Date: 201512
Local time of day: 0601-1200
Location: MDW.Tower, IL
Weather elements / visibility: Snow
Light: Daylight
Aircraft: Light Transport; Low Wing; 2 Turbojet Eng
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Tower MDW
Second aircraft: Large Transport; Low Wing; 2 Turbojet Eng
Reporter organization: Government
Reporter function: Local
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Confusion; Distraction; Human-Machine Interface; Situational Awareness
Anomaly: Conflict Ground Conflict; Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Incursion Taxiway
Detector: Person Air Traffic Control
When detected: In-flight; Taxi
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance; Air Traffic Control Separated Traffic; Flight Crew Executed Go Around / Missed Approach
Contributing factors: Procedure; Airport; Weather
Primary problem: Weather

Narrative: I was working Local Control. I had a relatively full final and a few departures. Outbound ground was taxiing Aircraft X for departure. It had been snowing for a few hours in the morning. The snow had stopped or tapered off; but the vehicles were still cleaning the surfaces. The City had a snow removal team sitting on TT; which would be the normal route from the FBO to any departure runway.Outbound ground taxied Aircraft X via Victor and Mike to hold short of Runway 27L. Taxiway M between Uniform and 27L appeared to have not been plowed yet that morning. It was completely snow covered with a very obvious distinction from where Uniform and 27L had both been cleared; either by a snow team or by the jet blast of arriving aircraft earlier that day. Aircraft X had stopped at the intersection of Mike and Uniform; appearing to be hesitant to taxi through the snow. I reached out to outbound ground to ensure he realized Aircraft X was hesitating. He was aware and Aircraft X did start to move through the snow. I had scanned 27L prior to the arrival of Aircraft Y and Aircraft X did not appear on the runway at that time. I am not sure if the aircraft was; in fact; past the hold short line at the time of my scanning. As previously mentioned; the taxiway was completely covered in snow and the pavement markings were completely obscured. The aircraft may have been past the line at this time; but appeared clear of the runway because it was in the area that had not been plowed and was not obviously over the line from this perspective. The ASDE-X issued an alert 'Runway 27L Go Around.' I complied with the alert and issued Aircraft Y go around instructions.I completed the proper coordination and informed the supervisor so that he could coordinate with TRACON. I crossed Aircraft X and asked him to cross without delay if able; as the next arrival was short final. I reached out to the next arrival and told them to expect a possible go around; as traffic was on the runway. Aircraft X cleared in a timely fashion and the next arrival landed without issue. While the pilot definitely crossed where the hold short lines were painted; those lines were obscured from view of the tower; and I'm assuming from the view of the pilots. I do not know for sure; but would not be surprised if the signs for 27L/9R on Taxiway M north of the runway were also obscured by snow. Not many aircraft would have taxied that way since the ice storm we had [earlier in the week] or even earlier that morning for anyone to report obscured signs; as it [was] not as common of a taxi route as the taxi routes the airlines use on the terminal side. I do not believe 27L had been cleared yet that morning; as the snow team was coming out shortly after to clear it. I believe they were staged on taxiway TT for that reason; blocking Aircraft X from leaving [the FBO] via TT; PP the normal taxi route to a departure runway on west flow in non-ILS critical conditions. The runway was mostly clear from the approach end to the A1 turnoff; but not clear beyond. If the city had cleared the runway; they should have also cleared the turnoffs; so that the hold lines would be visible to pilots. In my opinion; the quality of snow removal has deteriorated; even since last winter. In general; there seems to be less equipment and less of a plan for efficiently removing snow. If the snow had been removed from taxiway M; even just the part that would make the hold lines visible; I can almost guarantee this event would not have taken place. The aircraft would have stopped further back and the perspective from the tower would have been different; allowing me to recognize his proximity to the runway relative to where the hold lines should be. Obviously; during a snow event; we work with less than ideal conditions as do the pilots. Everyone moves a slower and more cautiously. Aircraft X; in my opinion; was taxiing with caution that morning. If the hold lines and possibly signs that are in place to alert the crew of the location of a runway were obscured; they did not have all the tools they should have had to do their job that morning. The ASDE-X functioned properly and a go around was issued preventing any possible incident. If anything; I would recommend a more efficient snow removal plan.




Synopsis: MDW Local Controller reported that an aircraft taxiing for departure crossed the hold short line. The hold short line and other taxiway/runway markers were obscured with snow. ASDE-X sounded an alert. An arriving aircraft was issued a go-around.