The stories about Paul Bunyan (the tall tale lumberjack) say he worked in northern Minnesota.

Looking at average temperatures in one northern Minnesota town show you that it isn't actually that hot. The average high for July (the hottest month) is 77.8F, while the average low for that month is 52.6F. The record low for that month is 32F. While International Falls is not the norm for northern MN, even a milder northern MN city like Brainerd has an average July temperature of 69F.

https://en.wikipedia.org/wiki/International_Falls,_Minnesota https://en.wikipedia.org/wiki/Climate_of_Minnesota

So, would flannel be too hot for the lumberjacks? For the ones in northern Minnesota, it wouldn't be.

Anecdotally, I've gotten medically diagnosed hypothermia in the rain during the summer in northern MN.