Carrier route record

WNSTL→SEA

Southwest Airlines · St. Louis to Seattle

Flights: 103
Period: July 2025 to June 2026
Share of route: 12%
Carriers: 2

Historical statistics for July 2025 to June 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 54.4% of flights arrive inside the 15-minute window, 23 points worse than the national rate of 77.6%. Late arrivals average roughly 45 minutes. Treat any tight connection here as a coin flip. That rests on 103 flights, with a median arrival 11 minutes late.

The national on-time rate over the same period is 77.6%, so Southwest on STL to SEA runs 23 points behind the country as a whole. Across a thousand flights that is about 232 extra late arrivals.

On time

54.4%

Arrivals under 15 minutes late, of 103 completed flights

Avg delay

43 min

Across delayed arrivals only, not all flights

Cancelled

0.00%

National rate is 1.81%

Whole route

77.6%

Every carrier on STL to SEA, 884 flights

These figures cover 103 flights between July 2025 and June 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.

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 54.4% on time is 23 points worse than the route as a whole, which runs 77.6%. Ranked by on-time rate it is last of two carriers on the route, where the range runs from Alaska at 80.7% to Southwest at 54.4%.

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 STL to SEA
Airline	Flights	On time	Avg delay	Cancelled	Grade
Alaska Airlines	781	80.7%	49 min	1.02%	B
Southwest Airlines (this page)thin sample	103	54.4%	43 min	0.00%	F+

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.

5am–8am—
8am–11am9 flights—
11am–2pm51 flights56.9%
2pm–5pm38 flights63.2%
5pm–8pm5 flights—
8pm–5am—

Scale runs 50% to 70%. 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 56.9% on time against 63.2% for 2pm–5pm. 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 51 flights scheduled in it.

Across the year

Southwest on STL to SEA through the year

Jul–Sep 202575 flights61.3%
Apr–Jun 202628 flights—

Scale runs 55% to 65%. 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 54.4% on time across 103 flights.

Split by quarter, Southwest on STL to SEA logged 75 flights in Jul–Sep 2025 and 28 flights in Apr–Jun 2026. Fewer than three of those quarters carried the 30 completed flights this site needs before printing a rate, so the 54.4% annual figure is the one to use.

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 2,039 reported delay minutes.

Late-arriving aircraft
40%
Carrier
31%
National Airspace System
28%
Extreme weather
1%

40% of delay minutes trace back to a late-arriving aircraft, meaning the plane was already behind before it got here. That is a cascade, and cascades start small and grow through the day. Across 2,039 reported delay minutes here, that single category outweighs weather by 39 to one. It is the strongest argument for booking the first departure of the morning, when the aircraft has been sitting at the gate overnight with nothing to inherit. The split is drawn from 2,039 reported delay minutes.

Cancellations are rare at 0.00%, below the national 1.81% 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

7.8% of arrivals land an hour or more late, close to the national 8.1%. That is the figure that decides a connection, and it says a 60 to 75 minute layover is reasonable here while anything under 45 minutes is a bet. Most delays on this route last roughly 45 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 64.7% on time across 103 flights, 10 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 10 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 54.4% 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.

Where to go next

Source: US DOT Bureau of Transportation Statistics, on-time performance data, July 2025 to June 2026. Last updated 15 September 2026; recomputed monthly. How this data is collected.

Key figures