The storm made landfall about 3 p.m. Wednesday afternoon on Florida's west coast as a major hurricane.
Tampa International Airport, where officials had prepared for a major impact before the storm hit farther south, suspended operations at 5 p.m. ET Tuesday.
The Tampa airport said there will be no departing flights through Thursday.
"We will share a reopening date and time when it is determined," the airport said on Twitter Wednesday. The airport typically handles 450 flights daily.
Miami International Airport on the east coast of Florida was still open at 8 p.m. ET Wednesday, according to a notice on the airport's website, although some flights had been delayed or canceled.
Operations ceased at 10:30 a.m. ET Wednesday at Orlando International Airport. The airport sees nearly 130,000 passengers daily, according to its website.
The terminal at St. Pete-Clearwater International Airport closed at 1 p.m. Tuesday "due to mandatory evacuation orders from Pinellas County and remain closed until the evacuation order is lifted," according to the verified tweet from the airport.
Sarasota Bradenton International Airport closed at 8 p.m. Tuesday night.
Key West International Airport is set to reopen at 7 a.m. Thursday, the airport said on its website.
Airport crews will work throughout the night to prepare the airport to reopen, Director Richard Strickland said in a Facebook post.
Florida airports lead in US cancellations
Orlando, Miami, Tampa and Fort Lauderdale airports were the top four trouble spots in the United States.
Effects could ripple through the southeastern United States with Atlanta and Charlotte already seeing cancellations.
Airlines canceling flights
American Airlines, which operates about 250 daily departures out of Miami, its fourth-largest hub, had canceled about 500 flights by 8 p.m. Wednesday, including mainline and regional service.
American customers traveling through 20 airports in the hurricane's path can rebook flights without change fees. The airline has also added "reduced, last-minute fares for cities that will be impacted" in hopes of helping people who are trying to "evacuate via air."
American is waiving change fees for customers flying to and from Georgia, North Carolina and South Carolina because of the path of Hurricane Ian. The airline had already waived change fees for flights to and from Florida.
United Airlines started to shutter operations on the Atlantic Coast of Florida in anticipation of Hurricane Ian's path after it makes landfall.
United says it would halt departures from West Palm Beach, Miami and Fort Lauderdale airports by Wednesday afternoon. United will not operate from Jacksonville starting on Thursday.
United said on Wednesday that it had proactively canceled 345 flights since Tuesday, swapping some outbound flights with larger airplanes to help customers who were evacuating from the storm's path.
United and Southwest Airlines also suspended operations at the Fort Myers and Sarasota airports.
United also canceled all Tuesday and Wednesday flights to and from Key West and canceled some flights out of Orlando "as to minimize crew layovers."
By 8 p.m. Wednesday, Southwest Airlines had canceled 525 US flights, according to FlightAware data. And Delta Air Lines had canceled just over 200 flights.
FAA closely monitoring Ian
The arrival and departures board lists numerous flight cancellations at Tampa International Airport before its scheduled 5 p.m. Tuesday closure.
Bryan R. Smith/AFP/Getty Images
The Federal Aviation Administration said it was "closely monitoring Hurricane Ian and its path," in a statement, underscoring that it does not cancel commercial flights.
"Before any storm hits, we prepare and protect air traffic control facilities and equipment along the projected storm path so operations can quickly resume after the hurricane passes to support disaster relief efforts."
Moving aircraft to safer places
Multiple airlines are moving aircraft out of harm's way and note it will take time to reestablish service after the storm. First, officials and the airlines must determine when and where it is safe to resume flights, and then they must have crews on the ground available.
"Our in-house weather forecasting is a powerful tool to aid in ops decision making, but equally important are the conditions of ground infrastructure after the storm passes," Delta spokesman Morgan Durrant told CNN.
Riding out the storm in Tampa
At Tampa International Airport, a team of 120 airport employees have volunteered to stay on site and ride out the storm, airport executive John Tiliacos said Tuesday. The team includes tradesmen like plumbers and electricians who will be essential to restoring service at the airport.
"Once the storm has passed, our team will conduct a damage assessment of our airfield and terminal facilities and determine whether we can reopen immediately or whether we have issues that we need to address as a result of the hurricane impact," Tiliacos said.
He raised the possibility of the runways reopening to essential flights before the passenger terminal reopens. The facilities are rated for a Category 4 storm, but the airfield could see flooding from the nearby bay.