Whether you're planning a wedding or a weekend outing, a forecast gives you trends seven to 10 days out and is 96 to 98% accurate one day out. Temperature holds up better further out than precipitation, which is notoriously hard to pin down — a problem for any outdoor event. Where you're holding it matters too: coastal regions and arid areas like the Southwest have steadier, more predictable weather than the middle of the country, and summer brings pop-up afternoon thunderstorms that are the hardest of all to forecast.