Okay, so I ran the test, and it seems like the chance of rain in Las Vegas is 10%, which is far too much for a track that's supposedly in the desert, with nothing around it.
If it's a track like Abu Dhabi or Qatar, which indeed do see rain in very rare occasions (I did see rain in Abu Dhabi in a Haas career in F1 22 before), I would understand that, but Las Vegas sees 5 inches of rain per year, and most of it comes from the monsoon season in Summer, and since the race is held in November, there should not be any rain at all.
Yeah, so the weather for Vegas should be set to dry only in Career Mode, maybe clouds here and there, but no rain.