Redder areas are those that are the farthest below normal rainfall the past two years. Right, from the forecast for this winter.
The National Weather Service station in Oxnard has formally predicted that this winter will probably be below normal for rainfall in the Los Angeles area and the Central Coast counties. If true, this would be the third winter of lower than historical average precipitation. Some areas of California have already set records for their rainfall deficit below the average over the past two rainy seasons, so this would not be a welcome turn of climatic events. The prediction is based on historic trends which show that the rainfall in Los Angeles is usually disappointing in years when two things are happening out in our weather generator: the Pacific.
First, the water temperatures in the southern ocean that are monitored for their weather impact — the famous El Nino Southern Oscillation in the subtropical waters below the equator — are in what the meteorologists call a neutral phase. They are neither El Nino nor La Nina, in weatherspeak. The so-called neutral condition, which is fairly common, usually accompanies a drier than normal winter rain season up here. Closer to home, the Pacific Decadal Oscillation out in the central North Pacific — which cycles between warm and cold phases every 20-30 years — is right now in the middle of a cold phase. This too "often is counterproductive for significant rain events across Southern California," says NWS meterologist Eric Boldt in the video below.
Taken together, the two observations from the Pacific and other historical records push the weather service to predict another down year for rain. "The majority of our staff believe that a drier than normal year is ahead of us, which would worsen the drought conditions for a third year in a row," Boldt says.
The big caveat of this and pretty much any predictions about seasonal SoCal rain is that there is rarely a true normal year. Instead, there are dry years of varying extremes and wet years — and sometimes extremely wet years, often associated with an El Nino or an atmospheric river of moisture flowing over us — when the state historically catches up to fill reservoirs and replenish aquifers. (Most rain here falls after January 1, leading into spring.) These dry and wet year rainfall totals are averaged together over time to calculate a historical "normal," but there is no typical rainfall amount.
In fact, Boldt cautions that while the conditions in effect this season suggest a dry year, they have occurred during previous wet winters. "It only takes one atmospheric river...to line up across the subtropical Pacific Ocean into Southern California to result in heavy rainfall and flash flooding," he says. But the odds are against it.
Boldt explains with graphics in this NWS video.