NOAA NWS API
The National Oceanic and Atmospheric Administration (NOAA) provides the public, government agencies, and commercial enterprises with XML data from the National Weather Service's (NWS) digital forecast database.
This API is available through SOAP or REST protocols, and provides an array of parameters allowing users to create customizable queries to the digital forecast database. Users can search the database by specific latitude/longitude, time, unit of measurement, and weather parameters. This service is provided free of charge by the U.S. government.