I have typically used The Weather Channel app. It is a good app but a bit cluttered. When it was announced that Apple had purchased Dark Sky I thought I would give the app another try. It needs some improvement in the graphics but overall I like it.
However, when checking the forecast for precipitation I noticed that the percentage that shows up for the day does not match the percentages throughout the day. See the attached.
Is this typical?
As I recall, this was one reason that I dropped one or the other of the weather apps. I could never quite understand exactly why the hourly precipitation percentages could not “match up with” the daily value. It was not that the math might have been beyond me, it was that I needed a bit more confidence that a value of 25% on the daily was not suddenly going to be because 15 min at lunch time was 100% thunderstorms.
I should hope as you that somewhere on this forum a meteorologist might explain what we should expect in these reports of rain percentages.
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JJW
Dark Sky uses GPS to give me an extremely local forecast which I’ve noticed frequently becomes more accurate as an event like rain approaches. At noon, it might predict rain at 3:00 pm, by 1:00p that might have changed to 2:40. By 2:00pm it’s usually on the money.
I rarely look at that percentage. I can’t recall when I purchased the app but it’s probably been at least 4 years ago. If it says its going to rain tomorrow, I pack my umbrella and check it again when making plans for lunch. 
According to the National Weather Service, if you see a 40 percent chance of rain, “there is a 40 percent chance that rain will occur at any given point in the area.” The “Probability of Precipitation” (PoP) simply describes the probability that the forecast grid/point in question will receive at least 0.01" of rain.
To expand on @bowline’s comment, your picture says, first, “There is a 50% chance it’s going to rain today (Sunday).” Second, “There’s a 1% chance it’s going to rain around midnight.” Third, “It’s not likely to rain between 2 A.M. and 8 A.M.” Later, “There’s a 10% chance it’s going to rain around 6 P.M.” And so on.
FWIW, I have found Dark Sky to be very inaccurate. Other third-party weather apps are also inaccurate, though less so.
In my experience, the most accurate weather app is the stock Apple app.
But I keep trying others because if I did not I would not be allowed here.
Here’s what I don’t like about the Apple app: I look at the forecast icon, it shows rain. Then I look at the detailed forecast, and it shows NO rain until 7 pm. I don’t get that.
The probability events for each hour are probability (ha!) not independent, so their sum won’t be a simple sum.
For the last 14 months on iOS I’ve been using the Hello Weather app, unlocked. (It’s free, but you can unlock extra features for $8.99/yr.) Unlocked, you can switch between real-time weather and extended forecasts from four different sources: DarkSky, TheWeatherChannel, AccuWeather, and AerisWeather. I’ve regularly checked and compared predictions between them all and found that TheWeatherChannel has been just as accurate as DarkSky… yet they differ at any given moment in their results! So when I’m out I switch between them if I think there’s imminent precipitation.
I’m mostly staying indoors as a result of the pandemic so I haven’t needed to consult the app much in the last few weeks, but having bought the DarkSky app years ago, I get Notifications through it when it’s about to rain or about to stop raining; perhaps something’s changed recently but those Notifications have gotten really good.
According to Hello Weather’s blog they already have plans in place to replace DarkSky’s forecasts for the pro-unlock users once its API stops working.
I’d like error bars on their estimates, but consumer weather apps don’t really do that. I do better intuiting what the graph means than trying to rigidly plan on the numbers.
I haven’t spent enough time yet with Dark Sky to know its accuracy but when I did a search, for my area The Weather Channel was rated as the most accurate. Apple’s weather app uses The Weather Channel for its data.
Possibly the Many Worlds Hypothesis explains it?
