A B727 CAPT DESCRIBED HIS ENCOUNTER WITH SEVERE TURB IN THE VICINITY OF BVT.

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B727 ENCOUNTERED SEVERE TURB AT FL310 ENRTE FROM SDF TO RFD. OUR RTE OF FLT WAS SDF-IIU-V53-STREP-BVT-EDN-DPA-RFD. THERE WERE CONVECTIVE SIGMETS ISSUED FOR OUR RTE OF FLT AND WE DEVIATED AROUND WX AS NECESSARY TO REMAIN CLR OF THE TSTMS. WE WERE IMC AT FL310; SLOWED TO 280 KIAS FOR LIGHT TURB WE WERE ENCOUNTERING AND THE AUTOPLT WAS ENGAGED. THE ACFT ENCOUNTERED SEVERE TURB ABOUT 35 MI W OF BVT VOR AT THIS FLT LEVEL AND SPD. THE ACFT GAINED AND LOST APPROX 700-800 FT WITHIN 5-7 SECONDS. THE CREW AND THE ACFT EXPERIENCED VERT LOADS OF 2.5G TO -1.0G. THE AUTOPLT DISCONNECTED ITSELF AUTOMATICALLY AND ACFT RECOVERY WAS MADE BY THE CAPT. WE REMAINED IN SEVERE TURB FOR ABOUT 2-3 MINS EXPERIENCING THE SAME GAIN AND LOSS OF ALT AND VERT LOADS FOR THE SAME AMOUNT OF TIME. ATC WAS ADVISED OF OUR SIT OF ENCOUNTERING THE SEVERE TURB. WE CONTINUED TO DEVIATE AROUND WX FOR ANOTHER 45 MI (65 MI NE OF BVT) THEN PROCEEDED DIRECTLY TO RFD. THE REST OF THE FLT WAS UNEVENTFUL. THE WX RADAR WAS NOT PAINTING (DISPLAYING) ANY WX ON THE SCREEN WHEN WE ENCOUNTERED THE SEVERE TURB.