ATC VOLUNTEERS THAT SMA HAS A CELL AT 12 O'CLOCK AND OFFERS A VECTOR. SMA OVERSHOOTS ASSIGNED ALT.

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IN IMC WE EXPERIENCED INCREASING TURB. A FEW SECONDS LATER; PRECIPITATION STARTED WITH INCREASING INTENSITY. CALLED ZAU ASKED FOR AND RECEIVED CLRNC TO 3000 FT TO PRECLUDE THE POSSIBILITY OF ICE. CTR QUICKLY CAME BACK AND ADVISED US TO SWITCH TO CHICAGO APCH BECAUSE 'THEY HAVE BETTER RADAR FOR CELLS.' AS WE WERE DSNDING; I CONTACTED APCH IMMEDIATELY; HE SUGGESTED A HDG OF 270 (FROM 350 FT) AS HE SHOWED A LEVEL 2 JUST N OF US. WE IMMEDIATELY INITIATED A L TURN AS WE CONTINUED OUR DSCNT. TURB AND PRECIPITATION WERE NOW MODERATE-SEVERE AND VERY HVY. AT THIS TIME I WAS UNCOMFORTABLE THAT 270 DEGS WAS GOING TO HELP US AWAY FROM THE CELL AND SO I ASKED FOR A 180 DEG BACK AWAY FROM IT. AS I RADIOED CTLR; WE WERE JUST PASSING THROUGH 3300-3400 MSL. HE APPROVED OUR REQUEST IMMEDIATELY AND SO WE COMMENCED OUR L TURN 180 DEGS THROUGH VERY HVY TURB. APPROX MIDWAY THROUGH THE TURN; I SCANNED THE GAUGES AND SAW THE ALTIMETER RACING PAST 3000 FT AND VSI NEARLY PEGGED. EVEN WITH FULL PWR APPLIED; THE SEVERE DOWN DRAFT FORCED THE ACFT TO 2500 MSL BEFORE STOPPING AND THEN EVEN THOUGH WE HAD CLB PWR AND AIRSPD; IMMEDIATE CORRECTIVE ACTION WAS TAKEN. CHICAGO APCH VECTORED US AROUND THIS CELL AND FLT PROCEEDED WITHOUT INCIDENT.