WHILE CLBG TO FL 350, USAIR FLT 95 ENCTRD CONT 'LIGHT CHOP,' SO CREW LVLD AT FL 310 & LEFT THE SEAT BELT SIGN ON. THEY INQUIRED ABOUT TURBC AHEAD & WERE ADZD OF RPRTD LGT/MOD CHOP & 1 ENCTR WITH SVR TURBC AT FL260. CLNC WAS RCVD TO DSCND TO FL 240. DRG DSCNT, THE ACFT ENCTRD SEVERE TURBC. THE CAPT DECLARED AN EMERG & THE DSCNT WAS CONTD TO FL 200. THE CAPTNOTED TURBC LASTED APRX 1.5 MIN BTN FL 270 & FL 210. THERE WERE NO CLDS; THE VIS (AT NGT) WAS UNLIMITED. THE FLT ATTEND-ANTS WERE UNRESTRAINED; 1 WAS SERIOUSLY INJURED & 3 RECEIVED MINOR INJURIES. A SIGMET WAS IN EFFECT WHICH FORCAST MOD TOOCLNLY SEVERE TURBC BTN FL 240 & FL 350 IN AN AREA WEST OF THE ACFT'S FLT PATH. INV SHOWED AN INCONSISTENCY IN THE DIS- SEMINATION OF CENTER WX ADVISORIES (CWA'S) & SIGMETS. ALTHO CWA'S & SIGMETS HAD THE SAME LVL OF PRIORITY FOR ATC & FSS USE, CWA'S WERE NOT DISSEMINATED OVR 604 & 135 WX CIRCUITS. USAIR RECEIVED WX INFO VIA FAA'S 604 & 135 CIRCUITS, SO DID NOT HAVE CWA INFO FOR WX BRIEFINGS. WX STUDY SHOWED TURBC WAS ENCTRD WHERE SUBTROPICAL & POLAR JET STREAMS CONVERGED.