Jesus reminds the people who are listening to him in the synagogue about Elijah and Elisha (4:27), who were prophets about whom they knew. (See: [[rc://en/ta/man/translate/writing-background]])
The people to whom Jesus was speaking would have known that Elijah was one of God's prophets. If your readers would not know that, you can make this implicit information explicit as in the UDB. Alternate translation: "when Elijah was prophesying in Israel" (See: [[rc://en/ta/man/translate/figs-explicit]])
This is a metaphor. The sky is pictured as a ceiling that was closed, and so no rain would fall from it. Alternate translation: "when no rain fell down from the sky" or "when there was no rain at all" (See: [[rc://en/ta/man/translate/figs-metaphor]])