Just in Case: Guidelines if it rains on the first nights of Succos

In general when it is raining one is not obligated to eat in the Succah. The rule for the first nights of Succos in unique in that many hold that there is a mitzvah to eat in the Succah even if it is raining. Thus we try to observe the mitzvah ideally by waiting a bit to see if the rain will stop, but we will eat a portion of bread in the Succah even if the rain doesn’t stop.

 
For the first night of Succos- If it is raining, we should wait to see if the rain will stop so that we can fulfill the Mitzvah to eat in the Succah in the ideal way, without rain. The extent that we will wait differs depending on family need. It may be 15 minutes for one family, or an hour or more for another family. Either way, we wait a bit, and then, if the rain has not stopped, we recite Kiddush, (wash) and have Hamotzie and eat a portion of bread in the Succah despite the rain. As part of Kiddush we do not recite “Leishev B’Succah” as some maintain that one cannot fulfill the obligation of Succah while it is raining. We do recite “Shehechiyanu” on the Yom Tov and on the Succah (as the Shehechiyanu on Succah can be recited even before Yom Tov). We then complete the meal indoors. 
If the rain stops before going to sleep then we go into the Succah to eat a portion of bread. If the indoor meal was already completed then a new Hamotzie is said. The bracha of “Leishev” would be recited.
 
For the second night- If it is raining, we should wait a few minutes to see if it will stop. If it doesn’t stop after a brief wait, we recite Kiddush indoors and begin the meal. Rain or not we eat a portion of bread in the Succah before completing the meal. If the rain has stopped by that time, the Bracha of “Leishev” should be recited. 
 
For further study see Shulchan Aruch/ Ramoh 639:5 with Mishna Berurah comments on pages 209-210.
 
With best wishes for a dry Succos and wonderful Yom Tov.