tn כְּצֵאתִי (kétse’ti) is the Qal infinitive construct of יָצָא (yatsa’); it functions here as the temporal clause before the statement about prayer.
sn There has been a good deal of speculation about why Moses would leave the city before praying. Rashi said he did not want to pray where there were so many idols. It may also be as the midrash in Exodus Rabbah 12:5 says that most of the devastation of this plague had been outside in the fields, and that was where Moses wished to go.
sn This clause provides the purpose/result of Moses’ intention: he will pray to Yahweh and the storms will cease “that you might know.…” It was not enough to pray and have the plague stop. Pharaoh must “know” that Yahweh is the sovereign Lord over the earth. Here was that purpose of knowing through experience. This clause provides the key for the exposition of this plague: God demonstrated his power over the forces of nature to show his sovereignty—the earth is Yahweh’s. He can destroy it. He can preserve it. If people sin by ignoring his word and not fearing him, he can bring judgment on them. If any fear Yahweh and obey his instructions, they will be spared. A positive way to express the expositional point of the chapter is to say that those who fear Yahweh and obey his word will escape the powerful destruction he has prepared for those who sinfully disregard his word.