LGT ENCOUNTERS SEVERE TURB; DECLARES AN EMER AND DSNDS TO FL290.

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SHORTLY AFTER DEPARTING SFO; WE RECEIVED AN ACARS MESSAGE FROM DISPATCH REGARDING MODERATE TURB FL310-FL350 OVER TULSA. WE WERE AT CRUISE ALT FL330. WE AGREED WITH DISPATCH ON CLBING TO FL370 W OF THE AREA AND TRANSITING AT FL370. THIS WAS BASED ON DISPATCH AND RPTED PIREPS INDICATING THE RIDE IMPROVED ABOVE FL350. THIS WAS ALSO CONCURRED WITH ATC DURING OUR NUMEROUS RIDE INQUIRIES. ALL WERE OCCASIONAL LIGHT OR 'NO RPT OF TURB.' AS A PRECAUTION; AND BEING IN CONTACT WITH DISPATCH WITH THE RPTS OF LIGHT TO POSSIBLE MODERATE TURB; I HAD THE FLT ATTENDANTS FINISH THEIR SVC EARLY AND STOW THE GALLEYS. FLT WAS SMOOTH AT FL330/370 UNTIL 20 MI N OF TULSA; WHEN WE SUDDENLY ENCOUNTERED SEVERE TURB FOR APPROX 1.0-1.5 MINS. AIRSPD PLUS OR MINUS 30-35 KTS; ALT PLUS OR MINS 800 FT. FO; PF; DID A GOOD JOB MAINTAINING ATTITUDE; AUTOTHRUST WAS DISCONNECTED AT THIS TIME. AFTER XMITTING OUR SIT TWICE TO ATC; I DECLARED AN EMER SINCE OUR RECEPTION WAS UNREADABLE DUE TO STATIC INTERFERENCE. I STATED WE WERE DSNDING IMMEDIATELY TO FL290 WHERE WE EXPERIENCED LIGHT TURB. AFTER CONNECTING WITH ATC; I GAVE PIREP AND ASKED LEAD FLT ATTENDANT STATUS OF CABIN -- NO INJURIES; NO ACFT DAMAGE. OPINION OF FLC; G-FORCES DID NOT EXCEED +2.5/-1. NO RADAR RETURN THROUGHOUT ENTIRE FLT. I'M CONVINCED THAT WHAT AVAILABLE INFO DISPATCH HAD AND RELAYED TO ME; BECAME VERY EFFECTIVE IN ELIMINATING ANY INJURIES; SINCE CABIN WAS SECURE AND PAX SEATED. THIS TURB WAS OBVIOUSLY UNKNOWN AND UNFORECAST BY METEOROLOGY DISPATCH AND ATC OR ELSE WE WOULD NOT HAVE BEEN ANYWHERE NEAR THIS LEVEL OF INTENSITY.