Adam Clarke on Exodus 9:28

It is enough - There is no need of any farther plague; I submit to the authority of Jehovah and will rebel no more.

Mighty thunderings - כלת אלהים koloth Elohim, voices of God; - that is, superlatively loud thunder. So mountains of God (Psa 36:6) means exceeding high mountains. So a prince of God (Gen 23:6) means a mighty prince. See a description of thunder, Psa 29:3-8 : "The Voice Of The Lord is upon the waters: the God of glory thundereth; the Lord is upon many waters. The voice of the Lord is powerful; the voice of the Lord is full of majesty. The voice of the Lord breaketh the cedars. The voice of the Lord divideth the flames of fire. The voice of the Lord shaketh the wilderness," etc. The production of rain by the electric spark is alluded to in a very beautiful manner, Jer 10:13 : When he uttereth his voice, there is a multitude of waters in the heavens. See Clarke's note on Gen 7:11, and Gen 8:1 (note).