Napa’s weather in september. The weather in napa during the month of september is neither too hot nor too cold. The average high temperature is 81°f (27. 2°c) and the average. Embrace wine country casual. You’ve probably seen the term “wine country casual” thrown around when researching what to wear in napa valley. Though a vague term, it. This style of top is classy without being too formal. Slip into your best suit when going to give an interview for a business. Wear a tie as well. Keep it formal instead of laid back.