The Department of Water Resources has finished the May 1, 2015 Water Supply Index (WSI) and Bulletin 120 (B120) forecasts. The forecasts include observed conditions through the end of April. The forecasts are posted at:
The projected median April-July runoff in the major Sierra river basins ranges from 3 percent on the Tule River to 44 percent on the Pit River. Forecasted median Water Year runoff in the Sierra ranges from 7 percent for the Tule River to 61 percent for the Sacramento River above Bend Bridge. The statewide forecasts for the April-July and Water Year periods are about 20 and 40 percent of average, respectively. Of the three Sierra regions, the Tulare Lake Basin has the lowest average runoff: 11 and 16 percent of average, respectively, for the April-July and Water Year periods.
For the runoff forecasts made by the Snow Surveys section (which excludes the Scott, Klamath, and Owens Rivers), all median forecasts except those for the Trinity River, Sacramento River at Delta, and the Cosumnes River predict new historic low April-July runoff volumes. In the cases of the San Joaquin River inflow to Millerton Lake, the Kings River below Pine Flat Reservoir, and the Kern River inflow to Lake Isabella, the April-July forecasts are about half of the lowest unimpaired runoff on record.
The WSI forecast is based on precipitation and flows observed through April 2015 and can be summarized as follows:
Sacramento River Unimpaired Runoff Water Year Forecast(50 percent exceedance)
9.0 MAF(49 percent of average)
Sacramento Valley Index (SVI)(50 percent exceedance)
San Joaquin Valley Index (SJI)(75 percent exceedance)
Unimpaired flows for the 2014-15 water year have occurred at the following rates of average:
October – April Runoff (%)
April Runoff (%)
Sacramento Valley Index (4 rivers)
San Joaquin Valley Index (6 rivers)
Tulare Lake Basin (4 rivers)
Through seven months of this water year, all rivers south of the Yuba River are flowing at rates less than 45 percent of average.
Precipitation for the 2014-15 water year accumulated at the following rates of average:
WY accumulated Precipitation (%)through April 30, 2015
Based on historical trends, less than 10 percent of the water year’s precipitation is expected after May 1.
Snowpack is monitored using two complementary methods: automatic snow sensor (or “pillow”) readings and manual snow course measurements. The snow sensors give us a daily snapshot of snow conditions while the manual snow course measurements provide a monthly verification of snow conditions in locations where snow has been measured in the same manner as far back as 100 years.
The snowpack based on the May 4, 2015 snow sensor readings are as follows:
Snow Water Equivalent (inches, May 4)
% of Average (May 4)
When looking at manual snow course data, the trend of the 2015 snow pack setting new historic lows continues. The May 2015 course measurements report shows a statewide average at just 2 percent of the historic average May 1 snow pack. Of the 166 courses measured, only 26 had any snow to report (16 percent of the courses measured). The following table compares the May 2015 snow survey results to those in May 1977 and May 2014.
1977% Average May
2014% Average May
2015% Average May
San Joaquin Valley
Statewide Average (weighted)
Lowest on record
Weather and Climate Outlooks:
A mix of light snow showers at the highest elevations, valley rain showers and thunderstorms affected California’s Central Valley and the Sierra Nevada to end the first week of May. Up to an inch of precipitation is expected for portions of the Northern and Central Sierra Nevada, although with watersheds so dry, no real significant rise in runoff is expected. Through the end of the six day forecast period there remains the possibility of localized thunderstorms in the Sierra Nevada, but no widespread precipitation associated with a significant weather pattern.
The NWS Climate Prediction Center (CPC) one-month outlook for May, issued April 30, indicates equal chances of above or below normal precipitation Statewide. The same outlook also predicts increased chances of above normal temperature Statewide.
The CPC three-month (May-June-July) outlook, issued April 16, indicates equal chances of above or below normal precipitation Statewide. The same outlook also predicts increased chances of above normal temperature Statewide.
El Niño conditions are present. Positive equatorial sea surface temperature (SST) anomalies continue across most of the Pacific Ocean. There is an approximately 70 percent chance that El Niño conditions will continue through the Northern Hemisphere summer 2015, and a greater than 60 percent chance it will last through autumn. It is imperative to remember that spring offers the least predictability in the state of the El Nino Southern Oscillation for the coming winter and that conditions that might point to such an event have occurred in the past and failed to produce. Late summer is a more realistic time frame to assess the potential of an El Nino event and if it might be large enough to have any expectation of the coming winter season. As a whole, El Nino offers zero predictability of hydrologic outcome for the northern part of the State.
A Bulletin 120 update for conditions as of May 12, 2015 will be available Thursday, May 14.
If you have any questions regarding this forecast, please contact a member of the Snow Surveys staff.