Starting In Cold Weather
Early NoBo hikers can expect to hike in cold weather depending on when they start. The cold in the Georgia mountains can last through March and into April in some cases. If you start in early March or before, expect snow.
Crowds of People
NoBo season is at its peak between March and April. Expect crowds of people at the beginning of your hike and a rare chance to camp alone (at least at designated camps).
Humidity & Bugs
I think this one is kind of unavoidable. Depending how quickly you move and how early you start, there's a high chance that you'll be hiking through the intense summer humidity of the east coast. Oh, and when you get to New England: BUGS!
It's pretty typical for a NoBo to complete their hike in September, and depending on the year, the foliage could be quite incredible. You may have driven through New England during peak foliage, but have you spent weeks hiking all day through it? It's something you'll never forget.
Early Fall Snow
Some late NoBo hikers could run into extreme cold and early fall snow storms in Maine. I finished on 10/31, so much later than most, but not the last. The nights had dropped to single digits and snow had fallen. Some of which was while we were on a bald mountain! If you find yourself in Maine later in the season be prepared for winter conditions.