Drought Update

The rainy season is quickly ending in California, and unfortunately, the drought has only continued to worsen over the course of the winter.  Lowland rainfall and mountain snowpack for winter 2013-14 are well below normal across all of the state, only adding to the precipitation deficits from the previous winter.  The latest US Drought monitor depicts the dire conditions, with extreme drought stretching from San Francisco southward to Santa Barbara and inland across the San Joaquin Valley.  The California Department of Water Resources just completed its final Sierra snow survey of the season, and found that the current snow water content was only 18% of normal for May 1.  For the northern Sierra region, the snow water content was just 7% of normal.  Snow water content is also well below normal across almost all of the southwestern US.  Late-season storms helped bring the snowpack to near- of above-normal conditions for the Pacific Northwest and northern Rockies.

A look at cumulative observed and average rainfall in San Francisco over the past four years clearly shows the mounting precipitation deficits.  For reference, San Francisco normally receives about 23 inches of rain per year, the vast majority of which falls between November and March.  Frequent storms brought above-normal precipitation to San Francisco and most of California during the 2010-2011 winter season, followed by slightly below-average precipitation during the 2011-2012 winter season.  Frequent storms and heavy precipitation returned during December 2012, but since then, the precipitation deficits have increased dramatically.  We ended the 2012-2013 winter season with about 50% of normal rainfall in San Francisco, and are now on track to end the 2013-2014 winter season with about 50% of normal rainfall.  With no significant precipitation on the horizon through mid-May, it is very likely that most areas in California will receive no substantial precipitation until this fall.  This all points to a summer of potentially extensive and numerous wildfires, poor air quality from wildfire smoke, and mandatory water restrictions in many areas.  An animation of the weekly US Drought Monitor since 2011 depicts the development and intensification of drought in California, going from no drought at the start of 2011 to extensive drought for much of the state today.

There is a dim light at the very end of the tunnel, as forecasts call for the development of El Nino conditions in the Pacific Ocean.  Many of California's wettest winters occurred during El Ninos.  However, it remains to be seen how strong the upcoming El Nino will be.  El Nino is a warming of the Pacific waters west of equatorial South America, but its atmospheric component is measured by differences in air pressure between Darwin, Australia, and Tahiti (in the Central Pacific).  Historically, moderate to strong El Ninos are typically associated with above-normal precipitation in California, but weak El Ninos have shown little correlation with precipitation here.