It’s about to start raining of Sukkoth
Question
On Sukkoth, if one sees gathering clouds and it seems that it’s about to start raining, however, it’s not raining yet, is one allowed to start eating or go to sleep outside the sukkah?
Answer
If one sees gathering clouds and it seems that it’s about to start raining, but it’s not raining yet, one is obligated to eat and sleep in the sukkah. However, one should not make the blessing “to sit in the sukkah”.
If it already started raining, and later the rain stopped, but it’s still overcast, and it seems that it’s going to start raining again, one is not obligated to sit in the sukkah.
Source
Rambam, Yad Ha-Chazakah, laws of sitting in the sukkah, ch. 6, Shulchan Oruch, section Orach Chaim, ch. 639; one should not make a blessing “to sit in the sukkah” because some commentaries hold that one is not obligated to sit in the sukkah, see Ritvo and Ran on tractate Sukkah, folio 29 and Meiri, folio 26