Generally speaking it doesn't rain in Israel on Succos. However the forecast is for rain late tonight and early tomorrow and therefore we will not be able to sit in the succa.
At first glance, we would say that the פטור of Succah when it is raining is the regular פטור of מצטער. However, if you look at both the Gemara and the Rambam it doesn't seem like that.
The Gemara in Succah on 26a in discussing the din of a חולה and his attendents posits the idea in the name of רבא of מצטער פטור מן הסוכה. However, the Mishna later (28b) discusses rain and gives a completely different reason, it gives a parable of a servant spilling water on his master (or vice versa see the Gemara 29a). The Gemara at no point brings this Mishna (discussing rain) as proof for רבא.
The Rambam also seems to split these 2 up. The Rambam (סוכה ו:ב) writes the halacha of a חולה and a מצטער that both are פטור מן הסוכה in halacha 2. However, he doesn't bring the halacha of rain until halacha 10 unconnected to מצטער. It seems clear from the placement that the 2 are unconnected.
Based on this we could say that the din of rain is an independent din and not based on מצטער. It could be that when it rains the succah loses the din succah because it is not a דירה with rain coming in. It is not related to the person's discomfort but the succah itself.