There are several Web pages devoted to the climate of the U.S. and specific regions of the country. At the national level, there is the National Climatic Data Center. A related resource is the Climate Prediction Center. [More information on these sites is available under the 'Government' menu selection.]
There are also centers -often located at major universities- which specialize in regional climatology; these centers include ones covering the West, the High Plains, the Midwest, the Northeast, the Southeast, and the South; you may go to them by clicking on the appropriate area of the map below which depicts each office's jurisdiction.
Additionally, Canadian climate information is available from the Canadian Climate and Water Information Office. Other data and climatology links are available from World Climate.