MD80 FLT CREW ENCOUNTERS MODERATE TURB AND PRECIP DURING CLB FROM ORD; DECLARES EMER AND TURNS TO EXIT WX.

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WE LEFT ORD AS USUAL WITH SCATTERED TSTMS TO THE NW AND E. OUR CLRNC WAS WESTERLY VECTORS WITH A CLB TO FL300. THE RADAR SHOWED NO KNOWN HAZARDS ON OUR RTE; BUT AT FL260 ZAU CLRED US FOR A R TURN TO ABOUT 300 DEGS. WE STARTED TO EXPERIENCE AN INCREASINGLY ROUGH RIDE TO A POINT THAT WE WERE IN OCCASIONAL MODERATE CHOP. AT THIS POINT OUR RADAR PICTURE SHOWED TURB AT ALL FORWARD QUADRANTS AND WHAT APPEARED TO BE PATCHY OUTLINES OF A LARGE PRECIP AREA THAT LOOKED TO BE THE REMNANTS OF A LARGE CELL THAT HAD RECENTLY DISSIPATED. THE RIDE CONTINUED TO DETERIORATE AS ATC DIRECTED A FURTHER TURN TO THE N THAT WOULD SEND US DEEPER INTO THE DISPLAYED PRECIP. BY THIS POINT WE WERE EXPERIENCING CONTINUOUS MODERATE TURB WITH EXTENSIVE ST ELMO'S FIRE ON THE WINDSCREEN. I HAD EVERY EXPECTATION AT THIS POINT TO EXPECT SEVERE TURB AHEAD. WE ASKED FOR VECTORS S TO EXIT THE PRECIP AREA AND ATC DENIED THE CLRNC. AT THIS POINT I DECLARED AN EMER WITH ATC AND TOLD THEM THAT WE WERE TURNING TO A HDG OF 240 DEGS TO EXIT THE PRECIP AREA TO AVOID PAX INJURY AND POSSIBLE STRUCTURAL DAMAGE TO THE ACFT. THEY DIRECTED US TO STOP OUR CLB AT FL260 AND WE COMPLIED. ONCE ON THE 240 DEG HDG; IT WAS OBVIOUS THAT THIS NEW HDG WAS STILL NOT ENOUGH TO EXIT THE AREA PROMPTLY SO I TOLD ATC THAT WE WERE COMING FURTHER S TO A 220 DEG HDG. THEY ACKNOWLEDGED THE CALL AND KEPT US A FL260 FOR ANOTHER 30-40 MI TO AVOID TFC OVERHEAD. ONCE WE EXITED THE PRECIP; WE WERE ABLE TO LOOK BACK AT A VERY DEFINED LINE OF CLOUDS THAT HAD BEEN THE CAUSE OF THE TURB. OUR RADAR DID DISPLAY OTHER WX IN THE AREA THAT WE COULD CONFIRM WITH LIGHTNING SO WE KNEW THE RADAR WAS ACTUALLY PERFORMING PROPERLY. THE REMAINDER OF THE FLT WAS UNEVENTFUL.