The website www.atweather.org provides weather forecasts for shelters along the AT. Information comes from the National Weather Service (NOAA) and is more relevant to hikers than forecasts for nearby towns. Even "nearby" towns are usually far away at much lower elevations, and so have very different weather than the AT. Conditions on the AT are typically cooler, wetter, and windier than in "nearby" towns.
The website is no-frills and wonderfully simple: choose a state from a drop down menu and then select a shelter (every AT shelter has a name). An up-to-date forecast then appears for that location that includes both the chance of precipitation and the predicted rainfall in inches.
Understanding Forecasts: a 20% vs. 80% chance of rain
During the summer months on the AT, you'll notice that nearly every day the forecast says, "chance of rain 30%" or "chance of thunderstorms 20%" and the like. Nevertheless the rain does not come. This is because individual storms form due to rising air caused by the uneven heating of the earth's surface, a process called convection. Conditions across the entire Appalachian mountain range favor the creation of thunderstorms but when and where a storm develops is dependent upon so many unmeasurable variables, it's essentially random. When a storm does appear and is detected on radar, the forecast for that specific location is changed and a weather alert may be issued.
When forecasts list the chance of precipitation at say 70-90%, that's usually because there is a large-scale weather system moving across the country. Unlike individual thunderstorms, these weather systems are driven by global meteorological patterns. Their movement can be plotted with more accuracy than a single storm and even the amount of precipitation can be predicted. So rather than telling you that conditions favor the creation of a storm, an 80% chance of rain means a large storm system is headed your way. Rain is essentially certain. Make hiking decisions based on the predicted rainfall amount. Rainfall of 1-2 inches isn't so bad but 5-6 inches is a deluge that's best avoided by remaining in a shelter or your tent.