DEPARTING EMB120 HAS A CONFLICT WITH A PRECEDING C-150.

Narrative

CLBING OUT ON THE FREES SID APPROX 3 TO 5 MI W ON THE 202 DEG RADIAL AND OUT OF APPROX 2500 TO 3000 FT; I NOTICED A CESSNA 150 AT OUR 1 O'CLOCK POS ON A COLLISION COURSE WITH OUR ACFT. I IMMEDIATELY ROLLED THE ACFT INTO AN APPROX 90 DEG L DSNDING BANK TO AVOID THE COLLISION WITH THE CESSNA THAT WAS BTWN 1/8 TO 1/4 OF A MI FROM US. MY FO ASKED THE TWR CTLR IF SHE KNEW OF ANY ACFT IN OUR VICINITY. SHE REPLIED SHE DID NOT. THE CESSNA PLT THEN CAME ON TWR FREQ AND REMINDED THE CTLR THAT HE WAS CLRED TO DEPART RWY 19 STRAIGHT OUT. IN MY OPINION. NO ONE PERSON WAS AT FAULT. HOWEVER; THE SIT MAY NEVER HAVE ARISEN IF THE CTLR HAD WARNED US THAT THE ACFT HAD DEPARTED IN THE SAME DIRECTION WE WERE HDG. THE WX WAS MARGINAL TO THE W OF THE ARPT DUE TO HVY RAIN SHOWERS; WE WERE BUSY USING THE RADAR TO PLAN A COURSE AROUND THE WX AND FLYING THE SID WHEN THE INCIDENT HAPPENED AND HAD NOT YET BEEN HANDED OVER TO ZOA. PROBS LIKE THIS ARE VERY COMMON IN STS. THE MIX OF HIGH SPD COMMUTER ACFT AND SMALL GA ACFT ARE; IN MY AND MANY OF MY COLLEAGUES OPINIONS; A TRAGEDY WAITING TO HAPPEN. IT WOULD SEEM THAT THE INSTALLATION OF RADAR IN THAT CTL TWR WOULD HELP THIS SIT GREATLY. INSPECTOR WITH THE FAA WAS IN THE JUMPSEAT CONDUCTING AN ENRTE INSPECTION AT THE TIME.