80 percent of the time a weather forecast a week out is pretty accurate but any further than a week out the weather forecasts are only about 10% accurate.
It's very hard to predict the weather any further out than 1 day and be very accurate.
I use weather.com to check weather forecasts and they've been pretty accurate a week out on the weather forecast but it's all guesses through science and a bunch of computer programs.
So weather can change from day to day and be very different from what a forecaster or weather forecast website states.