Before I answer your question, please explain how the "plain and obvious reading" necessitates that rain came from the first heaven when there's no indication that any water existed between "the waters which were under the firmament" and "the waters which were above the firmament" until after the flood?
I want to make sure I understand your question, and that we are talking about the same things. Let's break what I think you're saying into two parts. One: defining "the waters which were under the firmament". Two: rain before or after the flood.
ONE: How are you defining "the waters which were under the firmament"? To me, it means what it reads: water under the firmament. The firmament, or expanse, is the expanse containing stars, Sun and Moon (sequentially, the 2nd heaven). That's the dividing line. Water above that, and water below that. We agree on that, yes?
What's below the firmament? Answer: the sky, the surface of the earth and below the surface of the earth. This is the other side of the diving line. This is "the waters which were under the firmament". Are you disagreeing with this? If so, why?
Gen 1:20 "And God said, Let the waters bring forth abundantly the moving creature that hath life, and
fowl that may fly above the earth in the open firmament of heaven.” Birds fly in what area? Answer: the area above the surface of the earth and below the expanse/firmament of the stars, Sun and Moon. This is sequentially the 1st heaven. The Bible describes three heavens.
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TWO: Gen 6 begins the Flood account. Gen 2:5 reads, "And every plant of the field before it was in the earth, and every herb of the field before it grew: for the LORD God had not caused it to rain upon the earth, and
there was not a man to till the ground. 6But there went up a mist from the earth, and watered the whole face of the ground." There's no man to till the field yet. Then what happens? "7And the LORD God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul.". Now there's a man to till the soil. Then the Lord plants the garden.
In Gen 7:4 the Lord says to Noah, "For yet seven days, and I will cause it to rain upon the earth forty days and forty nights; and every living substance that I have made will I destroy from off the face of the earth." Gen 7:11 "In the six hundredth year of Noah's life, in the second month, the seventeenth day of the month, the same day were all the fountains of the great deep broken up, and the windows of heaven were opened. 12And the rain was upon the earth forty days and forty nights."
These scriptures are why I say the flood waters came from the depths of the earth and rain from the sky also known as the 1st heaven, where birds fly. Man has discovered there is vast amounts of water under the surface of the earth, and it rains from the sky. There's no scriptural account of Noah asking the Lord, "What's rain?" There is no scripture that reads The Flood Waters came from the waters above the firmament/expanse of stars/sun/moon. The Flood account is complete and sufficient. Why would anyone bring in a non-scriptural claim that the waters from above the firmament/expanse of stars/sun/moon, waters that Lord separated from the expanse and earth, was used for the Flood? Unnecessary and unscriptural.
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