Just posted this on the Large Format forum as well...
As a tool for planning, I've long been fond of the noaa.gov's website, especially the tabular data that includes projected hourly cloud cover, precipitation, temperature, etc.
Through REI, I've discovered a new tool that takes this data to a new level. Called weatherspark, it is an interactive site that provides not only realtime, but historic weather info.
Check it out at http://weatherspark.com/