Has there been no rain before in over 100 chapters?
There has been. They have encountered each other in rain on the way to school, hugging because she was depressed.
Then there was (before that) that time she was playing hooky in the rain and mc showed up with an umbrella and they decided to remain for a bit longer in the rain because they enjoyed the couples umbrella ("let's go all the way around the school building to use the front entrance instead <3").
And they encountered each other while simply out and about doing nothing (mc took shelter under an eave after looking at books iirc, since she had no umbrella) where hino took her under her umbrella (she went out walking for no real reason knowing it would rain from a forecast) and brought her home for that scene where she baths while her clothes are washed (panties handwashed). Iirc that one happened before hino visit mc's house for the first time, AND mc visit hino's house for first time (so whose house did she bath in?)?
There has been a couple of other occurrences too.