Simply put, this means the atmosphere has coupled with the warm ocean waters of the Pacific and has met criteria for an El Niño event to be declared. This particular El Niño event formed very late and is weak in strength.
A quick El Niño refresher (via CPC) -
What Is El Niño?
El Niño refers to conditions when the sea's surface in an area along the equator in the central and eastern Pacific Ocean gets warmer than usual. The average water temperature in that area is typically just 1-3°C (about 2-5°F) warmer than normal, but it has the effect of adding huge amounts of heat and moisture to the atmosphere, ultimately affecting patterns of air pressure and rainfall across the Pacific and beyond.
La Niña is the climatological counterpart to El Niño-a yin to its yang, so to speak. La Niña refers to periods when sea-surface temperatures in the equatorial central and eastern Pacific are cooler than normal. The lower surface temperatures suppress transfer of heat and moisture to the atmosphere, again affecting air pressure and precipitation patterns across a large region.
So, what does this mean - specifically, what does this mean for California?
• The prolonged drought in California will continue through 2015.
• Even if a couple of wet storms move through California this spring, there won't be nearly enough precipitation to impact the overall drought situation.
• Individual storms and precipitation amounts related to this El Niño will be episodic at best; essentially, there is no predictability for California precipitation during a weak spring El Niño event.
In other words, there is no relationship/correlation between enhanced precipitation in CA and a weak El Niño event.
• Even if we end up with several wet storms over the next month, they will likely be warmer storms with high snow levels.
• As far as water storage, the snowpack across the Sierra Nevada is dismal, at best, and remains below 20% statewide.
• How does this compare to past weak El Niño events?
Five weak El Niño years have been identified on the El Niño Blog: 1953-1954, 1958-1959, 1976-1977, 1977-1978, and 2004-2005.
Precipitation with each of these events was widely variable and no solid trends could be formed.
• The full report can be found by clicking the link below:
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.pdf
