@lgaetz is a bit of a dialplan demigod and has done some write-ups such as.
It may be worth looking at that and possibly searching the forums for from-internal-custom
Also helpful
https://wiki.asterisk.org/wiki/display/AST/Contexts%2C+Extensions%2C+and+Priorities
Asterisk + The National Weather Service = Free Weather Reports For 1,000 Cities From Any Touchtone Phone – Nerd Vittles may also be useful as a reference point but it uses TTS
Asterisk sound files - Asterisk Sounds, help Asterisk to speak your language has a listing of standard sound files.
search for weather
https://www.asterisksounds.org/en/sounds?s=weather
side note… If you use the standard sounds you can seamlessly use custom prompts from Allison (the voice of Asterisk) https://www.theivrvoice.com/