FLC OF MLG MISSED TURN AT 1500 MSL PER SID WHILE DEPARTING LGB.

Narrative

OUR CLRNC AFTER TKOF WAS TO FLY RWY HDG TO 1500 FT THEN TURN L TO 180 DEGS MAGNETIC (NOT SURE OF EXACT HDG) AND CLB TO 4000 FT. WE WERE CLRED FOR TKOF AND IMMEDIATELY ENTERED RAIN AND TURB. WX RADAR SHOWED RETURNS ON ALL SIDES WITH PURPLE DOPPLER RETURNS THROUGHOUT. THE CAPT AND I WERE BOTH OBSERVING WX RADAR AND COAST APCH WAS VERY 'BUSY.' WE REMAINED ON RWY HDG TO 4000 WHEN COAST ASKED US IF WE HAD STARTED OUR TURN. AT THIS POINT REALIZING OUR MISTAKE I IMMEDIATELY TURNED TO OUR ASSIGNED HDG. I BELIEVE WE MISSED MAKING OUR ASSIGNED TURN AT 1500 DUE TO HVY WORKLOAD IN COCKPIT AT THE TIME ALONG WITH FOCUSING OUR ATTN ON RADAR FOR WX AVOIDANCE. ADDITIONALLY; I HAD BEEN USED TO HEARING 'MAINTAIN RWY HDG FOR VECTORS' AND WHEN WE RECEIVED THIS PARTICULAR CLRNC; THE TURN AT 1500 WAS A TRAP THAT WE FELL INTO. SOMETIMES; I BELIEVE; WHEN YOU GET USED TO A 'PATTERN' AND YOU GET THROWN A 'CURVE' IT'S EASY TO MISS. IN SUMMARY; OR TO RECAP; I THINK THAT 3 FACTORS AIDED US IN OUR MISTAKE. TURB AND WX CAUSED US TO FOCUS MORE ON RADAR SCREEN THAN TO FLYING THE ACFT. HIGH VOLUME OF COMS IN THIS PHASE OF OUR FLT HELPED ADD TO OVERALL 'STIMULUS' IN COCKPIT. THE TURN AT 1500 WAS A SLIGHT CHANGE IN MY MIND FROM EITHER 'MAINTAIN RWY HDG' OR A TURN IMMEDIATELY AFTER TKOF THAT WE ARE MORE NORMALLY ACCUSTOMED TO. THE TIME TO CLB TO 1500 GAVE US A CHANCE TO FORGET MAKING OUR TURN -- AND WE DID.