So, one thing that occurs to me, is that it took a long, long, long time before weather prediction models could improve on "it will be about the same as yesterd

So, one thing that occurs to me, is that it took a long, long, long time before weather prediction models could improve on "it will be about the same as yesterday". So, you might want to test your code on something like, say, "it will move the same amount as yesterday, in the same direction" and see if that also gets 90%. Not saying it would, just that this is a thing you should check.