Southwest Airlines planes are seen in front of the Las Vegas strip, Nevada, United States in this April 23, 2015, file photo. REUTERS/Lucy Nicholson/Files

(Reuters) - Southwest Airlines Co LUV.N cut its forecast for a key revenue measure on Friday, citing "unexpected softness" in demand for last-minute flight bookings in the second half of February.

The No. 4 airline by passenger traffic said it expects first-quarter operating unit revenue to fall in the 2-3 percent range, compared with its previous forecast of flat to down 1 percent.

Unit revenue compares sales to the number of seats an airline flies and how far it flies them.