GAPS BTWN THE CELLS OF LIGHT TO MODERATE PRECIPITATION. WE MADE A DECISION TO TURN TOWARD OXI VORTAC TO PENETRATE THE WX IN WHAT WE THOUGHT WOULD BE ONLY LIGHT TO MODERATE RAIN. WE ENTERED THE AREA AND EXPERIENCED MODERATE TO HVY RAIN; A FEW HARD BUMPS; AND STRONG CONVECTION. THE CONVECTION CAUSED OUR INDICATED ALT TO VARY +/- 700 FT. I AM UNABLE TO KNOW EXACTLY HOW MANY FT THIS ACTUALLY CAUSED US TO DEPART FROM OUR ASSIGNED ALT OF 10000 FT (MSL). ATC NEVER QUESTIONED OUR ALT. WE USED SMOOTH PITCH CHANGES AND LARGE PWR CHANGES TO FLY TO MAINTAIN OUR ASSIGNED ALT. WE EXPERIENCED THESE ADVERSE CONDITIONS FOR ONLY A FEW MINS BEFORE EXITING THE SYS TO NORMAL FLT CONDITIONS. THE FLT WAS UNEVENTFUL FROM THAT POINT TO AN ILS APCH AND LNDG AT AZO. IN RETROSPECT; OF COURSE; I SHOULD HAVE MADE THE DECISION TO LAND AT FWA TO REASSESS THE SITUATION AND PROBABLY WAIT A FEW HRS BEFORE DEPARTING. HAD WE NOT HAD RADAR I KNOW THIS WOULD HAVE BEEN THE CASE. BUT OUR RELIANCE ON RADAR 'SUCKED' US INTO SINCERELY BELIEVING WE COULD PENETRATE A 'SOFT' RAIN AREA ONLY. 'THINKING' IT IS OK IS NOT GOOD ENOUGH IN AVIATION. PLTS NEED TO KNOW.