Solar storms often start because of a flare/eruption on the Sun. It's extremely challenging to predict when these eruptions are going to happen, so those predictions are given in terms of probabilities (for example - you might say there's a high or low chance of an eruption on a given day). Once the eruption has happened, there's about a 1-2 day time lag before it might get to the Earth and cause a solar storm, so that's the kind of notice we get. But not every eruption is headed our way, and not every eruption causes a big storm. So there's a lot of hard work that goes into figuring out whether we will get a storm or not.