With July Fourth less than a week away, U.S. airlines are already showing signs of a repeat of last summer's holiday travel nightmares.

More than 2,200 flights were canceled on both Monday and Tuesday, and over 16,000 flights were delayed on those days. Wednesday showed more warnings ahead of a hectic travel weekend.

As of 5 p.m. ET, 1,661 U.S. flights were canceled and there were more than 24,621 delays, according to FlightAware. Those numbers were expected to climb as the airline industry grapples with myriad issues.

Severe weather is receiving the brunt of the blame, with much of the country dealing with storms and excessive heat. But the issues run much deeper, including staffing shortages for the airlines and the Federal Aviation Administration (FAA). A lack of air-traffic controllers continues to plague the FAA and tight flight schedules.

The Northeast was experiencing the most issues, including New York (JFK and LaGuardia), Boston (Logan) and Newark (Liberty). Several international and cross-continental flights are being impacted, which could trigger a logjam like last summer's crisis that saw mass cancellations and passengers stranded for days.

Severe weather is expected to put more stress on the system.

"Thunderstorms in the New York area are blocking arrival and departure routes for La Guardia Airport, JFK International Airport and Newark Liberty International Airport," the FAA said. "You can expect delays in the New York metro area tonight."

Friday marks the start of the four-day July Fourth holiday weekend, which the Transportation Security Administration projected to be the busiest air travel day since the start of the COVID-19 pandemic. More than 2.82 million passengers are expected to be screened on Friday alone.