Thats not going to stop Trump supporters from voting though. They will even cross a field of land mines in a blizzard just to cast their vote for The Orange One.
Looking at the weather map, on the west coast, Washington and Oregon are going blue regardless of weather. Idaho, Montana, Wyoming and North Dakota are going red regardless of weather. I doubt weather affects Iowa, because Iowans are used to terrible weather, including freezing cold and snow. Plus they are really passionate about voting because they are a first-primary/caucus state. So rain isn't going to keep them from voting.
In the middle of the country, Missouri, Arkansas and Louisiana are going to be red regardless of weather. Minnesota and Illinois will be blue regardless of weather. In the east, Maine, New Hampshire and Vermont are used to cold/rain so the vote won't be affected. South Carolina will be red regardless of weather.
So the states where the weather might have an impact are Michigan, Wisconsin, Florida, Texas and North Carolina. In North Carolina specifically, the part that would be impacted by rain is probably mostly Trump voters. In Texas the rain is hitting all the big cities so that would theoretically hurt Democrats the most. In Florida the rain hits Miami, but I'm thinking Floridians are accustomed enough to rain to mostly just tough it out. That said, reduced turnout in Miami would be disastrous for Democrats.
Heavy rain in the big cities in Wisconsin and Michigan will definitely hurt Democrats, so that would be really bad news for the Harris campaign. So on balance, that weather forecast heavily favors Trump. I guess we will see if he is able to capitalize on it, or if people are so determined to keep him out that they brave the weather to do so.
The other thing(s) to consider, is that a lot of folks have already voted early or by mail (I did), so the lines at the polls are going to be shorter and there are just less people overall to be discouraged by weather. I'd guess that anyone who was waiting specifically to vote in person, for whatever reason, is passionate enough about voting in person, that they're not going to be dissuaded from voting at the last minute by some rain.