U.S. airlines are coming under fire for increased prices out of Miami ahead of Hurricane Irma, as a mandatory evacuation order comes in effect for the coastal areas of Miami-Dade county.

Flights out of Miami were particularly high for Friday, Sept. 8, a search by TheStreet showed, with Delta Air Lines (DAL) and American Airways (AAL) offering flights that were almost twice as expensive as normal and that would take double the time. American Airlines had the most expensive flight, coming in at $1,242 on Alphabet's (GOOGL) Google flight function.

American Airlines on Wednesday, however, said that fares would be capped at $99. 

"While there are limited seats remaining before the storm hits, we will cap our pre-tax fares at $99 for MainCabin seats on direct, single leg flights out of Florida for tickets sold through Sunday Sept. 10 for travel until Sept. 13," American said in a statement. 

The city of Miami is preparing for its largest evacuation in more than a decade Thursday as the U.S. National Hurricane Cents warned that Irma, potentially the most powerful storm to hit the American mainland since 1992, is maintaining its category 5 wind speed as it bears down on the south Florida coast.

Miami Mayor Carlos Gimenez has issued a mandatory evacuation order for residents in some coastal areas of Dade County, the metropolitan area of Miami, to move inland and order local schools to be closed for the remainder of the week. The evacuation order, which is set to take effect at 7am eastern Thursday, is the first since Hurricane Wilma in 2005 and is expected to affect around 150,000 people.

American Airlines has cancelled operations in Miami, Fort Lauderdale, Fort Myers, Orlando and West Palm Beach throughout the weekend. Flights due to arrive from Europe and South America have also be canceled on Friday.

United Airlines (UAL) was showing prices on more than $6,000 for a flight from Miami to Denver, according to the Telegraph. The airline, however, said the price was quoted in error, citing a computer glitch.

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"United did not change how we priced our seats for flights out of Florida, and we will be offering additional flights out of Florida today and tomorrow to help more customers in those affected areas," a spokesman told the newspaper.

The higher prices were due in part to an algorithm used by airlines, known as dynamic pricing, which sees prices go up when there is increased demand.

Airlines that work in the affected area are working to accommodate passengers. "Some carriers serving the impacted areas are adding flights and temporarily reducing or capping fares out of cities in the path of Hurricane Irma," a statement from industry group Airlines for America said.

JetBlue has also capped its flights at $99 and Delta said it has "reduced its highest fares, typically seen when customers are booking last-minute travel when inventory is low and demand is highest. Delta carefully examined fares from Florida as Irma's path became apparent and demand to fly out of the area surged."

American closed at $44.31 Wednesday. Delta gained 3.38% to end the day at $47.39 and United lost 1.26% to close at $60.33.

Airlines in Europe were having a buoyant day, with Air France KLM (AFLYY) up 1.33% to €12.98, British Airways owner International Consolidated Airlines Group (ICAGY) was flat 593 pence. Lufthansa (DLAKY) was up 0.41% to €21.90.

British Airways said that flights between London Heathrow and Miami on Friday, Saturday and Sunday could be cancelled.

(This story has been updated)

Stick with TheStreet for updated Irma coverage:

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