FYI, the iOS Weather app does have this now. If you click on a day in the 10-day forecast, you get a detail view with hour-by-hour forecast data for many metrics. One of them is pressure (although I don't normally look at barometric pressure so maybe this different than what you need).
Edit: You can also click on the "pressure" widget and it brings up the same hour-by-hour detail view.
"Chance of precipitation" isn't one of them though, they'll only give you a predicted inches of rainfall. Should I assume that if there's a nonzero amount of rain in the graph it's a definitely going to happen and I should carry around my umbrella?
Darksky used to make that really easy to check for the entire day and I'm not loving Weather as a replacement.
Glad I'm not the only one, I was starting to wonder if I was crazy because that felt like the main piece of data in Dark Sky to me. But I saw a lot of iOS news coverage saying basically "Dark Sky is shutting down, but don't worry about it because Weather app has its local forecast features now!"
Meanwhile Weather app completely missed the point and gives me a bunch of stuff I don't care about.
You can tell me that the wind at 4 PM is going to be 5 MPH west northwest but you can't tell me there's a 30% chance of rain? Gee that's great, thanks for shutting down Dark Sky!
Edit: You can also click on the "pressure" widget and it brings up the same hour-by-hour detail view.