WATER SUPPLY INDEX for December 1, 2023

Welcome to Water Year 2024! DWR has finished the December 1, 2023 Water Supply Index (WSI) forecast. These forecasts include observed conditions through the end of November.

The WSI forecasts are posted at: https://cdec.water.ca.gov/reportapp/javareports?name=WSI

Forecast Summary

The forecast for the Sacramento Valley Water Supply Index is “Below Normal” and the forecast for the San Joaquin Valley Water Supply Index is “Dry” due in large measure to lower-than-average precipitation in October and November despite both indices being categorized as wet last year.

The WSI forecast is based on the precipitation and runoff (full natural flow) through November 30, 2023 and can be summarized as follows:

Sacramento River Unimpaired Runoff Water Year Forecast (50 percent exceedance)	13.9 (79 percent of average)
Sacramento Valley Index (SVI) (50 percent exceedance)	7.22 (Below Normal)
San Joaquin Valley Index (SJI) (75 percent exceedance)	2.35 (Dry)

Runoff

After a historic water year which included a late season tropical storm impacting Southern California and the Southern Sierra, river flows for October were higher than expected despite the well below average precipitation for the month. While most northern and central basins were around average, in the southern Sierras many basins were flowing at several times the monthly average. During November, flows decreased considerably with all the northern and most of central basins below average for the month. In the Tulare Basin, the Kern and Tule, are still flowing near 200 percent of average but the Kings and Kaweah have fallen to around average for the month.

Unimpaired flows in Percent of Average for Water Year 2024:

River	Oct	Nov	Dec (Month to Date)
Trinity	81	63	34
Shasta	84	75	43
Sacramento at Bend Bridge	80	79	36
Feather	112	98	42
Yuba	106	77	30
American	108	72	26
Sacramento Region	90	83
Cosumnes	273	137	20
Mokelumne	92	83	28
Stanislaus	94	61	21
Tuolumne	87	57	19
Merced	97	82	23
San Joaquin	175	125	58
San Joaquin Region	120	84
Kings	208	105	50
Kaweah	191	117	33
Tule	526	189	72
Kern	226	198	161
Tulare Region	223	143

Precipitation: The entire state has started the new water year off with below average precipitation. California has experienced a few storm systems so far, but none have brought significant precipitation. Another storm system will be moving across the top of the northern coast, however it likely will not impact the entire state. Thus, the state will likely remain below average precipitation for the month for the three major indices. As of December 8, the Northern Sierra 8-Station Index stands at 17 percent of average while the San Joaquin 5-Station Index and the Tulare Basin 6-Station Index are lower – at 4 and 0 percent of average, respectively.

Precipitation for Water Year 2024 accumulated at the following rates of average:

Region	% of Average			Precipitation Index (inches)
	Oct	Nov	Dec 8 (Month to Date)	WY to Date	Dec 8 (Month to Date)	WY to Date
Northern Sierra 8-Station Index	28	60	17	52	1.7	5.6
San Joaquin 5-Station Index	9	54	4	31	0.3	2.3
Tulare Basin 6-Station Index	16	38	0	24	0.0	1.2

Monthly Precipitation in Percent of Average for Water Year 2024 by Hydrologic Region

Hydrologic Region	Oct	Nov
North Coast	59	66
San Francisco Bay	29	65
Central Coast	10	47
South Coast	14	64
Sacramento River	41	50
San Joaquin River	18	55
Tulare Lake	35	28
North Lahontan	49	71
South Lahontan	13	38
Colorado River	0	113
Statewide Weighted Average	36	56

Snowpack: As of the morning of December 8, statewide snowpack is currently 37 percent of average to date. The snow water content is 47, 41, and 19 percent of average for the date in the northern, central, and southern Sierra, respectively.

The snowpack as of the morning of December 8, 2023 stands at the following (based on snow sensors)

Region	No. of Stations	Snow Water Equivalent (inches)	% Average (Apr 1)	% of Average (Dec 8)
Northern	30	1.8	7	47
Central	53	1.9	7	41
Southern	26	0.7	3	19
Statewide	109	1.6	6	37

Reservoir Storage

After a historically wet water year that saw the highest snowpack in at least 40 years, reservoir storage is well above average across the state except for the North Coast which is at 90 percent of average. Statewide totals are 121 percent of average and a large increase from early last water year.

Summary of Storage in Major Reservoirs as of November 30, 2023

Weather and Climate Outlooks

According to the CNRFC 6-day forecast, there is little to no precipitation forecasted across the state except for a storm that will clip the top of the Northern Coast on Sunday.  The Northern Coast may see up to 0.6 inches of rain in parts. Freezing elevations are currently between 8,000 and 10,000 feet. These values will rise slightly over the next three days up to 12,000 feet across most of the state over the weekend.

The NWS Climate Prediction Center (CPC) one‐month outlook for December 2023, issued on November 30, 2023, predicts increased chances of above normal temperatures for the far-northern portion of California and equal chances of above or below normal temperatures elsewhere. The same outlook predicts increased chances of above normal precipitation for the far-northern portion of California and equal chances of above or below normal precipitation elsewhere.

The CPC three‐month (December-January-February) outlook, issued November 16, 2023, predicts increased chances of above normal temperatures for the entire state except for the Colorado River region along the California-Arizona border which will see equal chances of above or below normal temperatures. The same outlook predicts increased chances of above normal precipitation statewide.

According to the latest El Niño/Southern Oscillation (ENSO) discussion issued by the Climate Prediction Center on December 4, 2023, El Niño conditions are observed. Equatorial sea surface temperatures (SSTs) are above average across the central and eastern Pacific Ocean. The tropical Pacific atmosphere anomalies are consistent with El Niño. El Niño is anticipated to continue through the Northern Hemisphere spring (with a 62% chance during April-June 2024).

Subseasonal to Seasonal (S2S) Forecasts (from Center for Western Weather and Water Extremes (CW3E))

The December 1, 2023 CW3E S2S Outlook says: Week 2 forecasts (Dec 8-14): Models generally agree on low likelihood (< 30% probability) activity over Southern and Central California.

Models agree on strong Madden-Julian Oscillation (MJO) activity over the Maritime Continent during early Week 2 which is historically associated with an increased likelihood of wet extremes in Central and Southern California in the following weeks.

Ridging outlooks show high likelihood of persistent ridging activity across the southwestern US during weeks 1-2, but the International Research Institute for Climate and Society (IRI) weather regime outlooks show high likelihood of a Pacific Trough.

Week 3 forecasts (Dec 15-21): Models disagree on the likelihood of AR activity over California.

Uncertainty in frequency and location of ridging activity during Weeks 3-4.

Next Update:

The next WSI forecast for conditions as of January 1, 2024 will be available by Tuesday, January 9, 2024. The first Bulletin 120 (B120) forecast of the new water year, for conditions as of February 1, 2024, will be available on Thursday, February 8, 2024. If you have any questions regarding this forecast, please contact a member of the Snow Surveys and Water Supply Forecasting Unit.

Interpreting Bulletin 120 and Water Supply Index Water Supply Forecasts

All forecasts have uncertainty. For water supply forecasts, the sources of uncertainty include unknown future weather, model simplifications, and data limitations. To express this uncertainty, the forecast is presented not as a single value but as a range of values, each with a specific probability of occurrence. The wider the spread among these values, the more uncertain the forecast. As the season progresses, forecasts generally become more accurate tightening the spread among these values.

The Bulletin 120 provides a range of possible volumes of water that might show up in the April through July time period. It uses three points to characterize this range of outcomes: a 50% exceedance (median), a 10% exceedance, and a 90% exceedance. The median forecast is a value that represents where half the outcomes are expected to be above, and half are expected to be below. The 10% exceedance represents a higher end of the range where only 1 in 10 events would be expected to be higher. The 90% exceedance is a low mark where only 1 in 10 events would be expected to be lower. The space between these points represents 80% of the expected outcomes and is often referred to as the 80% confidence interval. These three points provide a guide for water resources planning covering the range of possible outcomes that may still transpire as the wet season winds down and the snowmelt season begins. There is still a 20% chance that the actual streamflow volume will fall either below the 90% exceedance forecast or above the 10% exceedance forecast.