I don't know if it is written anywhere but always allow for a flight that is delayed to be moved back up.
There are a few reasons for this:
I recommend FlightRadar24 to check "where is my plane coming from?" and you can track the inbound flight to see if the issue is a delay occurring previously on your route. Then especially if the flight continues on throughout the day it's likely to go as projected with the delays - plan to be at the gate around when the inbound flight is arriving.
Doing the same thing you may see your plane is already there, but there isn't crew available - this could change on the fly, if crew suddenly becomes available due to some other delay or cancelation the flight time can be moved back up in an instant.
Then also Southwest can arrange for a plane swap and leave earlier with a new plane taken from some other route and move the departure back up. Probably not if there is a big continuing group on the other flight - see if the flight number originates at your airport or if it continues throughout the day. I think this is less likely if the continuing flight will be coming within 1-2 hours.
In fact if the announcement was early enough, head the airport early, if your flight is delayed significantly you can maybe move up onto an earlier flight for free.
I hope this helps!
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