en_tn/1sa/12/17.md

Is it not the wheat harvest today?

Samuel knows it is harvest time. He is using a rhetorical question to emphasize that it does not usually rain at this time so the people will know that the rain that will ruin their harvest is a judgment from Yahweh. Alternate translation: "It is harvest time and does not usually rain at this time" (See: rc://en/ta/man/translate/figs-rquestion)

he may send thunder and rain

Samuel is asking Yahweh to punish Israel for asking for a king by sending a rainstorm during the harvest which will ruin the grain.