Western Regional Climate Center
Climate Diagnostics Center
Climate Prediction Center
National Climate Data Center
Atmospheric Sciences Department, University of Washington
Global data from Center for Climate Research, University of Delaware
US Historical Climate Network (best for trend analysis)
Monthly and seasonal climate composites with NCEP data

El Niño, La Niña
Pacific Decadal Oscillation
Global climate change

Oregon Climate Services
Idaho State Climate Services
American Association of State Climatologists