Typically, El Niño is first noticed along the South American west coast when warmer than normal temperatures appear within 1 to 4 months the warmer temperatures extend to the mid-Pacific. This is believed to alter the position of the jet stream and cause more intense storms in several regions in North America.
In the Pacific Northwest, El Niño tends to bring drier winters. The area affected in this manner includes Washington, Oregon, British Columbia and the more mountainous portions of Idaho, western Montana, western Alberta and northwest Wyoming.
The warming associated with an El Niño is a departure from long-term average ocean temperatures which are generally cool in the eastern Pacific due to upwelling. As the trade winds in the central and eastern Pacific the upwelling of cold water from far below the ocean surface slows and may even cease temporarily. These effects in turn causes the ocean to warm a degree or two (C) above its climatological average. A strong El Niño can warm by 3 or 4 degrees (C) over large areas, and even 5 degrees (C) in smaller regions.