PAX AND CABIN CREW INJURED DURING ENCOUNTER WITH TURB.

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WHILE ENRTE AT CRUISE AT 15000 FT TURB WAS ENCOUNTERED. THERE WERE NO SIGMETS FOR THIS ALT OR AREA. THERE WAS NO CONVECTIVE ACTIVITY OR PIREPS FOR TURB. THERE WAS A SIGMET FOR OCN/SEVERE TURB BELOW 11000 FT IN THE JFK AREA BUT NOTHING FOR THE AREA WE WERE FLYING IN. ATC PASSED ALONG 1 PIREP FOR LIGHT TO MODERATE TURB IN AN AREA S OF OUR RTE AND LOWER. CONDITIONS WERE SMOOTH BEFORE OUR ENCOUNTER. BASED ON THE PIREP THE SEAT BELT SIGN WAS ON AND THE PAX WERE INSTRUCTED TO CHK THE SECURITY OF THEIR SEAT BELTS. THE ACTUAL TURB ENCOUNTER WAS MODERATE TO SEVERE. THE AUTOPLT WAS ENGAGED AND STAYED ENGAGED. THE INITIAL ENCOUNTER LASTED LESS THAN 60 SECONDS. WE ENCOUNTERED AT LEAST 5 DEGS NOSE DOWN AND UP PITCH CHANGE PLUS 30 DEGS OF BANK. THERE WAS NEVER LOSS OF CTL OF THE ACFT. AFTER THE INITIAL CONTACT; TURB WAS ONLY LIGHT TO MODERATE. THE FLT ATTENDANT INFORMED US THAT SHE HIT HER HEAD ON THE ROOF OF THE CABIN. SHE ALSO STATED THAT 2 PAX WERE INJURED. WE DECLARED AN EMER AND LANDED IN PVD. THE 2 INJURED PAX AND FLT ATTENDANT WERE TRANSPORTED TO LCL HOSPITAL. IT IS MY UNDERSTANDING THAT ALL 3 WERE RELEASED WITHOUT SERIOUS INJURY. I THINK THE CAUSE OF TURB WAS LARGE TEMP CHANGES ASSOCIATED WITH THE BACK SIDE OF A VERY WEAK UNFORECAST FRONT. HAD WE KNOWN OF THE SEVERITY OF THE TURB WE COULD HAVE 1) TRIED TO AVOID IT; 2) SPECIFICALLY INSTRUCTED THE FLT ATTENDANT TO BELT HERSELF IN. WE DID NOT ANTICIPATE THIS MUCH TURB AND ALSO ASSUMED THAT MY ANNOUNCEMENT TO THE PAX WOULD HAVE INDICATED TO THE FLT ATTENDANT TO RETURN TO HER SEAT AND BUCKLE IN.