AN MD88 DEPARTING SAT; TX; HAS A POTENTIAL CONFLICT WITH A CESSNA TYPE ACFT UNDER IMC CONDITIONS. THE C150-172 HAD DEP FROM RWY 12R AND THE MD88 FROM RWY 3. THE CESSNA HAD A 090 DEG HDG ASSIGNED AND THE MD88 HAD A RWY HEADING ASSIGNMENT. DEP CTLR EXPLAINED THAT THE CESSNA TURNED TOO SOON; BEFORE THE END OF THE RWY.

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AFTER TKOF (SAT) ON RWY 3; WE RECEIVED A TCASII RA 'MONITOR VERT SPD' PASSING 2000 FT FOR 5000 FT. TCASII (RADAR) SCREEN SHOWED RED TARGET AT 1 O'CLOCK POS LESS THAN 1 MI; +200 FT. VSI GREEN ARC COMMANDED A DSCNT AS WE INITIATED AN IMMEDIATE L DSNDING TURN TO AVOID TFC IN IMC CONDITIONS. WE NOTIFIED ATC OF OUR TFC RA EVASIVE MANEUVER AND WERE ABLE TO ACCEPT DEP VECTORS AFTER TCASII 'CLR OF CONFLICT.' WE ESTIMATE THE TARGET PASSED TO OUR R LESS THAN 1 MI AND WITHIN 100 FT OF OUR ALT. BEFORE HDOF FROM SAT DEP CTL (127.1) THEY OFFERED US THEIR SYNOPSIS OF WHAT HAD HAPPENED. (REFER TO SAT 10-9 PAGE ARPT DIAGRAM FOR A BETTER UNDERSTANDING OF EVENTS.) PRIOR TO US; A CESSNA WAS CLRED FOR TKOF ON RWY 12R WITH INSTRUCTIONS TO TURN TO A 090 DEG HDG AFTER AIRBORNE. HIS TKOF ROLL WAS LESS THAN 2000 FT. NEXT; WE WERE CLRED FOR TKOF ON RWY 3 WITH RWY HEADING FOR VECTORS. ATC CONCLUDED THAT SINCE THE CESSNA'S DEP TURN WAS INITIATED EARLY; AT ABOUT 2000 FT FROM THE BEGINNING OF RWY 12R; HIS ACFT PROBABLY CROSSED OUR CLBOUT PATH AND BECAME OUR TCASII RA TARGET AFTER LIFT-OFF. SUGGEST SAT ATC REQUIRE ALL DEPARTING ACFT TO DELAY TURNS AFTER TKOF (WHEN INTERSECTING RWYS ARE USED SIMULTANEOUSLY) TO AVOID THIS DANGEROUS SIT IN THE FUTURE.