There is no reason to fear rain. It rains on trees every day. They're fine. If heavy rain is expected, prop one end of the pot up on a rock or fencing slat, so the pot is on a bit (not a lot) of an incline. That incline increases drainage to low end and help shed excess water.
FWIW, you will have to get a feel for how to water it all over again in its new location. It's not the same as inside. We can't really tell you how to do that because your local conditions and tree will dictate. One way to get a feel for when to water is to water the pot heavily until a lot of water runs out the bottom holes. Allow that water to drain and stop flowing. Then pick the pot up. Feel the weight? That is what a well-watered pot feels like. Let it sit for a day, pick it up. Feel the weight difference ,or you may not. depends on how much water the tree is using. Some days it will use a lot, other days it won't. Keep monitoring the pot though, it will get lighter, sometimes substantially lighter with windy dryer weather. Once it dries down an inch into the soil, water well again.
Also FWIW, NEVER rely on the rain to do your watering for you. It will not. If you do that, you will lose trees. Rain is NEVER a given. Enough rain to soak through the pot is an all day affair, or a heavy 1/2 downpour. A light rain that lasts an hour isn't going to do much. Rain is fickle and unreliable as a water source.
