If one knows that is going to rain later that night. can one start off sleeping inside or does one have to sleep in the Succa until it rains?
If one knows that it is going to rain, for example the sky is full of clouds, and it looks like rain is imminent, according to some poskim, he does not have to sleep in the succah. This is because it is most probable that he will be mitzaer, he doesn’t have to go to sleep there, since he will have to get up in the middle of the night. However others say that he is obligated to go to sleep even though he most probably will have to wake up and go into the house later. Everyone agrees though that this does not apply to eating, i.e. it appears that it will rain before the end of his meal. Also, everyone would agree that this also only applies if the sky looks like it is going to rain, but not if it merely the weather forecast.
Meiri Succah 26a D:H Shomrei Hair, (also see Ritza D:H Pirtza), Kovetz Halachos ( R’ S. Kaminetsky shlit”a) Succos 16-2. The opinions that say tht he has to sleep in the succah, Daas Torah 639-5, Shevet Hakehosi 1-199, and 3-192 (2) that he says that he asked R’ Eliyashiv zt”l about this and he agreed that the person has to go to the succah, also see Piskei Teshuvos 639-14.