What is El Niño?

https://wunderground.atavist.com/el-nino-forecast

El Niño refers to the abnormal warming of the sea surface temperatures in the eastern Pacific Ocean along the equator. This condition of the Pacific Ocean can help to produce a variety of global effects, including drier than normal weather in Indonesia and the Philippines, wetter than normal weather for Peru and Ecuador, a warmer than normal winter for the United States, and above normal precipitation for the southern tier of the United States.
The National Oceanic and Atmospheric Administration (NOAA) has warned that the ongoing El Niño episode could rival the strongest El Niños on record (1982-83, 1997-98) and is nearly certain to last into the Northern Hemisphere spring of 2016.
Bill Patzert, a climatologist with NASA’s Jet Propulsion Laboratory, gave this El Niño its “Godzilla” moniker. The “Godzilla” El Niño of 2015-2016 is expected to be one of the strongest on record, having large consequences on global weather. But not all of El Niño’s impacts are monstrous, and some can even be positive. Knowing what to expect can help people deal with the worst effects of El Niño.

Latest forecast

By Bob Henson

Weekly Update (October 19, 2015):
Strong El Niño conditions remain in place. The weekly sea surface temperature reading, taken within the Niño 3.4 region near the equator, currently sits at 2.4°C above average. NOAA provides an update on El Niño conditions each Monday.
Monthly Outlook (October 8, 2015):

According to NOAA’s latest monthly outlook, there is an approximately 95% chance that El Niño will continue through Northern Hemisphere winter 2015-16, gradually weakening through spring 2016. All models surveyed predict El Niño to continue into the Northern Hemisphere spring 2016, and all multi-model averages predict a peak in late fall/early winter. Outlooks generally favor cooler  and wetter than usual conditions across the southern U.S., and drier and milder than average conditions over the northern tier of states.
More background:
Unusually warm waters now extend from the South American coast westward to the International Date Line in a classic El Niño signature. For much of 2014, the atmosphere failed to respond to several brief warmings of the eastern tropical Pacific, but now both ocean and air are locked into the synchrony that builds and sustains the strongest El Niño events.

5 likely effects of an El Niño episode
1. Increased precipitation in California
2. Fewer hurricanes in the Atlantic
3. Below-normal tornado activity
4. Increased typhoon activity in the Pacific
5. Mild winter for much of the U.S.

Westerly winds bursts continue to kick up across the tropical Pacific, pushing warm water downward and eastward in the form of lumbering, downwelling Kelvin waves that push toward the shores of South America, where they act to suppress the normal upwelling of cooler water.
The only El Niño events in NOAA’s 1950-2015 database comparable in strength to the one now developing occurred in 1982-83 and 1997-98. A single pair of cases is a thin framework on which to build any projections of what El Niño may bring across North America this winter. However, three other episodes since 1950 are rated as “strong” (Niño 3.4 readings topping the SST threshold of +1.5°C for at least three overlapping three-month periods). Many of the far-flung atmospheric responses to El Niño become more reliable the stronger the event, so it’s wise to look especially closely at these cases, rather than simply averaging across all El Niño events.

El Niño is the abnormal warming of the equatorial ocean off the west coast of South America. La Niña is exactly the opposite, characterized by the abnormal cooling of the eastern Pacific Ocean.