WATER SUPPLY INDEX: Forecast for March 1, 2018

From the Department of Water Resources:

The Department of Water Resources has finished the March 1, 2018 Water Supply Index (WSI) and Bulletin 120 (B120) forecasts. The forecasts include observed conditions through the end of February.

The forecasts are posted at:

WSI:       http://cdec.water.ca.gov/cgi-progs/iodir/wsi
B120:     http://cdec.water.ca.gov/b120_summary.html

 

Forecast Summary:

The projected median April-July (AJ) runoff in the major Sierra river basins ranges from 19 percent on the Tule River to 71 percent for the Inflow to Lake Shasta.  There is a fairly significant drop in the forecast from Shasta at 71 percent to the Feather River at Oroville at 36 percent.  Much of this is due to the geologic features in the two watersheds.  Whereas the Inflow to Shasta forecast is heavily influenced by the prior two water year’s runoff due to the volcanic soils (which were near average in 2016 and well above average in 2017), the Feather River forecast is more influenced by the current snow pack, which of course is quite dry.

The statewide seasonal (April-July) median forecast is 5.67 MAF which puts it at 41 percent of the historic average.

The projected median Water Year (WY) runoff in the major Sierra river basins ranges from 24 percent on the Tule River to 62 percent for the Inflow to Lake Shasta. The projected statewide median WY runoff is 48 percent of the historic average.

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

  Sacramento River Unimpaired Runoff Water Year Forecast

(50 percent exceedance)

9.5 MAF

(53 percent of average)

Sacramento Valley Index (SVI)

(50 percent exceedance)

6.0

(Dry)

San Joaquin Valley Index

(75 percent exceedance)

1.8

(Critical)

The influence from the poor snow pack is evident in the WSI forecast when compared to last month.  The Sacramento River Runoff (SRR) median forecast dropped 2.2 MAF from the February 1 forecast. The Sacramento Valley Index (SVI) dropped to the Dry category, while the San Joaquin Index (SJI) dropped to Critical category.

Runoff:

Following a dry January, February Statewide runoff was 26 percent of average and the water year to date was 45 percent of average. In comparison, many individual rivers last February were flowing above 400 percent of average. This year, the highest rates of runoff in the State in February are all in the Eastern Sierra Nevada. The North Lahontan region runoff was 72 percent of average for February and 118 percent of water year average to date. The South Lahontan region runoff was 64 percent for February and 111 percent of water year average to date.

Even with above average precipitation and snow during the first week of March, all major rivers in the Western Sierra Nevada are flowing below 50 percent of average except for the Merced and San Joaquin Rivers which are flowing at 67 and 53 percent of average, respectively. The Mokelumne, Kaweah, and Tule Rivers are all flowing below 30 percent of average at 27, 27, and 28 percent of average, respectively.

Unimpaired flows for the 2017-2018 water year:

Region October-February Runoff (%) February Runoff (%)
Sacramento Valley Index (4 rivers) 52 29
San Joaquin Valley Index (6 rivers) 52 24
Tulare Lake Basin (4 rivers) 52 28

Precipitation:

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 February 28, 2018

Sacramento River 42
San Joaquin River 32
Tulare Lake 20
Statewide 35

 

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

March 8, 2018

Month-to-date precipitation

as a percent of month total (inches) through March 8, 2018

Northern Sierra 8-Station Index 64 (24.2 inches) 39 (3.0 inches)
San Joaquin 5-Station Index 52 (15.2 inches) 80 (4.9 inches)
Tulare Basin 6-Station Index 37 (7.6 inches) 52 (2.4 inches)

The 20.5 inches of precipitation measured during October-February in the Northern Sierra 8-Station Index ranks as the 12th driest total during that period over the entire record of the 8-Station Index dating back to 1921. The 0.9 inches of precipitation on the 8-Station Index in February ranked as the 7th driest on record.

The 10.3 inches of precipitation measured during October-February in the San Joaquin 5-Station Index ranks as the 4th driest total during that period over the entire record of the 5-Station Index dating back to 1913. The 1.1 inches of precipitation on the 5-Station Index in February ranked as the 7th driest on record.

The 5.2 inches of precipitation measured during October-February in the Tulare Basin 6-Station Index ranks as the 2nd driest total during that period over the entire record of the 6-Station Index dating back to 1922. The 0.7 inches of precipitation on the 6-Station Index in February ranked as the 3rd driest on record.

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.

A dry December followed by below average precipitation in January and February with few snow producing storms has resulted in a below normal snow pack as of March 1.  Statewide the snow pack is at 19 percent of average to date and just 17 percent of the expected April 1 total.

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

 Region No. Courses

Measured

Average WC (inches) % Average
April 1
% Average
March 1
North Coast 10 4.5 19 21
Sacramento 66 4.7 16 18
San Joaquin Valley 63 5.8 16 19
Tulare Lake 34 3.8 15 16
North Lahontan 11 7.2 21 24
South Lahontan 17 7.9 30 35
Statewide Average (weighted) 17 19

This month, the snow course readings from automated snow sensor network don’t quite match with the manual snow course readings.  The snow sensors give 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.

There is a slight difference in averages for the snowpack for conditions as of March 1, 2018 between the snow courses and snow pillows. The average for the snow courses depends on when each course was measured as shown on the snow course measurement report. Most snow surveyors decided it would be safest to measure early in the measurement window to avoid the storms. Since most of the snow courses were measured before the storms in late February and early March, the average will be lower than what the snow pillows are reporting. The automated snow pillows captured what snow had fallen in late February and reports conditions for all pillows on March 1.

The snowpack as of the morning of March 8, 2018 stands at the following (based on snow sensors):

Region Snow Water Equivalent (inches) % of Average (Apr 1) % of Average (Mar 8)
Northern 7.7 28 30
Central 11.7 39 42
Southern 8.7 34 37
Statewide 9.6 35 38

Since March 1, the Statewide snow pack went up by 3.8-inches of snow water content, which accounts for about 14 percent rise in the April 1 average.

Weather and Climate Outlooks:

The 6-day weather outlook from the CNRFC predicts precipitation over most of the State with totals ranging from 0.5 an inch at the lower elevations in the Central Valley up to 4 inches in the far North Coast. Precipitation falls pretty much on all 6 days. The majority of the precipitation falls over the far North Coast on Day 1. Day 3 and Day 6 will have the most coverage over the State. On Day 3, the majority of the precipitation falls over the northern part of the State. On Day 6, the majority of the precipitation falls over the north and central Sierra Nevada. The freezing elevations for the period are forecasted to range between 5,000 to 8,000 feet for the Northern and Central Sierras and 7,0000 to 9,000 feet for the Southern Sierra Nevada.

The NWS Climate Prediction Center (CPC) one-month outlook for March, issued on February 28, indicates increased chances of above normal precipitation for northern 4/5 of the State and equal chances above or below normal elsewhere.  The outlook also indicates increased chances of below normal temperatures for the northern 4/5 of the State and equal chances above or below normal elsewhere.

The CPC three-month (March-April-May) outlook, issued February 15, indicates increased chances of below normal precipitation for the entire State and increased chances of above normal temperatures for the southern 2/3 of the state otherwise equal chances above or below normal elsewhere.

According to the latest ENSO discussion from the CPC, La Niña conditions are present.  Equatorial sea surface temperatures (SSTs) are below average across the central and eastern Pacific Ocean.  A transition from La Niña to ENSO-neutral is most likely during the Northern Hemisphere spring (~55% chance of ENSO-neutral during March-May season).

Next Update:

A Bulletin 120 and Water Supply Index forecasts for conditions as of April 1 will be available Monday, April 9. The next Bulletin 120 Update for conditions as of March 13 will be available March 15. If you have any questions regarding this forecast, please contact a member of the Snow Surveys staff.

Important Links:

Full Natural Flow Data:

Daily FNF: http://cdec.water.ca.gov/cgi-progs/stages/FNF

Monthly FNF: http://cdec.water.ca.gov/cgi-progs/reports/FNFSUM

Seasonal FNF: http://cdec.water.ca.gov/cgi-progs/reports/FLOWOUT

Precipitation Data:

Latest Northern Sierra 8-Station Precipitation Index: http://cdec.water.ca.gov/cgi-progs/products/TAB_ESI.pdf

Latest San Joaquin 5-Station Precipitation Index: http://cdec.water.ca.gov/cgi-progs/products/TAB_FSI.pdf

Latest Tulare Basin 6-Station Precipitation Index: http://cdec.water.ca.gov/cgi-progs/products/TAB_TSI.pdf

Snow Data:

Latest Snow Sensor Report: http://cdec.water.ca.gov/cgi-progs/snow/PAGE6

Latest Statewide Summary of Snow Water Equivalents: http://cdec.water.ca.gov/cgi-progs/snow/DLYSWEQ

Extended Regional Forecasts:

California Nevada River Forecast Center 6 Day QPF and Snow Level Forecast: http://www.cnrfc.noaa.gov/awipsProducts/RNOHD6RSA.php

Climate Prediction Center One-Month Outlook Forecasts: http://www.cpc.noaa.gov/products/predictions/30day/

Climate Prediction Center Three-Month Outlook Forecasts: http://www.cpc.noaa.gov/products/predictions/long_range/seasonal.php?lead=1

U.S. Seasonal Drought Outlook: http://www.cpc.ncep.noaa.gov/products/expert_assessment/sdo_summary.html

Weather Forecast Office California Service Area-Products: http://www.cnrfc.noaa.gov/forecasts.php

El Niño Southern Oscillation (ENSO) Conditions and Weekly Discussion (including La Niña):http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf

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