In the Northeast: August is hot and sometimes muggy, but decreasingly so as the month winds down. September is the beginning of the transition--snow is very rare except in the mountains, but freezing weather is not out of the question in parts of northern New England and New York, and in the mountains.
Midwest: August is quite hot; September is starting to cool off. Again, snow is highly unlikely in September. The northern Great Lakes region is not as hot.
Southeast: Hot and muggy. September does not provide much relief from the heat in the deep south, especially in Florida. Summer continues until October down south.
West: Warm throughout (the closer to the coast, the more stable the climate is). You can get a heat wave any time, because of the downsloping. The Northwest is chilly and rainy (farther inland, in the mountains, it can and does snow in August and September; valleys can get pretty hot at times. The Desert Southwest is usually a furnace in August (and the monsoons can make it more humid); September is still very hot in the deserts.
