SMA ENCOUNTERED DOWNDRAFTS AND WAS UNABLE TO MAINTAIN ASSIGNED ALT.

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ACFT UNABLE TO MAINTAIN ASSIGNED ALT OF 7000' DUE TO EXTREMELY STRONG DOWNDRAFTS IN MODERATE TURB OVER HILLY TERRAIN APCHING MOUNTAINOUS TERRAIN. REQUESTED LOWER ALT; ATC ADVISED THEY WERE UNABLE TO ISSUE LOWER ALT. ADVISED ATC WE WOULD HAVE TO DECLARE AN EMER BECAUSE WE WERE UNABLE TO MAINTAIN 7000'. ATC REPLIED 7000' WAS MEA FOR THAT AREA. REQUESTED AND RECEIVED VFR ON TOP CLRNC.* ATC SAID THERE HAD BEEN OTHER RPTS OF TURB BELOW 10000' AND ASKED IF WE WANTED TO CLB. WE REPLIED WE WERE UNABLE. REQUESTED ABOUT 30 DEG RIGHT TURN TO GET AWAY FROM APCHING MOUNTAINOUS TERRAIN AND DOWNDRAFT. ATC GRANTED REQUEST. ATC THEN ASKED IF WE WERE STILL IN EMER CONDITION AND WE ADVISED WE WERE OK AND ABLE TO MAINTAIN 6500'. FREQ CHANGE TO SBA APCH AND WE ADVISED ATC WE WERE AGAIN ABLE TO CLB TO 7000'. WE WERE CLRED TO THAT ALT AND CONTINUED ON COURSE. *WE WERE VECTORED ON A ROUTE E OF ROUTE WE WERE ORIGINALLY CLRED ON AND IT TOOK US CLOSER TO MOUNTAINOUS TERRAIN. RECEIVED PREFLT BRIEFING FROM COMPUSERVE (EMI) THERE WERE 2 PIREPS ABOUT TURB NEAR GVO VOR; SO ON ARR AT ARPT I CALLED HAWTHORNE FSS AND ASKED FOR UPDATE ON TURB. BRIEFER'S REMARKS INDICATED THAT TURB WOULDN'T BE A SIGNIFICANT FACTOR (SEE ENCLOSED PRINTOUT). FLT WAS PLANNED FOR ROUNDTRIP SNA-SBP-SNA. GND TIME IN SBP WAS 1 HR; SO DIDN'T GET NEW BRIEFING FOR RETURN FLT DUE TO SMOOTH FLT FROM SNA-SBP (WHICH DID NOT INCLUDE FLYING OVER MOUNTAINOUS TERRAIN). ROUTE CLRNC RECEIVED FOR RETURN LEG WAS E OF INTENDED FLT PLANNED ROUTE AND THAT ROUTE FLOWS TO SBP.