@elfalas I'm not sure why you're mixing indoors and outdoors temperatures in this context. The warmer regions don't react well to sudden snowfalls or drops in temperature because the infrastructure there is not built for them; traffic speed slows down due to poor traction and/or visibility, it takes longer to clean up the snow because the snow-removal vehicle fleet is not remotely as numerous as in the northern regions, and many public institutions such as schools end up with freezing temperatures inside because they don't have central heating, which is why Tachibana had to light up the gas stove.
With that being said, many of the households in the warmer provinces of Japan don't have any heating other than a couple electric appliances, either, and many of the cheaper ones aren't very well-insulated, so during some of the winter months the indoor temps drop to 10–15 °C or even lower, which is why you constantly see people dozing off under the kotatsu or telling each other not to fall asleep without a blanket. In contrast, where I live we have snowfalls 4–6 months a year and outside temps often drop below −20°C around mid-January, but it's rare to see a household with inside temperature below 18°C (20–22 °C is the norm in most places) because they're all built with central heating in mind.