How deep of a precipitation hole is California in?

September 11, 2015

A BIG one.

………..(silence)…………

Wait, you want more detail than that? One sentence won’t suffice? I suppose that is reasonable request.

In January, after a series of rain events the previous month in California, I wrote an article using analysis from my Climate Prediction Center colleague Rich Tinker that described how much rain/snow was needed by the end of the California water year (the end of September) to get California out of its precipitation hole.

The answer was a lot of precipitation. It would have taken near record amounts of rain across the agriculture-dominated central California – the San Joaquin Valley – to bring the most recent four year period out of the driest 20 percent of years on record.  Flash forward to September and those rains did not happen last year. In fact, California remains extremely dry.

After four years of cumulative precipitation deficits, California and other parts of the West remain in the grip of exceptional drought (darkest red). Data Snapshots map based on data from the U.S. Drought Monitor project.

The most recent US Drought Monitor, released September 8, has 46% of the state under the most extreme drought category (D4-Exceptional Drought). Over 97% of the state is experiencing some degree of drought. Only areas in far southeastern California have received enough rain to simply be abnormally dry and not under drought.

So what will it take for the upcoming water year to put a big dent into California’s precipitation deficits?

Again the answer is a lot of rain. As of the end of August, California is running 5-year precipitation deficits (starting in October 2011) of 8 inches in the dry southeast to almost 50 inches along the north coast.  In California, four year rainfall amounts (2011-2014) have been between 54-75% of normal during that time frame. To put the deficits into another perspective, every region in California is missing at least a year’s worth of precipitation. In fact, the south coast of California Is missing almost two year’s worth of rain (1.82 years to be exact). This deficit isn’t so much a hole as a giant chasm.

One measure used by the U.S. Drought Monitor team to declare drought is whether precipitation totals are in the bottom 20 percent of the record. For five-year precipitation totals (October 2011 – September 2016) to get out of the bottom 20% of records dating back to 1928, precipitation totals from October 2015 through September 2016 must exceed 135-160% of normal in northern California, 160% of normal in the dry southeast to 198% of normal in the San Joaquin Valley. This is a ton of rain/snow.

Percent of normal precipitation required in the upcoming water year (October 1, 2015-September 30, 2016) in order to mitigate 5-year rainfall deficits. (left) Precipitation needed to emerge from the "bottom bracket"—the 20th percentile, or lowest 20% of values, for all 5-year periods in the historical record. (right) Precipitation needed to be restored to the middle of the historical pack (50th percentile). Maps by NOAA Climate.gov, based on analysis of Climate Division data by Rich Tinker, NOAA Climate Prediction Center.

In order for rains during the 2015-2016 water year to be 198% of normal in the agricultural-center of the state—the San Joaquin Valley—the upcoming water year would have to be the wettest on record.  And that is just to get five year precipitation deficits out of the bottom 20%! The only region of California that would not have to have a top-10-wettest water year since 1928 is the north coast. They would only need the 11th wettest October-September.

For these regions to bring five year totals to the 50th percentile—the middle of the pack—every region in California would need record-breaking amounts of rain. The south coast of California would have to receive precipitation over 300% of normal (nearly 53 inches of rain)! But hey, that’s only a mere 14.95 inches higher than the current record for wettest water year ever.

The San Joaquin Valley would have to break its previous water year record by 18 inches! Even the region closest to average in California (the north coast) would have to see precipitation totals over 17 inches higher than the previous water year record.  

This analysis is a relatively basic view on what constitutes “drought” or “drought recovery.” Drought in California is about much more than the total amount of rain falling from the sky. How long and hard it falls, whether it comes as rain or snow, and how resilient various ecosystems are to such extended stress are other factors that we have to take into account when we talk about drought recovery.

Explaining this nuance via email, California State Climatologist Mike Anderson wrote that as far as surface conditions go,  “We need sufficient rainfall to replenish used storage in the surface reservoirs and to begin to restore lost groundwater systems, and we need to have a healthy snowpack to deliver runoff in the spring and summer.  Looking at past years that ended multi-year droughts and comparing their characteristics, they ended up being years that had somewhere in the ballpark of 150% of average precipitation and about 150% of average snowpack.”  

But regardless of exactly how you define “drought recovery,” the point remains, California has been incredibly dry over the last four years, and it will take a great deal of rain and snow for meaningful recovery. Explaining this nuance via email, California State Climatologist Mike Anderson wrote that as far as surface conditions go,  “We need sufficient rainfall to replenish used storage in the surface reservoirs and to begin to restore lost groundwater systems, and we need to have a healthy snowpack to deliver runoff in the spring and summer.  Looking at past years that ended multi-year droughts and comparing their characteristics, they ended up being years that had somewhere in the ballpark of 150% of average precipitation and about 150% of average snowpack.”  

The “good news” part of this post

If there is any “good” news in this post, it would be that for many of the regions in central and southern California, the wettest water years on record were tied to El Niño events. And 2015 is in the midst of one of the strongest events since 1950. (See more about this on our ENSO blog). As such, the latest seasonal precipitation forecast from the Climate Prediction Center for the upcoming December-February period is what you would expect to see during an El Niño year: an elevated chance for above-average rains across the central/southern parts of the state. Rains that would help alleviate some of the deficits that have been built since 2011.

DJF 2015 precipitation outlook

Chances of possible temperature (upper map) and precipitation (lower) outcomes for December 2015-February 2016: above normal, below normal, or near normal. Above or below normal means temperatures in the upper or lower third of the range of historical temperatures. White does not mean "near normal;" it show places where the chances for above-, below-, and near-normal temperatures are equal. Maps by NOAA Climate.gov, based on data from the Climate Prediction Center.

The jury is still out, though, on northern California. In the past, the connection between wet winters and El Niño has been less reliable in the northern part of the state than the southern part. But according to a new analysis by the NOAA Drought Task Force, the odds for a wet winter across the entire state improve the stronger the El Niño event is, and the 2015-16 event is currently forecast to remain strong through winter.

Regardless, it is important to remember that forecasts are about probabilities, not guarantees. While it seems like this winter could see above-average rains, the name of the game in California is still to conserve water. But maybe make sure you have that umbrella ready, too.