I watch national precipitation maps quite often and I have never seen a time when precipitation was not occurring some place in the country. Has it ever happened?
You are correct to observe that precipitation, at least some precipitation, is always falling at some place in the United States. It is a huge country: The United States is the fourth largest in the world (Russia is largest followed by Canada, China and the United States). Because of its size and location, the United States occupies many diverse kinds of geography that create environments favorable for precipitation. Large bodies of water (the Pacific Ocean to the west, Gulf of Mexico to the south, the Atlantic Ocean to the east, the Great Lakes in the north) provide ample sources of moisture.