As a tool for planning (besides looking out the window) , the noaa.gov's weather data has been one of my favorite tools, especially the tabular data that includes hourly cloud cover, precipitation, temperature, etc.
I've just 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/
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