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El Nino Dramatically Affects West Coast Winters

October 13, 2009

By WeatherBug's Stephen Baxter

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El Nino, the abnormal warming of the tropical Pacific, is developing and will impact temperature and precipitation patterns along the entire West Coast this upcoming winter. Both the Northwest and the Southwest typically experience significant, but opposite, El Nino affects from changing jet stream patterns.

From excessive rain, flooding, and mudslides in southern California to drought in the Pacific Northwest, El Nino has historically changed winter climate in ways that impact the entire West.

Southern California, Desert Southwest

  • Powerful upper-level winds send storms barreling into southern California and through the Southwest, causing above normal precipitation and colder temperatures.

  • Cooler temperatures lead to lower snow levels and increasing snow packs, which in turn lead to increased stream flow in the spring. Since snow pack is a major source of water in the West, this can replenish the fresh water supply.
  • Major flooding can occur. Almost all of the major flood events on main rivers have happened during or following El Nino winters, such as the 1983 Colorado River flood that killed at least seven people and caused over $12 million in property damage.
  • Continuous rainfall can cause damaging, even deadly, mudslides, especially in and around Los Angeles and San Diego.
  • Some of the worst effects are felt during the strongest El Nino years. During the 1982-83 and 1997-98 El Ninos, many precipitation records were set throughout the West. The winter of 1982-83 set the record for the wettest and snowiest California winter as flooding caused approximately $850 million in damage. Frequent heavy rain in the 1997-98 season caused almost $550 million in storm damage statewide.

    However, there have been several El Nino winters that did not see continuous storms pummeling the Southwest. The recent 2002-03 and 2006-07 El Ninos actually saw below normal precipitation from southern California to the Desert Southwest

    Northwest

    • The changing jet stream guides storms south into California or well north into Canada.

  • Mild and dry Northwest winters result with significantly decreased mountain snow packs and spring stream flow. This can negatively affect water-sensitive industries like agriculture and power generation.
  • Drought conditions are more likely to persist through El Nino winters.
  • Like in the Southwest, not all El Ninos produce these patterns in the Northwest. The 1997-98 and 1982-83 El Ninos resulted in above normal precipitation all the way up the coast into the Pacific Northwest. The last three El Ninos, however, beginning in 2002, 2004, and 2006, did produce generally mild and dry winters here.

    Northern California resides between the two regions that are most affected by El Nino. Including cities like San Francisco and Sacramento, it is a battle ground between mild and dry to the north, and cool and wet to the south. Winter weather here will depend on the strength of the El Nino and other climate factors. Strong El Ninos tend to signal a cool and soggy season, while weaker El Ninos often result in a more average winter.

    Keep checking back with WeatherBug for the latest on the developing El Nino and how it will affect climate in the West as well as the entire U.S.

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