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El Nino's Intensity Helps Determine U.S. Impact

August 2009

By WeatherBug's Stephen Baxter

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El Nino, the abnormal warming of the Pacific Ocean off South America`s west coast, is developing and could become a major factor for the upcoming fall and winter. Its severity will have a direct impact on floods, droughts and wintertime arctic outbreaks across the U.S.

The above-normal warming of the Pacific triggering El Nino occurs every two to five years and lasts an average of 12 months. The last one occurred in 2006, a moderate one that lasted only seven months. El Nino alters climate pattern around the world and causes shifts in North America`s jet stream, the high altitude winds that steer storms. El Nino`s intensity guides the severity of these shifting weather patterns.

Strong El Nino

Strong Pacific warming producing a strong El Nino affects, even dominates, worldwide weather patterns. In the U.S., the northern Plains, upper Midwest and western Great Lakes experience a warmer than normal season with significantly less snowfall. El Nino keeps the brutal arctic air bottled up in Canada.

An active southern jet stream brings more rain and mountain snow to the South from California to the Gulf Coast, accompanied by cooler temperatures. This can lead to dangerous flooding in California and the Southwest. The very strong El Nino event of 1982-1983 produced the wettest and snowiest winter on record in much of California, while the equally strong 1997-1998 event set many monthly precipitation records. Major cities like San Francisco and Los Angeles set records for the wettest February on record in 1998.

The Southeast is more likely to see frequent winter storms and colder than normal temperatures. The displaced jet stream also steered storms away from the Ohio Valley, producing less rain and snow there. The Northwest is likely to see a warmer, drier season when the El Nino is strong; normal storm tracks aim farther south, barreling into California instead.

Weak to Moderate El Nino

Weaker El Ninos do not provide the same weather as its stronger brother. During a weak to moderate El Nino, the effects are smaller away from the tropics, and other climate factors outweigh El Nino on U.S. weather. The currently weak El Nino is forecast to remain weak to moderate through the winter.

A weak to moderate El Nino is less likely to produce a persistent crashing of winter storms into California. The last three El Nino`s, in 2002, 2004, and 2006, were weak to moderate. Those did not cause the excessive rains and flooding that were seen during the very strong 1997-1998 El Nino. In fact, the 2002-2003 and 2006-2007 seasons saw below-normal rainfall in the Southwest.

Portions of the Midwest into the interior Northeast typically experience much colder winters during weak El Nino seasons than during strong seasons, with more frequent arctic air outbreaks. The northern Plains and upper Midwest are likely to see a more normal winter during weaker El Nino`s. In fact, the weak El Nino`s of 1976-1977 and 1977-1978 coincided with two brutally cold winters in the U.S.`s eastern half.

In both weak and strong El Nino events, the Northwest can expect a warmer and drier season, while cooler and wetter weather remains likely in the Southeast.

WeatherBug will continue to be a reliable source to get up-to-date information on El Nino and how it can impact weather in your neighborhood.

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