Not necessarily since you could safely use the closest city which is in the weather database. As long as the ‘weather-city’ being used is mentioned, together with the distance from it, the user knows that it is an estimate.
On the weather-and-climate website for the US only 60+ cities are listed. There is probably an (open source) weather database available with more cities than this. (After a quick search I found http://www.worldclimate.com/sources.htm).