Carrier route record

WNSEA→STL

Southwest Airlines · Seattle to St. Louis

Flights: 103
Period: September 2025 to August 2026
Share of route: 12%
Carriers: 2

Historical statistics for September 2025 to August 2026, not live status. For today’s flights, check with Southwest Airlines.

Southwest on SEA to STL sits in the middle of the pack with a C+. Close to three flights in four arrive on time by the DOT's definition, 4 points worse than the national rate of 77.3%. The late ones land about 55 minutes behind schedule. That rests on 103 flights, with a median arrival 2 minutes late.

The US average over the same twelve months is 77.3%, so there is little to separate Southwest on SEA to STL from the country as a whole. The variation that matters here is internal: which carrier, and what time of day.

On time

73.5%

Arrivals under 15 minutes late, of 102 completed flights

Avg delay

55 min

Across delayed arrivals only, not all flights

Cancelled

0.97%

National rate is 1.89%

Whole route

84.4%

Every carrier on SEA to STL, 883 flights

These figures cover 103 flights between September 2025 and August 2026. On-time means an arrival less than 15 minutes after the scheduled gate time, the same threshold the Department of Transportation uses in its own reports. Cancelled and diverted flights are excluded from the on-time rate and counted separately, which is why the cancellation column matters on its own.

Fare search by Aviasales, a Travelpayouts partner. Booking through it may earn this site a commission at no cost to you; it changes no figure here.

Carrier by carrier

Southwest against every carrier on SEA to STL

Southwest flew 103 of the 883 flights on SEA to STL, about 12% of it. Its 73.5% on time is 11 points worse than the route as a whole, which runs 84.4%. Ranked by on-time rate it is last of two carriers on the route, where the range runs from Alaska at 85.8% to Southwest at 73.5%.

Only one carrier clears this site's 365-flight floor here, so there is no comparison to draw. The table below is the full picture.

On-time performance by carrier on SEA to STL
Airline	Flights	On time	Avg delay	Cancelled	Grade
Alaska Airlines	780	85.8%	48 min	0.26%	A+
Southwest Airlines (this page)thin sample	103	73.5%	55 min	0.97%	C+

Timetable by Aviasales, a Travelpayouts partner. Booking through it may earn this site a commission at no cost to you; it changes no figure here.

Departure window

When Southwest runs best on this route

Departure time matters more here than the choice of carrier. The gap between the best and worst window is 30 points, against half a point between the best and worst carrier.

5am–8am—
8am–11am49 flights89.6%
11am–2pm54 flights59.3%
2pm–5pm—
5pm–8pm—
8pm–5am—

Scale runs 50% to 95%. Rows with fewer than 30 flights show a dash.

The weak window here is the middle of the day, not the evening: 11am–2pm runs 59.3% on time against 89.6% for 8am–11am. That shape usually means a departure bank rather than an accumulating delay, so the recovery later in the afternoon is real rather than noise. The 11am–2pm window is the busiest, with 54 flights scheduled in it.

Across the year

Southwest on SEA to STL through the year

Jul–Sep 202521 flights—
Apr–Jun 202628 flights—
Jul–Sep 202654 flights75.5%

Scale runs 65% to 80%. Rows with fewer than 30 flights show a dash.

Monthly volumes are too low here to break the year into twelve reliable readings. What the twelve-month total says is 73.5% on time across 103 flights.

Split by quarter, Southwest on SEA to STL logged 21 flights in Jul–Sep 2025, 28 flights in Apr–Jun 2026 and 54 flights in Jul–Sep 2026. Fewer than three of those quarters carried the 30 completed flights this site needs before printing a rate, so the 73.5% annual figure is the one to use.

Arrival by arrival

How late Southwest on SEA to STL runs when it is late

The 27 late arrivals split 12 under half an hour, 9 between 30 and 60 minutes, and 6 an hour or more.

15+ min early9 flights8.8%
Within 15 min66 flights64.7%
15–29 min late12 flights11.8%
30–59 min late9 flights8.8%
1–2 hours late4 flights3.9%
2+ hours late2 flights2.0%

Scale runs 0% to 100%.

Share of 102 completed flights by arrival against schedule.

A late pushback is often recovered here: of 40 departures that left 15 minutes late or more, 15 still arrived on time. Only 2 left on time and still arrived late.

The worst days

Worst days for Southwest on SEA to STL

The latest arrival of the period was on 6 August 2026, 6 hours 29 minutes behind schedule; most of its delay minutes were filed as national airspace system delay. Three of the five worst days fell in August 2026.

The latest arrivals of Southwest on SEA to STL in the period
Date	Arrived late by	Largest filed cause
6 August 2026	6 hours 29 minutes	National Airspace System
5 July 2026	2 hours 31 minutes	Late-arriving aircraft
24 August 2026	1 hour 51 minutes	Late-arriving aircraft
4 July 2026	1 hour 37 minutes	Late-arriving aircraft
14 August 2026	1 hour 13 minutes	Late-arriving aircraft

Delay causes

Why Southwest runs late on this route

Late-arriving aircraft accounts for 47% of the 1,476 delay minutes carriers reported here, and the remaining four split what is left.

Late-arriving aircraft
47%
National Airspace System
36%
Carrier
15%
Security
2%

47% of delay minutes trace back to a late-arriving aircraft, meaning the plane was already behind before it got here. Across 1,476 reported delay minutes here, that single category outweighs weather by 47 to one. The split is drawn from 1,476 reported delay minutes.

36% of delay minutes are charged to the National Airspace System: air traffic volume, runway configuration, ground stops. No carrier can price its way out of that, which is why airport choice moves the number here more than carrier choice does. That is 536 minutes out of 1,476, against a national NAS share of 21%.

Cancellations sit at 0.97% against a national 1.89%, which is ordinary. Diversions add another 0.00%.

Connections and stage length

What a delay here costs

5.9% of arrivals land an hour or more late, close to the national 8.3%, so a 60 to 75 minute layover is reasonable here while anything under 45 minutes is a bet. Most delays on this route last about 55 minutes, which a normal layover absorbs.

At 1,709 miles this is transcontinental flying, and long routes get a quiet structural advantage. Scheduled block time on a flight this length carries more padding in absolute minutes, so a twenty-minute late departure often still becomes an on-time arrival. Read the on-time rate here as slightly flattering next to a short-haul route with the same operational quality.

The return leg is a different operation and performs like one: it runs 49.5% on time across 103 flights, 24 points below this direction. Departure banks, aircraft rotations and the congestion at each end all differ, so scoring the two directions together would average away a real 24 points of difference.

One caution: this is a thin sample. 103 flights over twelve months is enough to clear this site's floor but not enough to pin the rate down tightly, and a single bad week moves it by more than a point. Treat the grade as a rough reading.

What to do with this

Booking Southwest on SEA to STL

Put together: treat all of this as provisional. 103 flights over twelve months is enough to clear this site's floor and not enough to be confident to better than a couple of points. The 73.5% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 102 completed flights over 12 months sit behind that.

Where to go next

Source: US DOT Bureau of Transportation Statistics, on-time performance data, September 2025 to August 2026. Last updated 22 October 2026; recomputed monthly. How this data is collected.

Key figures