I survived winter on the AT about 10 years ago. Technology may have improved since then, but I definitely know what works. Here are my suggested layers for cold/wet days on the AT:

• Thick wool socks
• A buff or balaclava, a nice thick beanie
• Fingerless polyester glove liners
• Polyester thermal base layer (top and bottom). Synthetic t-shirt.
• Breathable nylon pants
• Midweight fleece jacket (Patagucci R1 is too light)
• Midweight puffy (I used down and only used it for emergencies or at camp, due to the wetness).
• Hardshell top and bottom. A real hardshell. Not Frog Toggs, and definitely not a poncho. This is your life saving equipment. If you are dying, it's what will save your life.

Your gear is extremely important. You need to be absolutely confident that it will still provide warmth even if it's completely drenched through.

The next key is to be keenly aware of what your body is telling you. You can't rely on shivering to warn you. When you're losing temperature while already active, it's very easy to miss it. A big part of this is about having your fuelling strategy dialled in. If you're taking in enough calories, then you should be burning hot.

The big risk is when you stop moving. You can go from very warm, to severely hypothermic in under 10 minutes. Be on the ball with your layer changes. Have your fleece and your hard shell nearby at all times and ready to put on the second you stop moving.

This is no joke. You can definitely die. The AT can get nasty. I hiked through more than a week solid of freezing rain and driving snow through the Smokies. Someone died of hypothermia in one of the shelters I had planned to shelter in. He was around your age, and an experienced hiker. It was incredibly sobering, especially since I had to choose between heading back out into the storm as the sun was setting or spending the night with a dead body.