Clear Sky Chart

http://cleardarksky.com/csk/

Based on the Canadian (GEM) weather model, Clear Sky Chart is perhaps the most accurate and the most usable forecaster of astronomical observing conditions for more than 1,900 observatories and observing sites in North America.

Satellite Imagery

http://weather.rap.ucar.edu/satellite/

This page, maintained by the National Center for Atmospheric Research, will provide you with the very latest GOES satellite images in the visible and infrared spectrum, as well as water-vapor imagery. You can even loop the clouds to indicate their motion over a time span of up to 12 hours.

Radar Imagery

http://radar.weather.gov/ridge/Conus/full_loop.php

This page depicts a Full Resolution National Mosaic Enhanced Radar Loop showing the movement of precipitation over the past 70 minutes. You can also adjust it to show a nonlooping image. And if you move the cursor to a specific spot on the map and click, you'll be directed to the local radar view from the nearest National Weather Service Forecast Office.

National Weather Service (NWS) Forecast Offices

http://www.nws.noaa.gov/organization.php

Speaking of the NWS Forecast Offices, this page provides links for about 150 offices across all 50 states, as well as Puerto Rico, Guam and American Samoa. Each Forecast Office provides the latest NWS weather forecast for its particular region as well as more weather info.

Weather Prediction Center (WPC)

http://www.wpc.ncep.noaa.gov/

Located in College Park, Maryland, the WPC is one of nine service centers under the umbrella of the National Centers for Environmental Prediction, a part of the National Weather Service, which is part of the U.S. National Oceanic and Atmospheric Administration. The Weather Prediction Center issues summaries on storm systems bringing significant rainfall and snowfall to portions of the United States.IT also forecasts precipitation amounts for the lower 48 states for systems expected to affect the country over the next seven days.