Carrier route record
WNSTL→SEA
Southwest Airlines · St. Louis to Seattle
- 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 STL to SEA earns an F+, which is as low as this site's scale goes for a reason. Only 49.5% of flights arrive inside the 15-minute window, 28 points worse than the national rate of 77.3%. Late arrivals average about 35 minutes. Treat any tight connection here as a coin flip. That rests on 103 flights, with a median arrival 15 minutes late.
The national on-time rate over the same period is 77.3%, so Southwest on STL to SEA runs 28 points behind the country as a whole. Across a thousand flights that is about 278 extra late arrivals.
Key figures
On time
49.5%
Arrivals under 15 minutes late, of 103 completed flights
Avg delay
35 min
Across delayed arrivals only, not all flights
Cancelled
0.00%
National rate is 1.89%
Whole route
78.6%
Every carrier on STL to SEA, 884 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.
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Carrier by carrier
Southwest against every carrier on STL to SEA
Southwest flew 103 of the 884 flights on STL to SEA, about 12% of it. Its 49.5% on time is 29 points worse than the route as a whole, which runs 78.6%. Ranked by on-time rate it is last of two carriers on the route, where the range runs from Alaska at 82.5% to Southwest at 49.5%.
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.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| Alaska Airlines | 781 | 82.5% | 50 min | 1.02% | B |
| Southwest Airlines (this page)thin sample | 103 | 49.5% | 35 min | 0.00% | F+ |
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Departure window
When Southwest runs best on this route
Departure time is the single lever a traveller controls, and it moves the odds more than most people expect. Each bar below is the on-time rate for flights scheduled to leave inside that window, across 103 flights.
There are not enough flights in each departure window to split Southwest's STL to SEA record by time of day without the percentages moving on noise. The headline rate of 49.5% is the reliable figure here.
Across the year
Southwest on STL to SEA through the year
- Jul–Sep 202521 flights—
- Apr–Jun 202628 flights—
- Jul–Sep 202654 flights48.1%
Scale runs 40% to 55%. 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 49.5% on time across 103 flights.
Split by quarter, Southwest on STL to SEA 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 49.5% annual figure is the one to use.
Arrival by arrival
How late Southwest on STL to SEA runs when it is late
Most of the lateness is short. 34 of the 52 late arrivals were under half an hour behind, and 7 reached an hour or more.
- 15+ min early3 flights2.9%
- Within 15 min48 flights46.6%
- 15–29 min late34 flights33.0%
- 30–59 min late11 flights10.7%
- 1–2 hours late4 flights3.9%
- 2+ hours late3 flights2.9%
Scale runs 0% to 100%.
Share of 103 completed flights by arrival against schedule.
6 of 38 late departures made up the time and arrived on time, 16% of them. In the other direction, 20 flights left on time yet arrived 15 minutes late or more, so that delay built up after pushback: in the air, in a holding pattern or on the ground at SEA.
The worst days
Worst days for Southwest on STL to SEA
The latest arrival of the period was on 22 June 2026, 2 hours 19 minutes behind schedule; most of its delay minutes were filed as a late-arriving aircraft. Four of the five worst days fell in June 2026.
| Date | Arrived late by | Largest filed cause |
|---|---|---|
| 22 June 2026 | 2 hours 19 minutes | Late-arriving aircraft |
| 19 June 2026 | 2 hours 14 minutes | Late-arriving aircraft |
| 13 June 2026 | 2 hours 8 minutes | Late-arriving aircraft |
| 5 July 2026 | 1 hour 23 minutes | National Airspace System |
| 18 June 2026 | 1 hour 17 minutes | Late-arriving aircraft |
Delay causes
Why Southwest runs late on this route
Carriers report a reason for every delay minute they cause, in five DOT categories. The split below is by minutes, not by number of flights, so a handful of very long weather delays weighs as much as many short ones. It covers 1,820 reported delay minutes.
- 34%Carrier
- 32%Late-arriving aircraft
- 31%National Airspace System
- 3%Extreme weather
No single cause dominates. carrier at 34%, late-arriving aircraft at 32% and national airspace system at 31% split the 1,820 reported delay minutes between them. An even spread like this usually means the operation is running close to its designed capacity with no one obvious weak point, and it is the hardest pattern for a traveller to route around. The split is drawn from 1,820 reported delay minutes.
Cancellations are rare at 0.00%, below the national 1.89% and low enough to earn the plus on the grade. Almost no flights are called off; the risk here is arriving late, not not arriving.
Connections and stage length
What a delay here costs
6.8% 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 35 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 73.5% on time across 103 flights, 24 points above 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 STL to SEA
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 49.5% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 103 completed flights over 12 months sit behind that.
Related
Where to go next
- STL to SEA, every carrier
- Southwest on SEA to STL, the return leg
- Alaska Airlines on STL to SEA82.5%
- Southwest Airlines network-wide record
- St. Louis Lambert International (STL) delay statistics
- Seattle-Tacoma International (SEA) delay statistics
- Why the first flight of the morning is statistically safest
- How to read this site: grades, sample sizes, and honest caveats
Source: US DOT Bureau of Transportation Statistics, on-time performance data, September 2025 to August 2026. Last updated ; recomputed monthly. How this data is collected.