Shares of American Airlines Group Inc. fell in morning trading after the U.S. carrier hiked its forecast for a key performance metric and fuel costs for the third quarter.

American Airlines is now expecting year-over-year growth of 2.0% to 3.0% in third-quarter total revenue per available seat mile, or TRASM, compared to a prior guidance of a 1.0% to 3.0% increase.

"The change in TRASM vs. previous guidance is due primarily to stronger domestic yields," the company said in an update to investors.

American Airlines also raised its fuel cost guidance for the third quarter, projecting an average fuel price of $2.28 per gallon to $2.33 per gallon, compared to the previous forecast range of $2.22 per gallon to $2.27 per gallon.

The company expects to record about $230 million in total pretax net special items in the third quarter.

American said Hurricane Florence caused the cancellation of 2,100 flights in September, reducing third-quarter revenue by $55 million and TRASM by 50 basis points.

The hurricane's negative impact on pretax income was estimated at $50 million, the company added.

American Airlines shares were down 4.01% to $34.46 at 10:15 a.m. ET.