Illinois State Water Survey - University of Illinois at Urbana-Champaign

State Climatologist Office for Illinois

El Niño and La Niña in Illinois

Dr. Jim Angel, State Climatologist

El Niño and La Niña refer to periods when sea-surface temperatures along the equator in the Pacific Ocean are either unusually warm (El Niño) or cold (La Niña). These events typically begin in the spring or summer and fade by the following spring. A more complete description of El Niño and La Niña can be found under Other Resources below.

Summary of Impacts of El Niño

El Niño events vary in size, intensity, and duration. As a result, the impacts can vary from one event to the next. In addition, there may be other factors that influence our weather during these events.

  • Summers tend to be slightly cooler and wetter than average
  • Falls tend to be wetter and cooler than average
  • Winters tend to be warmer and drier
  • Springs tend to be drier than average
  • Snowfall tends to be 70 to 90 percent of average
  • Heating degree days tend to be 80 to 90 percent of average. Lower heating degree days mean lower heating bills.
  • Tends to reduce tornado activity in the High Plains and Midwest and increases it in the South.

Summary of Impacts of La Niña

Generally, La Niña impacts are not as clear-cut because there are fewer strong ones in recent years (1970-71, 1973-74, 1975-76, 1988-89).

  • Summers have a tendency to be warmer and drier in Illinois
  • Falls have a tendency to be cooler in the north and wetter in the southeast
  • Winters are typically warmer and wetter than average with more snow and winter storms
  • Springs tend to be cooler across most of the state and drier in the west
  • Tends to increase tornado activity in the High Plains and Midwest, while reducing it in the South.

Impact of Strong and Weak El Niño and La Niña

This page explores the impacts of strong and weak El Niño and La Niña events on monthly temperature and precipitation in Illinois. Monthly temperature and precipitation are expressed as departures from the 1971-2000 average (i.e. normal).

The definition of events are based on the sea surface temperature (SST) anomalies in the Nino3.4 region of the Pacific Ocean region.

  • Strong El Niño (SST departure of +2°C or warmer):
    • Temperature - tendency for warmer-than-average weather from December through March
    • Precipitation - tendency for wetter-than-average weather in late winter and spring
  • Weak El Niño (SST departure between +1 and +2°C):
  • Strong La Niña (SST departure of -2°C or colder):
    • Temperature - tendency for warmer-than-average weather from January through March but not as strong or consistent as a strong El Nino.
    • Precipitation - tendency for drier-than-average weather July and August before becoming highly variable
  • Weak La Niña (SST departure between -1 and -2°C):
    • Temperature - tendency for warmer-than-average weather from October to December and again from February to May.
    • Precipitation - tendency for dry-than-average weather from November to January, and again from April to June.

Other Resources

Water Survey Resources

Outside Resources

Illinois State Water Survey

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Champaign, IL 61820-7463

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