Dowsing for Rain: The 2013-14 Southern California Winter Precipitation Outlook

Rocky Peak Road near its junction with the Chumash Trail

Updated October 17, 2013.

As I started up the Chumash Trail, a patchwork of clouds filled the sky, and the fresh smell of a recent shower filled my lungs. A strong and unseasonably cold low pressure system was producing the first widespread rainfall of Southern California’s 2013-14 rain season.

I hoped there would be another shower during my run. The plants and animals needed it and I needed it. It had been many months since I had run in the rain on a local trail.

As I worked up the trail I thought about how dry it had been. Downtown Los Angeles’ 2012-13 water year (Jul 1 – Jun 30) was the sixth driest on record. Rainfall in Los Angeles since January 1 has been about 25% of normal. The 2012-13 Sierra snowpack was one of the worst on record.

Will this rain and snow season be any better? For months I’ve been monitoring climate data and forecasts looking for something on which to base a 2013-14 Winter Outlook. Historically, ENSO has been the big dog in Southern California rain season weather with El Nino conditions generally producing wetter weather and La Nina conditions generally drier.

But ENSO conditions are currently Neutral and are expected to remain so through the end of the year. Most climate models forecast there will be slow warming of SSTs in the equatorial Pacific (NINO 3.4 region) over the next several months, but at this time of the year it would be very unusual to have substantial warming. The CPC/IRI ENSO Forecasts from IRI’s October Quick Look indicate the probability of an El Nino developing before the end of the year is less than 20% — and 20% seems high.

One computer model that at times has been forecasting above average precipitation this Winter in Southern California is the Climate Forecast System version 2 (CFSv2). The CFSv2 is fully coupled ocean-atmosphere-land-sea ice model used to forecast parameters such as sea surface temperature, temperature and precipitation rate. While skillful at predicting tropical SSTs, the CFSv2 generally performs very poorly when forecasting precipitation over land, so forecasts such as this one for Dec-Jan-Feb must be viewed somewhat skeptically.

Another glass half-full observation is that the Madden-Julian Oscillation (MJO) has been relatively active this year and if this activity continues it provides recurring opportunities for enhanced U.S. West Coast precipitation. The downside is that it can result in periods of dry weather as well.

With the ocean and atmosphere neutral there’s just not much on which to base a rain season forecast. According to the Climate Prediction Center, the current winter precipitation outlook for most of Southern California calls for an equal chance of below average, average, or above average rainfall. I know — that’s no help at all — you might as well flip a three-sided coin!

Update October 17, 2013. The Climate Prediction Center released its updated U.S. Dec-Jan-Feb Precipitation Outlook today. The outlook now indicates an equal chance of below average, average, or above average precipitation for all of Southern California. The October CPC outlook is usually the basis for the initial official NOAA U.S. Winter Outlook.

The title photo is of Rocky Peak Road, near its junction with the Chumash Trail. Here’s a larger version of the panorama.