Skyhawk 172/Cutlass 172
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 1
Situational Awareness; Distraction
ATC Issue All Types; Conflict NMAC
Human Factors; Procedure
Runway 16L and 16R were in use. I departed Aircraft X from 16L and departed Aircraft Y from 16R. Aircraft X was IFR on the Aersnal4 SID and Aircraft Y was VFR and was remaining in right closed traffic. I didn't think their flight paths would ever cross. Aircraft Y was in right closed traffic to runway 16R; which is the shorter runway. A normal rectangular pattern would have kept his turns inside of Aircraft X. Due to an increase in traffic volume in a short amount of time; I did not notice that he was flying a wider than normal pattern. When Aircraft X turned right his flight path conflicted with Aircraft Y; which was in the crosswind turn. When I noticed the conflict the aircraft were separated by 300 hundred feet. Aircraft X was no longer on my frequency. I immediately told Aircraft Y to enter the downwind; as it appeared he was still in a wide crosswind turn; to de-conflict their flight paths; however; I did not preface the call with 'Traffic Alert'.I would recommend emphasizing the importance of standard traffic pattern legs to the flight schools on the airport. If they want to fly a wider pattern than normal to please advise the controller so it can be expected.
HEF Tower Controller reported of a Near Mid Air Collision between two aircraft under his control. Controller issued traffic; but did not use the statement; 'Traffic Alert.' Reporter did not think aircraft would get close.