Intellicast stopped updating their iphone app and it was replaced by an app called Storm, which I love. The hourly forecasts show both percentage chance of precipitation and accumulation. This tells me if I should bring gear for a light drizzle or a downpour.

Dark Sky is good if your timing is flexible. It gives a graphic representation of precipitation probability for the next hour and is based only on GPS location. I can leave early or late from my work (and home), so I can wait until a shower passes or leave early to beat a storm that is rolling in.

Weather underground is too busy.