Updated El Niño forecast reaffirms fall and winter rain outlook

Updated El Niño forecast reaffirms fall and winter rain outlook

Why are forecasters expecting this El Niño event to be larger than others in the past?

National and international climate prediction forecasters have released an updated report on what they are terming "a very strong El Niño event" developing in Pacific waters, affirming previous forecasts favoring substantial late fall and winter rains for large areas of the U.S. including the western, southwestern and southern states of the U.S.

According to the latest World Meteorological Organization (WMO) prediction models, the current El Niño will continue to strengthen through the remainder of the year, peaking between October 2015 and January 2016. During that peak, the models reaffirm an earlier forecast update from the National Aeronautical and Atmospheric Administration (NOAA) three weeks ago indicating this El Niño could be among the top four strongest on record.

"We have had years of record Arctic sea ice minimum and have lost a massive area of Northern Hemisphere snow cover, probably by more than 1 million square kilometers in the past 15 years," said David Carlson, director of WMO's World Climate Research program, a situation he says introduces another factor in predicting a record El Niño Southern Oscillation (ENSO) event this year.

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El Niño is an anomalous warming of the central and eastern equatorial Pacific Ocean. But for reasons still unknown, every two to seven years this patch of ocean warms for six to 18 months.

Under normal circumstances, easterly trade winds near the equator push warm water into the western Pacific Ocean. In turn, an upwelling develops moving deep, cold ocean water to the surface creating a cooler eastern and central Pacific Ocean. But, according to Weather Channel climate specialists, during an El Niño year, these trade winds weaken, and may even reverse from west to east, bringing warmer western Pacific water back toward the central and/or eastern Pacific Ocean in what's known as an "equatorial-trapped Kelvin wave."

Since thunderstorms require warn, humid air near the water's surface to fully develop, during these periods, more rain showers can be expected across the western Pacific. The change in normal trade winds also causes changes in atmospheric circulation not just over the equatorial Pacific, but also can have far-reaching impacts on the atmospheric circulation across much of North America, disrupting normal weather patterns.

Why a larger El Niño?

Why are forecasters expecting this El Niño event to be larger than others in the past?

"Compared to the last major El Niño event in 1997-1998, there is much more information available [to forecasters]," said Maxx Dilley, Director of WMO’s Climate Prediction and Adaptation Division. "We have better models and are much more prepared."

He says this year’s El Niño event is expected to be the strongest since 1997-1998, and has the potential and energy to be even stronger. Already the developing El Niño has created a wetter than average spring and summer for much of the Southwest, including New Mexico and Texas.

Dilley told reporters at a recent press conference that the El Niño/Southern Oscillation is one of the main drivers of the climate system and contributes to extreme events like droughts and flooding in different parts of the world. Globally, it has a warming influence on average temperatures.

He points to record warm temperatures already this summer season as being, in many instances, the warmest on record, and attributes that to a changing climate and promoted by the current strong southern oscillation event.

Carlson, however, warns that while this developing El Niño has the potential to be one for the record books -- perhaps the strongest ever on record -- not every geographical region of the zone expected to be affected will live up to its full potential.

Unique El Niño events

He says no two El Niño events are the same, and cautions those hoping for drought relief next winter in California not to depend on it happening. He warns regional forecasters should not draw the conclusion that significant rains are ahead as other factors can play a role in influencing ENSO events.

Heavy rainfall is possible with or without the influence of a hearty El Niño, so concrete conclusions should not be assumed until more is known about the event and how it will affect each area.

A look back at past El Niño events however indicates some common weather expectations if other factors do not interfere or disrupt ENSO conditions. They include:

  • Wetter conditions for the Southern half of the U.S. from California to the Carolinas then up parts of the East Coast
  • Drier conditions for parts of the Ohio Valley, Great Lakes, Northwest and Northern Rockies
  • Cooler conditions for the desert Southwest, Southern Plains, northern Gulf Coast
  • Warmer conditions for the northern tier states from the Pacific Northwest to the Northern Plains, Great Lakes, and Northeast
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