The Jewish Calendar and Holidays (incl. Sabbath): Sukkos: Eating in the Sukka in the Rain:
Can I eat in the Sukka in my raincoat and hat when it's raining?
You are under no obligation to eat in a Sukka at all when it's raining (Shulchan Aruch O.C. 639:5). If you are
eating in the Sukka and it starts to rain, you may pick up and finish in your home. Once you've moved in,
even if the rain stops, you do not have to finish outside. The exception to this is the first night of Sukkos.
You are obligated to make Kiddush and eat an olive-sized piece of bread in the Sukka, even in the rain.
Preferably, you should wait some time to see whether it stops raining first. You should certainly don your
raincoat and rainhat for this situation, so that you don't ruin that new suit and tie.