Charles Spurgeon Commentary 1 Samuel 12:14-17

"If ye will fear Jehovah, and serve him, and hearken unto his voice, and not rebel against the commandment of Jehovah, and both ye and also the king that reigneth over you be followers of Jehovah your God, [well]: but if ye will not hearken unto the voice of Jehovah, but rebel against the commandment of Jehovah, then will the hand of Jehovah be against you, as it was against your fathers. Now therefore stand still and see this great thing, which Jehovah will do before your eyes. Is it not wheat harvest to-day? I will call unto Jehovah, that he may send thunder and rain; and ye shall know and see that your wickedness is great, which ye have done in the sight of Jehovah, in asking you a king." — 1 Samuel 12:14-17 (ASV)

This was to be a sign to them that Samuel was God's prophet.

On a previous occasion, in answer to his prayer, God had thundered against the Philistines but, this time, his thunder was his voice against Israel. In reading the Bible, we must always remember that it was not written in England but in Palestine. Wheat harvest there takes place about the month of May, when the weather is usually settled, and such things as thunder and rain are almost unknown. It was extraordinary, therefore, as we speak of "a bolt out of the blue."