Date: 201705
Local time of day: 1801-2400
Location: ZDV.ARTCC, CO
Flight conditions: Marginal
Weather elements / visibility: Thunderstorm; Turbulence
Aircraft: Medium Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZDV
ATC / advisory: Center ZDV
Second aircraft: Large Transport
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Workload; Situational Awareness; Training / Qualification
Second reporter function: Handoff / Assist; Enroute
Anomaly: ATC Issue All Types; Airspace Violation All Types; Conflict Airborne Conflict; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Separated Traffic; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Company Policy; Airspace Structure; Human Factors; Staffing; Weather
Primary problem: Weather

Narrative: I was working Sector 25R combined with Sector 65 during busy and complex traffic for nearly 2 hours with no D side. It is an arrival sector and we had multiple shutoffs to Denver airport during that time. I issued multiple reroutes and had the majority of aircraft deviating for weather in my sector and adjacent sectors. Aircraft Y flew through my sector at FL370 and was handed off to Sector 30 eastbound and deviating for weather. After the aircraft left my airspace I dropped the data block.Aircraft X came from Sector 5 headed eastbound to Sector 30 at FL370. The aircraft was in handoff status to Sector 30 for approximately 7 minutes. Aircraft Y deviated back toward my airspace; but I had not been shown a data block or received any coordination. The conflict alert went off when Aircraft X was about 2 minutes from the boundary. I called Sector 30 and told him that I would turn Aircraft X 15 degrees right. He said I didn't need to; that he would descend Aircraft Y. I turned Aircraft X when I didn't see Aircraft Y descend in a timely manner. Aircraft Y began a slow descent; but when the aircraft reached FL364 and was on the edge of the bubble it began to climb again to FL370 when we had less than 5 miles.We were very short staffed and didn't have enough staffing to open up my D side or split off Sector 65. The supervisor took 4 or 5 people back to have a 'break out session'. The supervisor made the comment during breakout that he knew there wasn't enough staffing to have them out; but he was proving a point to the evening supervisor because he approved shift changes that were outside of his shift. I feel that safety was compromised due to supervisors disagreeing with one another and trying to get even. Staffing has been falling below numbers a lot lately and we very rarely get overtime called in to help. Now that summer weather patterns have begun this is going to be a serious issue soon.We need more staffing and we need supervisors to make better choices.


Second narrative: The Radar Controller; who was training; was working aircraft Y heading northwest into some weather near Denver. Aircraft Y asked to deviate left or west and was permitted to do so. Aircraft Y was getting close to Sector 25 and I had not pointed him out to Sector 25 yet. Aircraft Y turned further to the left and the Radar Controller saw the traffic (aircraft X who was being worked by Sector 25) and turned aircraft Y 20 degrees right for traffic.After both his trainer and I said that that was not going to be enough; he initiated a descent to Aircraft Y to FL350. At that time; Aircraft Y and Aircraft X started flashing and Sector 25 called me telling me that they were turning Aircraft X to the right 15 degrees. I told them that we were taking Aircraft Y down to FL350; but we would turn Aircraft X back. When I hopped back off the line; I heard Aircraft Y stating that he did not want to descend because of weather and would rather take the turn. The Radar Controller climbed Aircraft Y back up to FL370 and turned him 20 degrees to the right (the original plan that I knew was not going to work). I told him that Sector 25 had turned Aircraft X 15 degrees right and that they were shipping him. There was nothing else I could do as 25 had shipped Aircraft X. I did not think that it was going to work at that time; but it was too late as they were already too close.The Radar Controller measured it and said that separation was not lost and so I stopped writing down the call signs and went on with the business of the sector. I did not find out that there was a loss of separation until [the next day;] when my supervisor told me.The trainer for the R side should have never allowed the trainee to climb the Aircraft Y back up! When the loss of separation occurred; the Aircraft Y was climbing back up into Aircraft X. If he had just kept him descending; there would not have been a loss in separation. When I went off line to talk with Sector 25; there was a plan in place that would have worked! When I came back to the sector after coordination; the R side ruined the plan!


Synopsis: Center Controllers reported a loss of separation between two aircraft deviating for weather.