South Fork American River, 2016.  Photo by DWR.

From the Department of Water Resources:

DWR staff has finished the May 1, 2018 Water Supply Index (WSI) and Bulletin 120 (B120) forecasts. The forecasts include observed conditions through the end of April.

The forecasts are posted at:

WSI:       http://cdec.water.ca.gov/cgi-progs/iodir/wsi

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

Forecast Summary:

The projected median April-July (AJ) runoff ranges from 44 percent on the Tule River to 107 percent for East Carson River. The statewide seasonal (April-July) median forecast is 11.2 MAF which puts it at 80 percent of the historic average. Compared to April 1, April-July forecasts have increased for all basins forecasted.

The projected median Water Year (WY) runoff ranges from 42 percent on the Trinity River to 90 percent for the American River. The projected statewide median WY runoff is 74 percent of the historic average.

The WSI forecast is based on precipitation and flows observed through April 2018 and can be summarized as follows:

Sacramento River Unimpaired Runoff Water Year Forecast

(50 percent exceedance)

12.8 MAF

(72 percent of average)

Sacramento Valley Index (SVI)

(50 percent exceedance)


(Below Normal)

San Joaquin Valley Index (SJI)

(75 percent exceedance)


(Below Normal)


The Sacramento Valley Index Below Normal Year Type remained unchanged compared to last month. The San Joaquin Valley Index Below Normal Year Type also remained unchanged compared to last month.


April statewide runoff was 134 percent of average and the water year to date was 73 percent of average. During April, the Feather through Kaweah Rivers all flowed above average with the Feather at 127 percent of average ranging up to the Tuolumne River at 177 percent of average. The North Lahontan Region continues to flow well above average at 197 percent of average for April and 145 percent of average for the water year to date. The significant runoff in April was mostly attributed to the atmospheric river event during the beginning of the month.

During the first week of May, rivers are flowing below average to start the month. All rivers are flowing below 70 percent of average except for the Cosumnes and Tuolumne Rivers which are flowing at 84 and 72 percent of average, respectively. With the expected warming trend settling in over the weekend, flows should increase due to increased snowmelt.

Unimpaired flows for the 2017-2018 water year:

Region October-April Runoff (%) April Runoff (%)
Sacramento Valley Index (4 rivers) 75 123
San Joaquin Valley Index (6 rivers) 109 169
Tulare Lake Basin (4 rivers) 89 136


Precipitation for the 2017-2018 water year has accumulated at the rates of average shown in the table below.

Region WY-to-date precipitation (%)

through April 30, 2018

April precipitation (%)
Sacramento River 88 147
San Joaquin River 83 116
Tulare Lake 69 80
Statewide 74 117


Region/Index WY-to-date precipitation as a percent of average (inches) through

May 8, 2018

April precipitation

as a percent of monthly average (inches)

Northern Sierra 8-Station Index 81 (38.8 inches) 126 (4.8 inches)
San Joaquin 5-Station Index 79 (29.4 inches) 105 (3.7 inches)
Tulare Basin 6-Station Index 65 (17.4 inches) 96 (2.4 inches)

Despite a very wet start to April with the unseasonably strong atmospheric river that came ashore April 5-7, the monthly precipitation for April ended up near average for the San Joaquin and Tulare regions.


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.

While March’s prolific storm activities significantly boosted the statewide snowpack from meager February observations, early April’s atmospheric river temporarily accelerated the snow melt as snow elevations were generally above 11,000 or 12,000 feet.  It is difficult to say how much snow was added above those elevations during this storm without direct measurements available, but melt began in earnest for elevations below this and by the end of a relatively warm April, much of the snow below 6,000 feet in the north and 8,000 feet in the south had melted out.

The results of the May 1, 2018 statewide snow surveys are as follows:

Region No. Courses


Average WC (inches) % Average
April 1
% Average May 1
North Coast 5 2.2 7 10
Sacramento 60 6.5 16 23
San Joaquin Valley 55 12.0 31 38
Tulare Lake 31 7.1 23 30
North Lahontan 4 7.4 27 31
South Lahontan None
Statewide Average (weighted) 21 27

The measurements on April 30 from the snow sensor network generally agree with the manual snow course measurements.  On May 1, the sensors showed a snow pack that was at 20 percent of the April 1 average for the Northern Sierra (comparable to the Sacramento region in the course report), 29 percent for the Central Sierra (comparable to the San Joaquin Valley), and 19% for the Southern Sierra (comparable to the Tulare Lake region).  Steady melt has occurred since that time (see the third column in the table below).

The snowpack as of the morning ofMay 7, 2018 stands at the following (based on snow sensors):

Region Snow Water Equivalent (inches) % of Average (Apr 1) % of Average (May 7)
Northern 3.7 13 21
Central 6.1 21 29
Southern 3.9 15 21
Statewide 4.8 17 25


Weather and Climate Outlooks:

The 6-day weather outlook from the CNRFC predicts dry conditions for most of the state except for the North Coast, Klamath, and Southern Sierra where extremely light amounts of precipitation are forecast. Today, less than 0.1 inch of precipitation is predicted for the North Coast and Klamath River watershed, and the same amount for Southern Sierra on Saturday. Freezing elevations will start out quite high today (above 12,000 feet) and remain so for the first part of the week before dropping to the 8,000-10,000 foot level in the Northern half of the state.  The Central and Southern Sierra will remain quite warm throughout the week as snow melt peak runoff dates are quickly approaching.

The NWS Climate Prediction Center (CPC) one-month outlook for May, issued on April 30, indicates increased chances of below normal precipitation for the northern third of the State, and equal chances of above or below normal precipitation elsewhere. The outlook also indicates increased chances of above normal temperatures for entire State.

The NWS CPC three-month (May-June-July) outlook, issued April 19, indicates increased chances of below normal precipitation for the northern half of the State, and equal chances of above or below normal precipitation for the southern half of the State. The outlook also indicates increased chances of above normal temperatures for the entire State.

According to the latest ENSO discussion from the CPC, issued on May 7, 2018 , La Niña conditions are present.  Equatorial sea surface temperatures (SSTs) are near-to- below average across the east-central and eastern Pacific Ocean.  La Niña is expected to transition to ENSO-neutral during April-May, with ENSO-neutral then likely (greater than 50% chance) to continue through the Northern Hemisphere summer 2018.

Next Update:

A Bulletin 120 update for conditions as of Tuesday, May 8, will be available on Thursday, May 10.  This is the last issue of the Water Supply Index (WSI) forecasts for Water Year 2018.  The next WSI will be available in December 2018.

Important Links:

Full Natural Flow Data :


Precipitation Data:

Snow Data:

Extended Regional Forecasts:

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