California residents are being warned to prepare for El Niño - as Nasa revealed the phenomenon will be the most watched in history.

There are predictions it could bring as much as 35 inches of rain during the upcoming winter season in some part of California.

However, El Nino this winter will leave a big wet but not necessarily snowy footprint on much of the United States - but won't solve California's drought crisis it has been claimed.

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This visualization shows side by side comparisons of Pacific Ocean sea surface height (SSH) of 2015 with the Pacific Ocean signal during the famous 1997 El Niño. These 1997 and 2015 El Niño animations were made from data collected by the TOPEX/Poseidon (1997) and the OSTM/Jason-2 (2015) satellites.

WHAT IS EL NIÑO El Niño is caused by a shift in the distribution of warm water in the Pacific Ocean around the equator. Usually the wind blows strongly from east to west, due to the rotation of the Earth, causing water to pile up in the western part of the Pacific. This pulls up colder water from the deep ocean in the eastern Pacific. However, in an El Niño, the winds pushing the water get weaker and cause the warmer water to shift back towards the east. This causes the eastern Pacific to get warmer. But as the ocean temperature is linked to the wind currents, this causes the winds to grow weaker still and so the ocean grows warmer, meaning the El Niño grows. This change in air and ocean currents around the equator can have a major impact on the weather patterns around the globe by creating pressure anomalies in the atmosphere. Advertisement

'We could see upwards of 200 percent of average rainfall event,' said Bill Croyle of the California Department of Water Resources, according to ABC7.

'We do expect an extremely wet season in Southern California.'

Last week, in what has been considered the fourth year of drought, flooding and mud flows trapped hundreds of motorists in northern Los Angeles County and forced the temporary closure of the 5 Freeway through the Tehachapi Mountains.

In August, flash flooding west of Desert Center washed out a 10 Freeway bridge.

'Make sure you heed any warnings of local officials,' said Chief Scott Edson of the Los Angeles County Sheriff's Department.

'If we're in your area asking you to evacuate because we're concerned for the area, please do your best to respond to our request so that you don't become part of the problem.'

Nasa said it plans to study the phenomenon in unprecedented detail.

This year's El Niño is already strong and appears likely to equal the event of 1997-98, the strongest El Niño on record, according to the World Meteorological Organization.

'El Niño is a fascinating phenomenon because it has such far-reaching and diverse impacts,' said Lesley Ott, research meteorologist at NASA's Goddard Space Flight Center, Greenbelt, Maryland.

'The fact that fires in Indonesia are linked with circulation patterns that influence rainfall over the United States shows how complex and interconnected the Earth system is.'

Using NASA satellite observations in tandem with supercomputer processing power for modeling systems, scientists have a comprehensive suite of tools to analyze El Niño events and their global impacts as never before.

For instance, scientists are learning how El Niño affects the year-to-year variability for fire seasons in the western United States, Amazon and Indonesia.

El Niño may also affect the yearly variability of the ground-level pollutant ozone that severely affects human health.

Researchers will be keenly focused on how the current El Niño will affect the drought in California.

This visualization shows side by side comparisons of Pacific Ocean sea surface height (SSH) anomalies of what is presently happening in 2015 with the Pacific Ocean signal during the famous 1997 El Niño. These 1997 and 2015 El Niño animations were made from data collected by the TOPEX/Poseidon (1997) and the OSTM/Jason-2 (2015) satellites.

'We still have a lot to learn about these connections, and NASA's suite of satellites will help us understand these processes in a new and deeper way,' said Ott.

Earlier this month the National Oceanic Atmospheric Administration issued its official winter forecast and 'the driver of this winter's outlook is El Nino,' said Mike Halpert, deputy director of NOAA's Climate Prediction Center.

However, forecasters say that although California is set for a wet winter, it won't be enough to ease its drought crisis.

While it is good news that drought improvement is predicted for California, one season of above-average rain and snow is unlikely to remove four years of drought,' said Halpert.

WHAT TO EXPECT THIS WINTER Precipitation Outlook: Wetter-than-average conditions most likely in the Southern Tier of the United States, from central and southern California, across Texas, to Florida, and up the East Coast to southern New England. Above-average precipitation is also favoured in southeastern Alaska. Drier-than-average conditions most likely for Hawaii, central and western Alaska, parts of the Pacific Northwest and northern Rockies, and for areas near the Great Lakes and Ohio Valley. The National Oceanic Atmospheric Administration issued its winter forecast, saying El Nino will leave a big wet but not necessarily snowy footprint on much of the United States. Temperature Outlook: Above-average temperatures are favoured across much of the West and the northern half of the contiguous United States. Temperatures are also favored to be above-average in Alaska and much of Hawaii. Below-average temperatures are most likely in the southern Plains and Southeast. Drought Outlook: The U.S. Drought Outlook shows some improvement is likely in central and southern California by the end of January, but not drought removal. Additional statewide relief is possible during February and March. Drought removal is likely across large parts of the Southwest, while improvement or removal is also likely in the southern Plains. However, drought is likely to persist in the Pacific Northwest and northern Rockies, with drought development likely in Hawaii, parts of the northern Plains and in the northern Great Lakes region. The seasonal outlooks for December 2015-February 2016 reflect the typical influence of El Niño on average winter temperature and precipitation: warm and dry across the North, and cool and wet in the South. Advertisement

'California would need close to twice its normal rainfall to get out of drought and that's unlikely.'

El Nino changes weather worldwide, mostly affecting the United States in winter.

The weather pattern happens every few years when the Pacific Ocean warms up around the equator.

This year's is one of the strongest El Ninos on record.

NOAA expects a cooler and wetter winter for the South.

For California, more precipitation than usual is expected during the critical time that its reservoirs usually fill, but there's no guarantee.

Only northern tier states, the Ohio Valley states and Alaska should be dry.

While California's drought is likely to lessen in January, even the wettest winter on record — 33 years ago — didn't have enough rain to wash out the current four-year drought, said NOAA hydrologist Alan Haynes of the California Nevada River Forecast Center.

Forecasters see a milder, warmer winter north of the Mason-Dixon line and for all of California and Nevada.

Texas and the Deep South are forecast to be cold.

Overall, the nation should have 2 percent fewer days when people have to fire up their furnaces, said Halpert.

He said the Northeast, where it was chilly and snowy last year, should see 6 percent fewer heating days.

Because of El Nino, NOAA is more confident than usual that its forecast is on target — 70 percent for a wet South, Halpert said.

The federal winter forecast doesn't address snow, just wet or dry and warm or cold.

Even though it's likely to be both cooler and wetter in the South, it is usually so warm there that it needs a blast of Arctic air for snowstorms and that's not looking likely, Halpert said.

And while the north is likely to be warmer, past El Ninos have had some big snowstorms.

Historically, because there's more storminess during El Ninos, there's been a slight but not great increase in snowfall in the Northeast during El Ninos, said NOAA El Nino expert Michelle L'Heureux. But that could be skewed by a few big years in the past like the winter of 2009-10, she said.

The Great Lakes area tends to get less snow during El Ninos, she said.

Private forecast firm Weather Bell Analytics predicts a swath from New Mexico across to the Carolinas and up the coast to Connecticut will get 50 percent more snow than usual.

AccuWeather, another private firm, sees severe thunderstorms in Florida, but forecasts less lake-effect snow around the Great Lakes, occasional mild days for the Midwest and says it will be 'not as brutal' for the Northeast.

The three-month temperature, left, and precipitation forecasts for the U.S. Forecasters say this winter El Nino is about to leave a big wet but not necessarily snowy footprint on much of the United States, including parched California. NOAA on Thursday issued a winter forecast, heavily influenced by one of the strongest El Ninos on record. (NOAA via AP)

El Niño is strengthening and is now 'too big to fail', forecasters have warned.

The latest analyses from the National Oceanic and Atmospheric Administration and from Nasa confirm that sea surface heights and temperatures, as well as wind patterns, show surface waters cooling off in the Western Pacific and warming significantly in the tropical Eastern Pacific.

'Whether El Niño gets slightly stronger or a little weaker is not statistically significant now - This baby is too big to fail,' said Bill Patzert, a climatologist at Nasa's Jet Propulsion Laboratory.

October sea level height anomalies show that 2015 is as big or bigger in heat content than 1997.

'Over North America, this winter will definitely not be normal,' Patzert said.

'However, the climatic events of the past decade make 'normal' difficult to define.'

Researchers say the phenomenon will get even stronger.

'The trade winds have been weakening again,'

THE EL NINO OF 1997 AND 1982 Both he El Nino of 1997-98 and 1982-83 were known in California for relentless rain, strong winds and heavy snow. Waves pounded the coast, mudslides rolled down mountainsides and floods swamped homes and claimed lives. Storms blamed on El Nino in 1997-98 killed at least 17 people, wiped out strawberry and artichoke crops, pushed houses off hillside foundations and washed out highways. Damage was estimated at more than $500 million. The 1982-83 tempests left 36 people dead, damaged or destroyed more than 7,900 homes and businesses, and caused $1.2 billion in losses, according to the weather service. This 18 March 1998 aerial view shows the city of Portoviejo partly flooded following consecutive days of torrential rains. City officials, who blame the meteorological phenomenon 'El Nino' for the disastrous weather, report that most residents are now without potable water and electricity. Advertisement

'This should strengthen this El Niño.'

Weaker trade winds out of the eastern Pacific allow west wind bursts to push warm surface waters from the central and western Pacific toward the Americas.

In its October monthly update, scientists at Noaa's Climate Prediction Center stated: 'All multi-model averages predict a peak in late fall/early winter.

'The forecaster consensus unanimously favors a strong El Niño...Overall, there is an approximately 95 percent chance that El Niño will continue through Northern Hemisphere winter 2015–16.'

The July–September average of sea surface temperatures was 1.5°C above normal, NOAA reported, ranking third behind 1982 (1.6°C) and 1997 (1.7°C).

Both Patzert and NOAA forecasters believe the southern tier of North America, particularly southern California, is likely to see a cooler and wetter than normal winter, while the northern tier could be warmer and drier.

But the sample of El Niños in the meteorological record are still too few and other elements of our changing climate are too new to say with certainty what the winter will bring.

'In the abstract,' he said, 'El Nino seems like our savior.' But if floods and mudslides develop, it's 'not going to look like the great wet hope charging across the landscape on a white horse.'

The July?September average of sea surface temperatures was 1.5°C above normal, NOAA reported, ranking third behind 1982 (1.6°C) and 1997 (1.7°C). The plot above shows sea surface temperatures in the tropical Pacific for all moderate to strong El Niño years since 1950.

A strong El Nino arrives about once every 20 years.

Ocean temperatures show this one to be the second-strongest since such record keeping began in 1950, said Eric Boldt, a meteorologist with the National Weather Service.

That would make it weaker than the El Nino of 1997-98 but stronger than the El Nino of 1982-83.

Both of those winters were known in California for relentless rain, strong winds and heavy snow.

Waves pounded the coast, mudslides rolled down mountainsides and floods swamped homes and claimed lives.

Storms blamed on El Nino in 1997-98 killed at least 17 people, wiped out strawberry and artichoke crops, pushed houses off hillside foundations and washed out highways. Damage was estimated at more than $500 million.

The 1982-83 tempests left 36 people dead, damaged or destroyed more than 7,900 homes and businesses, and caused $1.2 billion in losses, according to the weather service.

El Ninos can be unpredictable.

Some have produced little rain, and some of the most damaging storms have come in non-El Nino years.

Weather models this year show a 60 percent chance of above-average rainfall in Southern California, but that figure declines farther north, Boldt said.

From the San Francisco Bay Area to Sequoia National Park, there's a 50 percent chance of above-average rainfall.

From Eureka to north of Reno, Nevada, that estimate drops to 33 percent.

It's likely to be drier in the Pacific Northwest and the northern Rocky Mountains.

California public agencies have been warned to prepare for large storms.

Boldt said he can't count all the meetings he's been to with emergency managers and local officials.

'That's been pushed hard, and people understand this is going to potentially be a bad winter for water issues,' he said.

State officials are watching weather models and updating emergency plans, said Kelly Huston, deputy director with the Governor's Office of Emergency Services.

'We worry that people won't take it seriously because they're so desperate for water,' Huston said. 'If it downpours heavily over a short period of time, it's going to be dangerous, not just a welcome relief they perceive to be helping the drought.'

The risk of rain is heightened in some areas by recent wildfires that have stripped away the trees and other vegetation that protect steep slopes from erosion.

Work is already being done to prevent debris flows in Northern California's Lake and Amador counties.

Heavy rain will bring some drought relief, but it is not expected to erase the state's water deficit, particularly if it doesn't rain as much in Northern California, home to the state's largest reservoirs.

There's also the chance that El Nino will be followed by its sister, La Nina, a different phenomenon that generally brings cooler temperatures in the Pacific and a drier winter.

El Nino can affect weather well beyond the West Coast.

The outlook for winter generally favors below-average temperatures and above-median precipitation across the southern U.S. and above-average temperatures and below-median precipitation over the northern U.S., according to the report.

The much-anticipated El Nino will add another year of drought in the north while bringing devastating floods to the south, experts have warned. This NOAA satellite image taken Thursday, September 10, 2015 shows a cold front over the Intermountain West and northern Plains moving southward with scattered rain showers

'We torture the data until it tells us what we want to hear,' said Alan Haynes, service coordination hydrologist at the California Nevada River Forecast Center.

'Hopefully, it is wet everywhere.'

The CPC/IRI ENSO forecast says there's an approximately 95 per cent chance that El Niño will continue through Northern Hemisphere winter 2015-16, gradually weakening through spring 2016

To compare the two, researchers created a side by side animation to show the development of both, and say they are 'shocked' by how similar they are.

The current El Nino weather phenomenon is expected to peak between October and January and could turn into one of the strongest on record, experts from the World Meteorological Organization have warned.

The video animation was created by Matt Rehme at NCAR's Visualization Lab, part of the Computational & Information Systems Lab.