As a professional gardener, who views a number of weather forecast sources because my income and rollercoastering depend on it, I can say they are all sh!te.
It is very difficult to guess where heavy cloud becomes rain, it is often a best guess, and even at the last minute they can get it completely wrong.
Big weather systems are easy to forecast, and they often get them bang on.
The weather for the next few days is a wiggly front, that is going to move up and down a bit, warm air below, cool air above, and a wet bit in the middle.
The wet bit in the middle is Alton...ish.
The queues are shorter, the staff kinder, the coasters faster in the rain.
Skin is waterproof.
Get on with it...make sure you do all rides with a roof, sadly, not as many as there used to be.