The future is a very hard thing to predict with any degree of specificity and accuracy. General predictions are easy and safe: The sun will rise tomorrow, you will take a short trip (to the mailbox, to the grocery store,...) tomorrow. Weather forecasters work with many, many, any I mean MANY scientific models, patterns, and probabilities. The accuracy decreases significantly for how many days out, one day out maybe 85% accurate, two days out maybe 60% accurate, and so on.
Basically, it's a calculated guess.