"How cool my body can get by sweating" is the best explanation of wet bulb I've seen, lead with that. And for travel, warn me the night before, not at 2pm when im already outside.
I did in fact, have alerts set up for the day before, in early versions. In practice though, the exact times were a bit shifty because weather forecasts update constantly. (I discovered that even the weather record for past hours updates after the time has passed, as the data flows in asynchronously)
So firing notifications in advance became about a balance between accuracy, annoyance and too much info, plus the apps scheduling them with iOS.
Eventually I figured showing a timeline for the day in the app and widget made more sense, and alerts firing as they happened. Thanks for the feedback. I'll keep looking for a balance between usability vs technical constraints for this.