Weather data is often cited as a "huge success" for the open data community. Where does this data sit, and how does one parse the data into something readable, like the 5 day forecast?
JSON is a simple text based data format than can be converted to arrays or any structured data to access it in your applications. Try RESTClient extension in mozilla firefox to get an idea of how this works if you are new to JSON. You can view any JSON data online, e.g. at JSON Viewer online
Here is an interesting project -- open weather map
Besides the NCDC data for forecasts, you can also get pretty detailed meterological information for airports (US and international) in METAR format:
... but to make use of it, you need to be able to decode the station codes to find airports near the location of interest.
NOAA provides weather data. You can see the general information and visualization at http://www.weather.gov/ Specific data products are found at http://www.ncdc.noaa.gov/most-popular-data When you click on a dataset you are interested, there is technical documentation and material to guide you in the use of the data. For example, local climatological data can be found at http://cdo.ncdc.noaa.gov/qclcd/QCLCD?prior=N with detailed notes at http://cdo.ncdc.noaa.gov/qclcd/qclcdfaq.htm