SoCal Rain Update: Keep it Coming

After 5 long years of drought, a series of powerful storms in January and February 2017 finally brought heavy rain and snow to California. Let’s take another look at where we stand in Los Angeles, and at water supplies around the state. As always, remember that in California we measure precipitation from October through the following September; this period is called the water year.

Currently, downtown LA is at 18.50” of rain for the water year. This is about 3.5” greater than the yearly average, and well past any of the drought years.


February 2017 finally brought a storm that put the Central Coast in the bullseye, and the effect on Lake Cachuma, the largest reservoir for Santa Barbara County, was incredible. So far this winter, Lake Cachuma is up from 7% full to 42% full, and on one day in February gained nearly 30,000 acre-feet of storage. (One acre-foot is enough water to cover an acre of land with one foot of water, about what two average households in CA use in a year).


Precipitation indexes for the Sierra Nevada show it’s been a very wet year throughout the range. The north Sierra, corresponding to the Sacramento River drainage, has had 76.3”, already well above the water year average of 50.0”, and with all-time records within reach.


The central Sierra, corresponding to the San Joaquin River drainage, is also already well above the water year average, with 60.4” to 40.8”. It too is on pace to chase some of the wettest years on record.


The south Sierra, corresponding to the Tulare Basin (Kings, Kaweah, Tule, & Kern Rivers), is now well above the water year average, with 40.9” to 29.3”.


Let’s look over to the other side of California, the east Sierra, corresponding to the Owens Valley. Snow water content has not only already doubled the April 1 average (50.4” to about 24”), it’s already tied the wettest year on record. This is where the water in the LA Aqueduct comes from, so it’s good news for city water supplies, as we’ll have to buy less water from the State Water Project and Colorado River Aqueduct.


The juxtaposition of such a wet year following the worst drought in the state’s history has highlighted that California, especially southern California, is the land of extremes. Annual precipitation in SoCal is almost comically variable, with the wettest year having over 10 times as much rain as the driest year.


Variability is expressed using coefficient of variation (standard deviation divided by mean). LA’s coefficient of variation is 0.48.

To demystify the variability of SoCal rainfall, I thought it might be interesting to do a comparison between Los Angeles and Portland, widely considered to be a pretty rainy place. Here’s the average rainfall for each city by month.


Portland gets about 36” of rain every year to LA’s 15”, though surprisingly enough LA is, on average, wetter than Portland in the month of February. However, when we look at daily rainfall records, a striking pattern emerges.


The all-time daily record rainfall in Portland is 2.69”; in LA it’s over twice that much at 5.88”. It’s never rained 3” or more in one day in Portland; there are 31 daily rainfall records greater than 3” in LA. The 4” mark has been hit ten times and 5” three times. LA’s daily rainfall record is greater than Portland’s for 165 days out of the year, despite Portland getting nearly 2.5 times the annual rainfall and being wetter in nearly every month, and LA being so reliably dry in summer that 19 days have never seen measurable rainfall and the last 140 Julys having delivered a grand total of 1.55” of rain.

The last 6 years are a reminder that for SoCal the faucet can turn on just as quickly as it turns off – and vice versa. The forecast for the next week or so is dry and in fact once, water year 1996-97, LA had no measurable rain between March 1 and the end of the water year. So now that I’ve sufficiently jinxed things, you’d better hope extra hard for some more drought relief this year!


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