The incident was classified as a "loss of separation" event, indicating the two aircraft breached the minimum safety distance required by air traffic control.
The Boeing 737-700 and Eurocopter were reportedly separated by just over half a mile at their closest proximity, leading the Southwest plane to stop its descent and turn right.
The medical helicopter, operated by Metro Aviation, was en route to a nearby hospital to collect a patient but had no patients onboard at the time.
The National Transportation Safety Board (NTSB) has launched an investigation into the incident, which occurred amid concerns over the federal government shutdown and air traffic control understaffing.