It was clear early in the day on Friday that the air traffic system in the northeastern United States was having some serious problems. So, let's dive into what actually happened and why it wreaked such havoc with the system.
First, there's the possible precipitating event. The US Office of Personnel Management (OPM) on Thursday released new guidelines for essential personnel. The new guidance stated that essential personnel were allowed to take leave during the shutdown. Before this, anyone deemed essential was barred from taking leave during a shutdown. The very next day, the ATC system in the northeast saw a significant staffing shortage. While no actual figures were released, it was enough to cause serious problems.
Staff shortages were reported at both Washington Center (ZDC) and Jacksonville Center (ZJX) on Friday. This led to the ATC centers having a lack of capacity for the number of flights that needed to use the airspace. To deal with the lack of capacity a ground stop had to be issued.
A ground stop simply means that flights are halted temporarily, and the most common reason for a ground stop is weather, which already happened to be a problem at LGA on Friday. If you have ever flown through Dallas (or any southern US airport) on a summer afternoon, you may have experienced a ground stop due to thunderstorms. It's simply a way to hold traffic in place.
But ground stops can have ripple effects throughout the system. When a ground stop is in place, planes that are scheduled to depart to the affected airport are held in place as well. ATC won't give a takeoff clearance to head to an airport that isn't currently accepting traffic. For flights en route already, a ground stop could lead to a diversion. A single delay on an aircraft's route for the day can lead to every flight after that on its route being delayed or, in a worst case scenario for passengers, cancelled flights.
By 1pm, FlightRadar24 had LGA, a highly congested airport on the best of days in one of the busiest airspaces anywhere in the United States, listed as the worst airport in the world for delays on Friday. It was likely this combination of an already busy airport, congested air space, lack of ATC capacity and weather (winds) that led to the FAA calling the infamous LGA ground stop.
H/T: NYC Aviation
