Why does the Torah write "ka'Eis Machar" (and not just 'Machar', as it did by Makas Dever [See Pasuk 5])?
Rashi: Moshe made a niche on the wall,, and said to Pharaoh that the following day, when the sun reaches that niche, the hail will begin to fall. 1
See Sifsei Chachamim.
Why does the Torah add "be'Egypt" (See also Pasuk 24)?
Ramban: This suggests that there were plagues of hail that were just as heavy as this one, 1 and it is only in Egypt, where it never rains, and where hail was a mind-boggling phenomenon, that the plague was unique. 2
The current Pesukim relate what Hashem instructed Moshe to say to Pharaoh. Why does the Torah not inform us that Moshe passed on the message?