- The FAA warned it could slow inbound flights to New York City; Philadelphia; Charlotte, North Carolina; and Washington, D.C.
- Heavy smoke from wildfires in Canada was cutting visibility.
- More than 4,200 U.S. flights were delayed Thursday, according to FlightAware.
"The FAA will likely need to take steps to manage the flow of traffic safely into New York City, [Washington] DC, Philadelphia and Charlotte [North Carolina] due to reduced visibility from wildfire smoke," the agency said.
The FAA briefly halted inbound traffic to Philadelphia International Airport earlier Thursday.
Hundreds of flights to and from LaGuardia Airport in New York and nearby Newark Liberty International Airport were delayed Wednesday due to the heavy smoke. The FAA had briefly paused traffic altogether into LaGuardia during the day.
By Thursday evening, more than 4,200 flights to, from and within the the U.S. were delayed, according to flight tracker FlightAware.
More than 280 LaGuardia-bound flights, almost half of the day's schedule were delayed as of 6:40 p.m. ET Thursday, while another 161 departures were delayed, FlightAware's tally showed.
At Newark, 129 departures were delayed, or 19% of the schedule, and 143 flights headed for the New Jersey airport were delayed, around a fifth of the total.