AT LTT AND AN SMA HAD AN NMAC.

Narrative

WE ARRIVED INTO THE PSP AREA FROM OUR DEP ARPT (LAX). HAD DSNDED OVER THE MOUNTAINS MAKING OUR BASE LEG TO RWY 31L AT PSP. WE HAD JUST SWITCHED FROM APCH TO TWR. ADVISED TWR WE WERE ON L BASE LEG FOR 31L. THEY INSTRUCTED US TO IMMEDIATELY TURN L AND LEVEL OFF DUE TO A TWIN SMA IN OUR 12 O'CLOCK POS. AS THE TFC WAS CALLED; I SAW THE TFC AND YELLED TO THE CAPT TO TURN HARD L. AT THAT POINT HE SAW THE TFC AND CONTINUED THE HARD L TURN AND DROPPED THE NOSE DOWN TO AVERT THE COLLISION. AT THAT POINT WE WERE APPROX 7 MI TO THE L OF 31L CTRLINE. THE TWIN SMA BANKED TO THE R AND CLBED. I BELIEVE OUR PATHS CROSSED -- THE TWIN SMA ABOVE US AND US BELOW THE SMA. I BELIEVE THAT IF THE TWR HAD NOT ADVISED US WE WOULD HAVE COLLIDED WITH THE TWIN SMA. FACTORS IN THIS SIT: I FEEL THAT; WITH OUR VERY STEEP DSCNT ON BASE LEG OVER THE HIGH TERRAIN; WE COULD NOT SEE THE SMA IN OUR NOSE LOW ATTITUDE. ALSO; APCH NEVER ADVISED US OF THE TFC LINING UP FOR THE SAME RWY AS WELL AS WE WERE JUST BEING HANDED OFF TO THE TWR. THIS INCIDENT SURE HAS ME LOOKING OUT THE WINDOW A LOT MORE NOW.