Melted snow saturates the ground at Phillips Station in the Sierra Nevada Mountains, the site of the California Department of Water Resources final snow survey of the 2020 season. The survey was held approximately 90 miles east of Sacramento off Highway 50 in El Dorado County. Photo taken April 30, 2020.
Kelly M. Grow / California Department of Water Resources, FOR EDITORIAL USE ONLY
WATER SUPPLY INDEX for May 1st: Check out the new Tableau dashboards!
The projected median April-July (AJ) runoff in the major Sierra river basins ranges from around 34 percent for the East Walker to 76 percent for the Cosumnes River. The statewide seasonal AJ median forecast is 7.85 MAF which is 56 percent of the historic average. This value is a 3 percent increase from the April 1 Bulletin 120 Forecast.
The projected median Water Year (WY) runoff in the major Sierra river basins ranges from 36 percent on the Trinity River to 68 percent for the Pit River. The projected Statewide median WY runoff is 51 percent of the historic average.
The WSI forecast is based on precipitation, snow, and flows observed through April 2020 and can be summarized as follows:
Sacramento River Unimpaired Runoff Water Year Forecast
The American, Cosumnes, Mokelumne, Stanislaus, Tuolumne, and Merced watersheds all flowed greater than 90 percent of average for the month of April. All other watersheds in the Sierra Nevada flowed less than 90 percent of average.
With increasing temperatures and ripe isothermal snowpack, most snowmelt runoff is expected to peak within the next week if it has not already.
April full natural flow rates updated through May 6-7, 2020:
Percent of Historic Average
Sacramento at Bend Bridge
Precipitation for Water Year 2020 accumulated at the following rates of average
WY accumulated precipitation (%) through April 30, 2020
The San Joaquin 5-Station and Tulare 6-Station Precipitation Indices both accumulated more precipitation during March-April than during December-February; typically, these three are the wettest months of a water year.
Monthly Precipitation to date in Percent of Average for Water Year 2020 for Regional Precipitation Indices
Regional Precipitation Indices
Northern Sierra 8-Station Index
San Joaquin 5-Station Index
Tulare Basin 6-Station Index
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.
May snow course measurements show the statewide average at 39 percent of average to date. The results of the May 2020 statewide snow surveys are as follows:
No. of Courses Measured
Average Snow Water Content
% Average April 1
% Average May 1
San Joaquin Valley
Statewide Average (weighted)
As of May 7, the statewide snowpack based on the automated snow sensor network is 25 percent of average to date and 17 percent of the April 1 average. The snowpack as of the morning of May 7, 2020 stands at the following (based on snow sensors):
Snow Water Content (inches)
% of Average (Apr 1)
% of Average (May 7)
The statewide snowpack snow water content has decreased by 3.1 inches from 8.0 inches on May 1. On May 1, the snow sensor statewide snowpack was at 36 percent of average to date, similar to the percent of average as determined by the April snow course measurements.
Weather and Climate Outlooks:
According to the CNRFC 6-day forecast, there are chances of precipitation in northern California over the last two days of the forecast. The North Coast basin is forecasted to receive the greatest amount of precipitation at an average of 1.3 inches. Between 0.4 and 0.8 inches of precipitation is predicted in the Klamath, Russian, Sacramento, Yuba, and American basins. Light precipitation between 0.1 and 0.3 inches is predicted in the San Joaquin and North San Joaquin basins. Forecasted precipitation in all other basins is negligible.
Freezing elevations range from 11,000 feet in northern California to 15,000 feet in southern California to start the period; by the end of the 6-day forecast, freezing elevations decrease to 5,000 feet in northern California and 13,000 feet in southern California.
The NWS Climate Prediction Center (CPC) one‐month revised outlook for May 2020 issued on April 30, 2020, points to chances of above normal temperatures across the state. This same outlook suggests equal chances of above or below normal precipitation for all of California.
The CPC three-month (May-June-July) outlook, issued on April 16, 2020, points to increased chances of above normal temperatures across the State. The same outlook forecasts equal chances of above or below normal precipitation in central and southern California, and chances of below normal precipitation in northern California.
According to the latest El Niño/Southern Oscillation (ENSO) discussion issued by the Climate Prediction Center on May 4, 2020, ENSO-neutral conditions are present. Equatorial sea surface temperatures (SSTs) are near-to-above average across most of the Pacific Ocean. The tropical atmospheric circulation is consistent with ENSO-neutral. ENSO-neutral is favored for Northern Hemisphere summer 2020 (~60% chance), remaining the most likely outcome through autumn.
A Bulletin 120 update for conditions as of May 12 will be available by Thursday, May 14. This is the last issuance of the Water Supply Index (WSI) forecasts for Water Year 2020. The next WSI will be available in December 2020. If you have any questions regarding this forecast, please contact a member of the Snow Surveys and Water Supply Forecasting Section.