In the United States, the El Niņo/Southern Oscillation (ENSO) phenomenon frequently has a profound influence on the nation's autumn, winter, and spring weather patterns. Most significantly, a strong sub-tropical jet stream associated with El Niņo (ENSO warm phase) often produces cooler- and wetter-than-normal conditions across the nation's southern tier from autumn into the spring, beginning during the year of warm-phase onset. Other typical warm-phase effects during the U.S. cold season include above-normal temperatures across most of the nation's northern tier and unusually dry conditions on the northern Plains and from the Ohio Valley to the lower Great Lakes region. La Niņa (ENSO cool phase) typically has the opposite effect on U.S. weather patterns, resulting in warmer- and drier-than-normal conditions across most of the South, wetter-than-normal weather in the Ohio Valley, and cold, wet weather in the northern Plains and upper Midwest. In addition, cool, wet conditions often extend southward during La Niņa along the West Coast into California.

Due to the timing and area affected by the phenomenon, winter wheat is the U.S. crop most susceptible to the effects of the ENSO, which can ultimately determine critical environmental factors such as snow cover and temperature and wind extremes. ENSO's impact on moisture supplies can also influence spring planting across the South and affect irrigation reserves in the West. In addition, cold, wet weather associated with the cold phase can increase stress on livestock across the northern Plains and upper Midwest, and contribute to spring snow-melt flooding in the upper Mississippi Valley and adjacent river basins. While the overall health of crops and livestock is dependent upon a variety of factors, departures from a region's normal weather patterns that can occur during a distinct warm or cool phase of the ENSO phenomenon can be significant.