I hope this question isn’t too ridiculous for GQ, but is it theoretically possible for rain to be falling on every square inch of the United States at the same time? I’m sure the answer’s probably no, but why is the answer probably no?
I’d guess the opposite. It’s theoretically possible (assuming (i) you mean something like a raindrop hits each square inch at least once a second (ii) you allow snow to count in case there’s some place in Alaska where it’s never warm enough to rain). There’s certainly enough water on earth for that to be true as oceans cover much more area than the U.S.
It’s incredibly unlikely however.
The storm of all storms
The largest diameter hurricane ever was Typhoon Tip at 1,380 miles in diameter. That would cover about half the continental US.
It would be a large storm system, but i don’t think it is impossible
We got heavy rain in Michigan when Hurricane Andrew hit.
It’ll happen this August 21. 
Here’s another scenario to consider: Raindrop It’s not raining everywhere, but it’s raining all at once. It’s still bad.
A semi-serious answer: I’d agree with post #2. There’s no physics theoretical reason it couldn’t rain over that large an area. It’s vanishingly unlikely though with the current climate.
In one sense, the atmosphere consists of sections that are dry and therefore cloudless, sections that are a little humid and therefore cloudy, and sections that are too humid to keep all the water in suspension, so some of it falls as rain/snow.
Typical weather is about gathering the widely distributed moisture into a much smaller area, thereby concentrating it enough so it starts falling out.
For all the air over the US to be that highly concentrated there’d have to be a very, very large area nearby that’s extremely dry. Very unlikely.
Ah, but, hurricanes have eyes.
And maybe 4000 years ago too. Guy named Noah knew all about it but he wasn’t specific about whether rain actually fell on every inch at the same time or not.