Jamieson, Fausset and Brown on 1 Samuel 12:17-25

17-25. Is it not wheat harvest to-day?--That season in Palestine occurs at the end of June or beginning of July, when it seldom or never rains, and the sky is serene and cloudless. There could not, therefore, have been a stronger or more appropriate proof of a divine mission than the phenomenon of rain and thunder happening, without any prognostics of its approach, upon the prediction of a person professing himself to be a prophet of the Lord, and giving it as an attestation of his words being true. The people regarded it as a miraculous display of divine power, and, panic-struck, implored the prophet to pray for them. Promising to do so, he dispelled their fears. The conduct of Samuel, in this whole affair of the king's appointment, shows him to have been a great and good man who sank all private and personal considerations in disinterested zeal for his country's good and whose last words in public were to warn the people, and their king, of the danger of apostasy and disobedience to God.