A rain proof jacket with hood or a poncho. Get one that is guaranteed waterproof. Woolen shirts and thick sweaters. During the winter months, December through February, these items are essential. Thick sweaters can be purchased in Kathmandu.
Heavyweight trousers are useful higher up in the mountains in the morning and at night.
These are necessary on all treks going above 10,000 feet. These are excellent to sleep in at night. In the winter months thermal underwear are quite invaluable. A track suit. Useful for wearing in camp and in the tent.