Is morning the time to offer Minchah?
Rashi (citing Tanchuma Pinchas 3): The rain began falling in Eretz Edom in the morning, and at the time of Minchah (the afternoon 1 ) it came; it overflowed and came to the valley. This is like he said (verse 17) "you will not see wind or rain."
Radak, Malbim: Yes; it refers to the time to offer the morning Minchah 2 .
In the Torah, 'Minchah' is a gift, or flour offered with a Korban. "Alos ha'Minchah" implies a known Minchah, i.e. of the Tamid. Regarding Eliyahu on Har ha'Karmel, it explicitly says that it was afternoon (Melachim I, 18:29). Perhaps that is Chazal's source to call the afternoon Tefilah 'Minchah'. (PF).
From where did the water come?
Rashi: Rain fell in Eretz Edom, and came to here. Radak - it was far away; the Machaneh did not see wind or rain.
Radak: A river of Edom overflowed due to Hashem's will, and the water spilled to this valley.
