Sea surface temperatures over the tropical Pacific in late September were generally warmer than average, but not warm enough for official El Niño conditions. (NOAA)

El Niños - which typically develop every 3 to 7 years - are well-known for their ripple effects on global weather patterns. For example, the extra heat the warm Pacific pumps into the atmosphere often manifests itself in stormy winter weather across the southern tier of the U.S.

But on Thursday, NOAA lowered the odds that an El Niño will develop in October from 70 to 55 percent.

“During September 2012, the trend towards El Niño slowed in several key oceanic and atmospheric indicators,” NOAA said.

Nevertheless, the Pacific remains on the cusp of El Niño conditions, and an El Niño watch, first issued in June, remains in effect.

For an El Niño to officially develop, sea surface temperature over a large block of the central tropical Pacific - known as region Niño3.4 - would have to average 0.5 C above normal over the course of three months.

Sea surface temperatures compared to average over Niño3.4 region. They have - for brief periods of time - moved into El Niño territory - but not sustained themselves. In September, they cooled squarely into neutral territory. (NOAA, adapted by CWG.)

In its current state, the Pacific is considered to be in the neutral phase of the El Niño Southern Oscillation (ENSO), which some refer to as “La Nada.” Prior to this neutral phase, sea surface temperatures in tropical Pacific had been colder than normal through last winter, in the La Niña phase.

Since the summer, computer model simulations have been quite bullish an El Niño will develop.

“[B]y July, more than 70% percent of models were calling for an El Niño,” wrote NCAR’s Henson.

As of mid-September, about 80 percent of computer models predict El Nino will develop, but at a weak intensity. (International Research Center for Climate and Society)

“Due to the recent slowdown in the development of El Nino, it is not clear whether a fully coupled El Niño will emerge,” NOAA said.

NOAA now calls for “borderline” El Niño conditions (between neutral and weak El Niño conditions) into winter with some possibility for strengthening.

What would need to happen for the onset of a full-fledged El Niño ?

National Center for Atmospheric Research (NCAR) climate scientist Kevin Trenberth told UCAR’s Henson the cycling of a phenomenon known as the Madden-Julian Oscillation (MJO) - a complex of weather systems that moves across the tropics every 30 to 60 days - could push atmospheric winds in the right direction to sustain warm Pacific waters.

“It only takes one MJO event to kick off a major El Niño,” Trenberth said. “That is still possible, if not looking likely.”

If El Niño conditions fail to develop or are weak, it is potentially bad news for parts of south central U.S. suffering from exceptional drought conditions. Winter storms tracking across the southern U.S. would likely be infused with deep tropical moisture under moderate or stronger El Niño conditions.

“The bigger the El Niño, the bigger the effect,” David Neelin, a professor of atmospheric and oceanic science at the University of California, Los Angeles, told the New York Times.

Precipitation compared to average during El Niño (left) and La Niña (right) across the U.S. 1950-2010. (NOAA)

In a neutral ENSO scenario or “La Nada”, it’s essentially anyone’s guess how winter weather patterns will evolve.

“I don’t like La Nada,” NASA climatologist Bill Paltzert told the LA Times. “Why is that? Because in La Niña or El Niño, you can actually make a forecast, because they provide some structure to the climate system for an entire winter -– you can make a fairly decent long-range forecast….

“With La Nada, it’s like teenagers without rules. It’s unconstrained and unpredictable,” Patzert said.

Related reading: The Winter Forecast and a Wimply El Nino