PHOENIX (AP) — American Airlines is warning passengers that it may have to ground flights in Phoenix during a heat wave that could send the temperature soaring to 120 degrees.

The airline is letting Phoenix passengers flying during the peak heat Monday through Wednesday to change flights without a fee. The forecast calls for a high of 118 on Monday and 120 on Tuesday in Phoenix.

The heat will have the biggest impact on smaller regional jets flying out of Phoenix.

Extreme heat creates changes in the air density that make it harder for airplanes to take off. Airlines respond by imposing weight restrictions, such as carrying less cargo and fuel. But in some cases, they will ground flights during the peak heat.