Southwest Airlines narrowed its third quarter revenue guidance Thursday as leisure travel demand remained strong but business revenues languished well below 2019 levels.

However, there are signs that corporate travel may be picking up, with Southwest (ticker: LUV) noting a significant improvement in the early weeks of September. Delta Air Lines (DAL) flagged a similar emerging trend Wednesday.

Southwest now expects operating revenue in the third quarter to be between 9% and 11% up on the same period in 2019, compared to a previous forecast of 8% to 12% higher.

"The company experienced strong travel demand for the Labor Day holiday and continues to experience strong revenue trends in third quarter 2022," the airline said in a filing Thursday.

Leisure revenue trends remain above 2019 levels and are beating the company's expectations, however, the same could not be said for business travel, at least over the summer.

Revenue trends for business travel were softer than expected from late July through August, Southwest said, down approximately 26% to 32% on 2019 levels, before improving so far in September. The company now sees third quarter business revenue to be between 26% and 28% down on 2019 levels, worse than the 17% to 21% down range in a previous estimate.

The improvement in the early weeks of September – an estimated 8 to 10 percentage points relative to August – may prove significant, particularly if that continues in the months ahead as more people return to the office.

"Although early in the booking curve, the company continues to experience strong revenue trends in fourth quarter 2022," it added.

An uptick in business travel has also been felt by Delta, the airline's President Glen William Hauenstein said at Morgan Stanley's Laguna Conference Wednesday. He said a "step-up in business post-Labor Day," has materialized. "It looks to us now as though business is going to have a very strong fall, which is always great for October."

United Airlines (UAL) raised its third quarter revenue outlook last week, citing continued strong demand following a "robust summer." United now expects total operating revenue in the quarter to be up around 12% on 2019 levels, an increase from a previous forecast of 11%.

Southwest stock fell 1% in premarket trading, and is 11% down so far this year as of Wednesday's close.