Related: Weather apps could in general be better, not just in terms of visualization. One thing which always annoys me is how they handle rain predictions. They give "probabilities" for rain. But what do those mean? The probability that it rains for at least one minute in the given time frame? And with at least x mm/inches of rain coming down? And isn't a low probability of short and weak rain totally different from…
I also do a lot of cycling, and total duration spent in the rain tends to matter more to me than the amount of rainfall. I don’t mind riding through a quick downpour or two on a ride if I can dry out in between, but spending several hours in constant rain can be miserable (and hypothermia becomes a concern) - even if it’s light rain.
Of course expected amount definitely has uses, too. For example if there is rain forecast for tonight, and I’m trying to determine the probability that a field will be usable for practice in the morning.
Anyway I would say that both probability and expected amount are important, and any competent weather app should offer both. The Apple Weather app only shows hourly forecasts for expected amount, and that alone would be enough to be a dealbreaker for me.