Well, the fact that they were not professional pilots is really irrelevent. Even a well trained airline pilot would have a hard time hitting a building that s/he couldn't see.
All they needed was weather good enough to see the towers from a distance. Even an overcast day with a ceiling of 3000' would work. After all, from what I have heard, they got the airline pilots to get them within sight of NYC by saying that they wanted to go to JFK, then they took over.
All that said, I'd guess that you're right that they were waiting for severe clear. What I said above would work for one plane, but the best way to maximize the possibility of all of them hitting their targets would be to make the hit on a CAVU day (Clear And Visibility Unlimited).
I'd bet that they had a range of days planned and that , as you said, the weather was, indeed, a factor.
As a side note, there has been much speculation about the fact that Tuesday and Wednesday are the least traveled days and, therefore, would have the least number of passangers per plane who could put up a fight.
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