ALTDEV DURING DEP FROM ARPT.

Narrative

AFTER DEP WE WERE CLRED TO MAINTAIN 6000 FT VIA THE RIVER 5 DEP. THE TWR ASSIGNED RWY HDG RATHER THAN THE 085 DEG RADIAL AS THE DEP CALLS FOR. AFTER CONTACTING DEP; A HDG OF 140 DEGS WAS ASSIGNED AND WAS BEING FLOWN TO AN ALT OF 6000 FT. AS WE APCH 6000 FT DEP INFORMED US OF TFC 12 O'CLOCK. I CALLED OUT OUR ALT TO THE FO; THE PF; AND I STARTED TO LOOK OUTSIDE FOR THE TFC. AS I SPOTTED THE TFC ACFT; I REALIZED AS DID THE FO AND ATC THAT WE HAD DRIFTED OFF OUR ALT. WE MADE IMMEDIATE; POSITIVE CORRECTIONS; WHICH INCLUDED PWR REDUCTIONS AND LOWERING OF THE NOSE OF THE ACFT. THIS WAS FOLLOWED BY ATC NOTIFYING US OF THE ALTDEV. ATC ASSIGNED A HDG OF 090 DEGS WHICH WAS EXECUTED. TFC PASSED WELL CLR OF US BY 1-2 MI HORIZLY. BOTH ACFT HAD VISUAL CONTACT WITH EACH OTHER. SHORTLY THEREAFTER; ATC RETURNED US TO A HDG OF 140 DEGS WITH A CLB TO 10000 FT. ATC MADE NO OTHER COMMENTS CONCERNING THE SIT. SOME OF THE CONTRIBUTING FACTORS WERE WX RELATED. WITH THE OVCST SKIES; LACK OF VISUAL HORIZON; AND LOOKING FOR A WHITE ACFT AGAINST AN OPAQUE BACKGND; THE NORMAL VISUAL CUES TO BEING LEVEL OR CLBING WERE NOT READILY APPARENT. ALL ACTIONS BY THE CREW TO CORRECT THIS DEV WERE DONE WITH IMMEDIATE ACTION BY THE CREW. SUPPLEMENTAL INFO FROM ACN 289174: AFTER CONTACTING DEP CTL; WE WERE INSTRUCTED TO TURN R 140 DEGS AND CLB AND MAINTAIN 6000 FT. I THEN EXECUTED TURN TO 140 DEGS AND CLB TO 6000 FT. JUST PRIOR TO REACHING THE ASSIGNED ALT; WERE INSTRUCTED BY DEP CTL TO LOOK FOR TFC; A CITATION JET; AT 12 O'CLOCK AT 7000 FT. AFTER POSITIVE CONTACT WITH TFC; I REALIZED THAT I WAS OFF OF THE ASSIGNED ALT AND TOOK APPROPRIATE; IMMEDIATE ACTION AND EXPEDITED A DSCNT BACK TO THE ASSIGNED ALT.