1 Samuel 12:17
Is it not wheat harvest to day? I will call unto the LORD, and he shall send thunder and rain; that ye may perceive and see that your wickedness is great, which ye have done in the sight of the LORD, in asking you a king. Is it not wheat harvest to day? I will call unto the LORD, and he shall send thunder and rain; that ye may perceive and see that your wickedness is great, which ye have done in the sight of the LORD, in asking you a king.
The verse belongs to the elders' petition for a king, in which the injustice of Samuel's sons furnished a pretext while the real motive was discontent and pride, and the demand itself was deliberate. The wheat harvest comes in May and June, when in the East the skies are normally cloudless with no rain. At that season the weather is normally calm and mild.
At Samuel's call to the Lord, God sent thunder and rain as an extraordinary, unusual and violent sign of His displeasure with Israel for asking for a king. The storm also proved the truth of Samuel's words. The sign was meant to expose and make plain Israel's wrongdoing, so the people would perceive that asking for a king was great wickedness and sin done before the Lord. That sin rejected His rule and wise arrangement and added to their sins.
The people responded in terror, fear, and humility, confessing their sin in asking for a king and asking Samuel to pray for them. Israel could have withdrawn its demand for a king at that point, but pride kept the people from doing so.