A link to that full enumerated listing right from the conditionCode section in the REST API documentation would make things so much easier!
That said, after a good bit of digging I found this page on the Swift side of the house; combining that with the partial response above, I think this may be the answer:
https://developer.apple.com/documentation/weatherkit/weathercondition
I believe each case string can be leading-capital-camel-cased to determine the string to expect?
SteeleMage
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution