Light Transport; Low Wing; 2 Turbojet Eng
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 5.0
Situational Awareness; Communication Breakdown; Workload
ATC Issue All Types; Conflict Airborne Conflict; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Issued New Clearance; Air Traffic Control Separated Traffic
Procedure; Weather; Airport; Airspace Structure; Company Policy; Human Factors
Aircraft X was level at 12000 feet direct TOXEE waypoint. An unknown aircraft less than 6 miles in front of and slower than Aircraft X was also direct TOXEE issued a descent to 13000 feet. I observed unknown aircraft descend below 13000 feet and alerted the Radar Controller. Conflict alert went off and the Radar Controller attempted to expedite the unknown aircraft to 11000 feet. I observed a closest proximity of 4.9 miles. Very complex traffic with bad weather. Due to heavy traffic volume and bad weather the traffic began to overwhelm the Radar Controller's and my ability to give control instructions due to frequency congestion both air to air and air to ground.This happens every year. KSUN is a popular destination. The airport and airspace is dangerous during these times. It is a matter of time; not chance; that aircraft collide with either terrain or each other. KSUN should set a maximum operations a day and hour limitation. During times where approaches are conducted arrivals should be spaced a minimum of 10 minutes apart or in VMC 5 minutes. To prevent collision of opposite direction aircraft; arrivals should only be allowed during the first half of the hour; departures the second half. This may seem like overkill but this is not a suggestion it is a must. Lives are at risk and it is not taken seriously by anyone other than the controllers.
A Center Controller reported an unknown aircraft descended into the sector with less than the required 5 miles lateral separation.