There was also unexpected softness in close-in demand in the second half of February that has since rebounded in March,
The budget airline now estimates first quarter 2017 operating revenue will decline in the two to three per cent range, compared with previous estimates of a 1% decline.
Bookings and unit revenue trends beyond the first quarter are encouraging, Southwest added.
The airline flew 8.7bn revenue passenger miles (RPMs) in February, an increase of 1.1% on a year ago.
Shares dropped 3% to US$54.62.
Story by ProactiveInvestors