There are different answers to that.
The easy one: it will either rain or not rain - therefore 50 %.
Another easy: it will most likely rain somewhere in the US - therefore almost 100%.
The hard one: learn a lot about meteorology and collect a lot of data about rain. Do a lot of calculation.
Give Us Tomorrow was created in 1978.
There is no specified rain season in the US. Most US states have a series of seasons that contain rain.
tomorrow
tomorrow
rain on us is by Ernest Pugh
tomorrow
Tomorrow.
Mexico us going to play with Venezuela tomorrow at 10:00pm
Our doctor told us the probability was 4%.
It shows us the likelihood of the occurrence of specified events.
The phrase "us ever after tomorrow's " is an awkward phrase, plus it makes "tomorrow's " a singular possessive. It shouldn't be used. You might say, "us for ever after", but even that becomes awkward in a sentence.
Tomorrow is the adverb in the sentence. It describes when Uncle Rico will visit us.