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!

From the Department of Water Resources:

(Note: scroll down to “Important Links” for links to new Tableau dashboards.  Take me there.)

DWR has finished the May 1, 2020 Water Supply Index (WSI) and Bulletin 120 (B120) forecasts. These forecasts include observed conditions through the end of April.  

The forecasts are posted at: 

               WSI:       http://cdec.water.ca.gov/reportapp/javareports?name=WSI 

               B120:     http://cdec.water.ca.gov/b120.html 

Forecast Summary: 

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  

(50 percent exceedance)  

 9.2 MAF 

(52 percent of average) 

Sacramento Valley Index (SVI)  

(50 percent exceedance)  

6.0 

(Dry) 

San Joaquin Valley Index (SJI)  

(75 percent exceedance)  

2.2 

(Dry) 

Runoff: 

Unimpaired flows in Percent of Average for Water Year 2020 are as follows: 

Hydrologic Region 

Oct 

Runoff 

Nov 

Runoff 

Dec 

Runoff 

Jan 

Runoff 

Feb 

Runoff 

Mar Runoff 

Apr Runoff 

Oct-Apr 

Runoff 

Sacramento River Region  

84 

45 

74 

45 

33 

31 

74 

50 

San Joaquin River Region  

92 

33 

84 

26 

26 

41 

96 

57 

Tulare Lake Region 

110 

77 

91 

42 

41 

38 

78 

62 

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: 

River Basin 

Percent of Historic Average 

Trinity 

38 

Shasta Inflow 

53 

Sacramento at Bend Bridge 

55 

Feather 

62 

Yuba 

76 

American 

85 

Cosumnes 

89 

Mokelumne 

90 

Stanislaus 

85 

Tuolumne 

85 

Merced 

77 

San Joaquin 

83 

Kings 

93 

Kaweah 

76 

Tule 

56 

Kern 

68 

 

Precipitation: 

Precipitation for Water Year 2020 accumulated at the following rates of average 

Region 

WY accumulated precipitation (%) through April 30, 2020 

Sacramento River Valley 

57 

San Joaquin River Valley 

72 

Tulare Lake Basin 

73 

Statewide  

70 

Regional Precipitation Indices 

WY average to date  

as of May 7, 2020 

Northern Sierra 8-Station Index  

56 (27.1 inches) 

San Joaquin 5-Station Index  

60 (22.4 inches) 

Tulare Basin 6-Station Index  

65 (17.4 inches) 

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 

Oct 

Nov 

Dec 

Jan 

Feb 

Mar 

Apr 

May 

Northern Sierra 8-Station Index  

3 

43 

107 

55 

2 

81 

73 

4 

San Joaquin 5-Station Index  

0 

69 

109 

19 

4 

101 

125 

0 

Tulare Basin 6-Station Index  

0 

106 

91 

12 

12 

76 

208 

0 

 

Snowpack: 

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: 

Region 

No. of Courses Measured 

Average Snow Water Content 

(inches) 

% Average 
April 1 

% Average May 1 

North Coast 

8 

3.8 

11 

15 

Sacramento  

46 

9.3 

29 

40 

San Joaquin Valley  

35 

12.8 

36 

43 

Tulare Lake  

32 

9.0 

33 

43 

North Lahontan  

3 

6.2 

20 

23 

South Lahontan  

1 

11.5 

52 

61 

Statewide Average (weighted) 

30 

39 

 

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): 

Region 

Snow Water Content (inches) 

% of Average (Apr 1) 

% of Average (May 7) 

Northern 

3.1 

10 

16 

Central 

6.2 

21 

29 

Southern 

4.6 

18 

25 

Statewide 

4.9 

17 

24 

 

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. 

Next Update: 

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. 

 

Important Links: 

Full Natural Flow Data

Precipitation Data: 

 

Snow Data: 

Extended Regional Forecasts: 

Bulletin 120: 

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