(Reuters) - This year’s El Niño may not deliver drought-busting rains to California as is popularly believed.

The ongoing drought in California has been one for the record books. Southern California has not met yearly average rainfall since 2010, and the central and northern parts have not achieved this since 2012.

Califrnians are hoping that this rainy season, which lasts from November through April, will be wetter than normal thanks to the record-strong El Niño, which has delivered sizable winter rainfall to the state in the past.

Reservoirs have been drying up since last year, particularly since snowfall in the Sierra Nevada mountain range has been far below average in recent years. This past spring, record-low snowpack was recorded in the Sierras.

California is a major hydroelectric power producer, and one-fifth of the state's electricity comes from this source. But this process is highly dependent on adequate rainfall.

California also leads in agricultural production. It produces roughly 80 percent of the world’s almonds, and each almond requires nearly one gallon of water.

With the stakes so high, California is in desperate need of rain, but we need to be prepared in case rains disappoint.

Not all El Niños are created equal, and they may not ensure a rainy winter in California. However, there is certainly reason to be optimistic about the outlook, particularly come the beginning of the year.

WET OUTLOOK, IN THEORY

When the Pacific Ocean is in an El Niño state, the warmer waters around the equator cause the jet stream to track farther south than usual during the Northern Hemispheric winter months (tmsnrt.rs/1lZP2K7).

This causes storm tracks to be regularly aimed toward Southern California during this time, often boosting wet season rainfall.

During the previous two times that El Niño was similarly as strong as this year's, 1982 and 1997, rainfall between December and February was particularly enhanced throughout the state.

However, the situation is different this year because the entire northern Pacific Ocean is quite warm in addition to the El Niño-characterizing anomalies at the equator (tmsnrt.rs/1Qi7gSl). It is not certain precisely how the widespread ocean warmth may alter atmospheric circulation.

But the slow start to the rainy season in California already sets 2015 apart from 1982 and 1997.

November precipitation was considerably lower than both of those years, and below average overall, which is not a good start to the supposedly “flood-laden” winter (tmsnrt.rs/1Qi6Z1L).

NO GUARANTEES

However, the El Niño phenomenon may not ensure bountiful rains for California.

Of all El Niño events since 1981, roughly half produced near average rainfall between December and February, and some even fell below average (tmsnrt.rs/1Qi7s3Q).

Several of the anomalously high-precipitation winters, particularly in Southern California, coincided with El Niño, including 1982 and 1997.

But in northern and central California, where the drought is the worst, winter deluges tend to occur more often when El Niño is absent.

El Niño conditions do not seem to have a bearing on soil conditions in November, though soils in Northern California are approaching record dry. Central California, where the bulk of agriculture production lies, is also near driest levels.

Recent rains in Southern California, which has been fairly dry for the past five years, have certainly chipped away at a small portion of the drought. July through October was the rainiest such period since 2006.

If El Niño unloads on California this winter, it could put a substantial dent in the drought, but there is a long way to go before the drought could be eliminated.

Currently, 45 percent of the state is classified as being in the most severe stage of drought. Prior to 2014, this stage had not been observed in the state since at least 2000. Thus, a drought of this magnitude could not possibly be erased in one season.

RAIN TO RETURN?

Despite disappointing November precipitation across most of the state, snowfall in the Sierra Nevada mountain range is already off to a good start.

Snowpack in the mountains is already above average for this time of year, prompting ski resorts to open exceptionally early. This is already a major turnaround from this past April, when snowpack was just 5 percent of normal.

But it is still quite early in the season and if this pattern cannot persist, the recent gains in moisture will be erased just as quickly as they arrived.

A possible explanation for the recent drier-than-expected conditions is the enhanced storm activity in the Indian Ocean, which has in turn held back some of the low pressure activity from reaching both North and South America.