WATER SUPPLY INDEX: Forecast for May 1st

Aerial view looking North along Old River, in the center is Fay Island, left top is Palm Tract, left bottom is Orwood Tract, bottom is Woodward Island and top right is Bacon island all part of the Sacramento-San Joaquin River Delta in San Joaquin County, California. Photo taken March 08, 2019 by Ken James, DWR

The Department of Water Resources staff have finished the May 1, 2019 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 forecasted river basins ranges from 140 percent on the Merced River to 192 percent for the Cosumnes River. The statewide seasonal AJ median forecast is 22.1 MAF which is 158 percent of the historic average. Compared to the April 1 B120 forecast, all forecasts south of the Sacramento River at Bend Bridge decreased, except for the Cosumnes River which increased. The largest decrease was forecast on the Tule River with a drop of 16 percent. All forecasts north of the Feather River increased – from 5 percent for the Pit River to 23 percent for the Sacramento River at Delta.

The projected median Water Year (WY) runoff in forecasted river basins ranges from 123 percent for the Trinity River to 183 percent for the Cosumnes River. The projected Statewide median WY runoff is 42.1 MAF which is 139 percent of average.

The WSI forecast is based on precipitation and flows observed through April 2019 and can be summarized as follows:

Sacramento River Unimpaired Runoff Water Year Forecast

(50 percent exceedance)

24.4 MAF

(137 percent of average)

Sacramento Valley Index (SVI)

(50 percent exceedance)

 10.2

(Wet)

San Joaquin Valley Index (SJI)

(75 percent exceedance)

4.2

(Wet)

 

Runoff:

April runoff was above well above average, even though precipitation was below average, due to the melting of the above average low snowpack. The total observed April runoff volume for the Trinity River reached a new record of 455 TAF compared to the previous record of 416 TAF set in 1915.

Unimpaired flows in Percent of Average for Water Year 2019

Hydrologic Region Oct Runoff Nov Runoff Dec Runoff Jan Runoff Feb Runoff Mar Runoff Apr Runoff Oct-Apr Runoff
Sacramento River Region 78 56 45 102 176 165 197 135
San Joaquin River Region 65 56 43 80 229 179 180 152
Tulare Lake Region 65 55 51 69 183 196 182 146

 

May full natural flow rates updated through May 6-7, 2019:

River Basin Percent of Historic Average
Trinity 127
Shasta Inflow 115
Sacramento at Bend Bridge 124
Feather 163
Yuba 135
American 130
Cosumnes 182
Mokelumne 134
Stanislaus 142
Tuolumne 128
Merced 152
San Joaquin 135
Kings 141
Kaweah 152
Tule 159
Kern 178

 

Precipitation:

Precipitation for Water Year 2019 accumulated at the following rates of average

Region WY accumulated precipitation through April 30, 2019

(percent of average to date)

Sacramento River Valley 131
San Joaquin River Valley 121
Tulare Lake Basin 130
Statewide 126
Regional Precipitation Indices Average to date

as of May 8, 2019

Northern Sierra 8-Station Index 128 (61.4 inches)
San Joaquin 5-Station Index 118 (44.0 inches)
Tulare Basin 6-Station Index 118 (31.7 inches)

 

Monthly Precipitation for individual months (May values are through May 7) in Percent of average for Water Year 2019:

Regional Precipitation Indices Oct Nov Dec Jan Feb Mar Apr May
Northern Sierra 8-Station Index 34 104 54 148 264 127 102 0
San Joaquin 5-Station Index 36 152 34 118 238 129 45 0
Tulare Basin 6-Station Index 125 163 40 103 193 163 24 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 over 100 years.

The results of the May 2019 statewide snow surveys are as follows:

 Region No. of Courses Measured Average Snow Water Content

(inches)

% Average
April 1
% Average
May
North Coast 9 36.8 106% 152%
Sacramento 63 39.7 120% 167%
San Joaquin Valley 57 43.1 131% 159%
Tulare Lake 41 32.4 130% 166%
North Lahontan 4 30.6 116% 136%
South Lahontan 1 32.5 147% 171%
Statewide Average (weighted) 124% 161%

 

The snowpack as of the morning of May 8, 2019 stands at the following (based on snow sensors):

Region Snow Water Content (inches) % of Average (Apr 1) % of Average (May 9)
Northern 24.7 83 139
Central 28.3 98 140
Southern 21.5 87 125
Statewide 25.3 91 135

Since April 1, the Statewide snowpack has decreased about 20 inches of snow water content, which accounts for about a 70 percent decrease in the percent of April 1 average. As of May 8, 2019, the snowpack in the Northern, Central, and Southern Sierras is less than what it was on the same day in WYs 2017, 2011, 2006, and 1998 based on the snow sensor network.

The statewide snowpack peaked for the water year on March 31 with a snow water content of 45.4 inches (162% of the April 1 average).

 

Weather and Climate Outlooks:

The CNRFC 6 day forecast shows chances of precipitation across areas south of I-80, tomorrow and Friday. Chances of precipitation along the Sierra crest down into the southern California on Sunday and Monday. Precipitation totals over the next 6 days will be tricky because of the convective nature of the storms. Places that are forecasted to receive the heaviest precipitation amounts are in the southern Sierra where amounts of 1.0 to 2.0 inches are possible.  In southern California, higher elevation areas can expect 0.25 to 0.75 inch of precipitation, whereas the lower elevation areas can expect to receive up to 0.25 inch.

The freezing elevations for the next 6 days are forecasted to vary from 10,000 to 12,000 feet for all major basins in the State including the North Coast and Klamath.

The NWS Climate Prediction Center (CPC) one‐month outlook issued on April 30, 2019, indicates increased chances of above normal temperatures statewide. The same outlook indicates increased chances of above normal precipitation along the central and southern Sierra, and southern California; and equal chances of above or below normal precipitation in northern California and central coast.

The CPC three‐month (May-June-July) outlook issued on April 18, 2019, indicates increased chances of above normal temperatures statewide. The same outlook indicates equal chances of above or below normal precipitation statewide except for the areas east of Highway 395 and eastern Sierra basins where increased chances of precipitation is forecasted.

According to the latest El Niño/Southern Oscillation (ENSO) discussion issued by the Climate Prediction Center on May 6, 2019, El Niño conditions are present. Equatorial sea surface temperatures are above average across most of the Pacific Ocean. The pattern of anomalous convection and winds are consistent with El Niño. A weak El Niño is likely to continue through the Northern Hemisphere summer 2019 (~65% chance) and possibly fall (~50-55% chance).

Next Update:

A Bulletin 120 update for conditions as of Tuesday, May 14 will be available Thursday, May 16. This is the last issue of the Water Supply Index (WSI) forecasts for Water Year 2019. The next WSI will be available in December 2019. If you have any questions regarding this forecast, please contact a member of the Snow Surveys staff.

 

Important Links:

Full Natural Flow Data:

Precipitation Data:

Snow Data:

Extended Regional Forecasts:

 

Click here to view all posted announcements.

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