By: Dr. Jeff Masters ,
January 23, 2015
The Western U.S. winter rainy season has reached its halfway point, and there is only bad news to report for drought-beleaguered California. November through March marks the period when California receives its heaviest rains and snows, thanks to the wintertime path of the jet stream, which dips to the south and brings wet Pacific low pressure systems to the state. The rainy season started out promisingly, with several December storms bringing precipitation amounts close to average for the month over much of the state. Troublingly, though, record-warm ocean temperatures off of the coast meant that the December storms were unusually warm. This resulted in snow falling only at very high elevations, keeping the critical Sierra snow pack much lower than usual. The jet stream pattern shifted during January 2015, bringing disastrously dry conditions to the state. January usually brings 4.19″ of rain to San Francisco, but no rain at all has fallen in January 2015 in the city–or over much of Central California. The dryness has been accompanied by near-record warmth at higher elevations in the Sierras, with temperatures at Blue Canyon and South Lake Tahoe averaging nearly 8°F above average for the month of January. As a result, the snowpack in the Sierras–a critical reservoir of water that is used throughout the rest of the year–is abysmally low, running about 30% of normal for this time of year. California’s eight largest reservoirs are 33% – 86% below their historical average, and the portion of the state covered by the highest level of drought expanded in mid-January–a very ominous occurrence for the height of the rainy season.
Figure 1. The Enterprise Bridge passes over a section of Lake Oroville that was nearly dry on September 30, 2014, in Oroville, California. Lake Oroville, California’s 2nd largest reservoir, was at 49% of average (30% of capacity), the second lowest level on record (behind 1977.) Heavy rains in December 2014 allowed lake levels to recover slightly–as of January 23, 2015 Lake Oroville was at its 7th lowest level of the past 35 years. Image credit: California Department of Water Resources.