BULLETIN 120 for February 1, 2023

DWR has finished the February 1, 2023 Water Supply Index (WSI) and Bulletin 120 (B120) forecasts. These forecasts include observed conditions through the end of January.

The forecasts are posted at:

WSI: https://cdec.water.ca.gov/reportapp/javareports?name=WSI
B120: https://cdec.water.ca.gov/reportapp/javareports?name=B120
B120 WY Distribution: https://cdec.water.ca.gov/reportapp/javareports?name=B120DIST

The B120 and WSI forecasts now incorporate the 6-day forecasted precipitation from the California-Nevada River Forecast Center. After the 6-day forecasted precipitation period, forecasts assume median climatology for the remainder of the forecast period.

Forecast Summary:

Water year 2023 has so far provided another example of the extreme variability that can happen as the water year evolves. October and November continued the narrative of extreme drought. December and January changed course as a series of storms battered California leading to record flooding in some places. For the seasonal snowpack, it meant a great jump in conditions from the beginning of January to the beginning of February. With the storms’ greater focus in the Central and Southern Parts of the state, the snowpack and projected runoff as a percent of average increase from north to south. Since mid-January drier than average conditions have returned. As history has already shown, a lot can happen between now and April 1, the historical average date of peak snowpack and the beginning of the spring melt. Updates to the forecast reflect those changes as they unfold.

The projected median April-July (AJ) runoff in the major Southern Cascades/Sacramento River basins ranges from 91% of average for the Pit River basin to 137% of average for the American River basin, in the major San Joaquin River basins ranges from 139% of average for the Cosumnes River basin to 170% of average for the San Joaquin River basin, and in the major Tulare Lake river basins ranges from 163% of average for the Kaweah River basin to 187% of average in the Kern River basin. The statewide seasonal AJ median forecast is 19.5 MAF which is 138% of the historic average.

The projected median Water Year (WY) runoff in the major Sierra Nevada watersheds ranges from 94% on the Total Inflow to Lake Shasta to 224% for the Cosumnes River. The projected statewide median WY runoff is 40.5 MAF which is 135% of the historic average.

The WSI forecast is based on precipitation, snow, and flows observed through January 2023 can be summarized as follows:

Sacramento River Unimpaired Runoff Water Year Forecast (50% exceedance)	20.0 MAF (113% of average)
Sacramento Valley Index (SVI) (50% exceedance)	7.9 (Above Normal)
San Joaquin Valley Index (SJI) (75% exceedance)	4.2 (Wet)

Runoff:

Water Year 2023 runoff began at a very low rate in October and November except for nearly average flows in the San Joaquin and Kings River watersheds in November. In December, flow rates for the Feather River southward picked up considerably due to the first in a series of atmospheric rivers that arrived in late December with a significant percentage of the total runoff for the month arriving after December 27. For the Trinity, Lake Shasta, and Sacrament River at Bend Bridge watersheds, the storms in late December did not reach that far north, and thus runoff remained at the same pace observed in October and November.

The series of moderate to strong atmospheric rivers continued into mid-January, and as the table below shows, each region benefitted from the copious amounts of rain that fell and created substantial runoff during that period. The areas that received the largest amount of precipitation are evident in the runoff rates shown below for January. Most notably the watersheds ranging from the Cosumnes southward to the Tule all experienced runoff 300% to nearly 500% of the January average, while the Feather, Yuba, American, and Kern all had runoff rates at or above 220%

For unimpaired flows through January 2023, the Sacramento Valley (includes the Sacramento River at Bend Bridge, Feather River, Yuba River, and American River) accumulated unimpaired runoff of 7,159 TAF is 142% of average. For the San Joaquin Valley (includes the Stanislaus River, Tuolumne River, Merced River, and San Joaquin River), the accumulated unimpaired runoff of 3,117 TAF is 294% of average. For the Tulare Lake Basin (Kings River, Kaweah River, Tule River, and Kern River), the accumulated unimpaired runoff of 782 TAF is 215% of average.

Unimpaired flows in Percent of Average for Water Year 2023 are as follows:

River	Oct	Nov	Dec	Jan	Oct-Jan	Feb (Month to Date)
Trinity	64	42	58	163	108	40
Shasta	63	59	54	153	97	41
Sacramento at Bend Bridge	53	58	62	173	110	39
Feather	63	69	125	220	158	67
Yuba	46	46	149	228	173	60
American	32	37	239	259	226	69
*Sacramento Region*	54	58	110	203	142
Cosumnes	96	98	589	499	498	110
Mokelumne	1	51	259	316	255	91
Stanislaus	46	57	276	310	257	84
Tuolumne	22	71	256	331	258	92
Merced	62	72	325	394	326	82
San Joaquin	46	105	314	359	282	138
*San Joaquin Region*	38	76	310	359	294
Kings	45	96	187	335	229	106
Kaweah	18	71	178	420	277	121
Tule	14	65	137	416	283	122
Kern	44	58	85	228	131	93
*Tulare Region*	41	78	154	334	215

Precipitation:

As noted in the discussion on full natural flow, moderate to exceptional atmospheric rivers began arriving in California around December 27 and continued into mid-January. The table below shows that both December and January precipitation accumulations were well above average with the San Joaquin and Tulare Lake Basins experiencing the most precipitation between late December and mid-January. Since the series of atmospheric rivers ended in mid-January, there has been lighter to moderate accumulation of precipitation leading up to the end of January.

After the last two months of extreme precipitation, February is starting off much drier than average. With only one small storm event so far; all three precipitation indices, the Northern Sierra 8-Station Precipitation Index (8SI), the San Joaquin 5-Station Precipitation Index (5SI), and the Tulare Basin 6-Station Precipitation Index (6SI), are recording below 20% for average to date. With only minor precipitation forecasted for the next six days, the first half of February will be well below average.

Precipitation for Water Year 2023 accumulated at the following rates of average:

Region	% of Average						Precipitation Index (inches)
Region	Oct	Nov	Dec	Jan	Feb 8 (Month to Date)	WY to Date	Feb 8 (Month to Date)	WY to Date
Northern Sierra 8-Station Index	0	88	167	176	16	130	1.5	39.0
San Joaquin 5-Station Index	0	111	243	180	18	161	1.3	35.1
Tulare Basin 6-Station Index	0	134	204	200	13	160	0.7	24.6

Monthly Precipitation in Percent of Average for Water Year 2023 by Hydrologic Region

Hydrologic Region	Oct	Nov	Dec	Jan	Oct-Jan	WY
North Coast	6	66	128	155	111	61
San Francisco Bay	0	66	185	266	175	95
Central Coast	1	105	228	205	178	92
South Coast	67	192	104	301	196	99
Sacramento River	1	83	165	203	147	76
San Joaquin River	1	112	245	256	201	102
Tulare Lake	0	123	195	244	185	92
North Lahontan	11	102	244	207	176	85
South Lahontan	1	130	144	284	193	89
Colorado River	162	38	27	104	78	33
Statewide Weighted Average	11	94	165	209	154	79

Snowpack:

California’s snowpack doubled between January 1 (16.7 inches of SWE) and February 1 (33.7 inches of SWE) thanks in part to the seven different atmospheric rivers that hit California during January. Statewide snowpack is off to its greatest start (212% of February 1 avg) dating back 54 years trailing only water year 1969 (230% February 1 avg) based on February snow course measurements.

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.

Snow course measurements were conducted during the February 10-day measurement period which ran from January 25 through February 3. Some snow courses that were scheduled for measurement were not measured due to limited staff or access issues.

Results of the February 1 snow course measurements are as follows:

Region	No. Courses	Avg WC (inches)	% Average, Apr 1	% Average, Feb 1
North Coast	11	29.0	105	170
Sacramento River	68	32.4	124	193
San Joaquin River	63	39.5	134	212
Tulare Lake	40	35.7	160	263
North Lahontan	11	26.6	149	237
South Lahontan	19	34.1	173	275
Statewide Average (weighted)			134	212

Based on the automated snow sensor network, the statewide snowpack measured 33.7 inches on February 1 which was 129% of the April 1 average and 206% of average for the date.

February has brought scattered precipitation across the northern and central areas of the state leading to accumulation while the southern Sierra has seen slight drops in snowpack as that area has seen minimal precipitation so far this month.

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

Region	No. of Stations	Avg. SWC	% of April 1	% of Average
Northern	31	32.8”	113	168
Central	49	37.2”	138	204
Southern	31	36.8”	160	236
Statewide	111	35.8”	136	201

Airborne Remote Sensing of Snow (ARSS):

During the month of January and into early February, the ARSS program initiated its first Airborne Snow Observatory (ASO) flights of the year. Flights were conducted in the Yuba, American, Tuolumne, Merced, San Joaquin, Kings, Kaweah, and Kern Rivers at the time of forecast issuance. Currently ASO flights are happening in the Feather River watershed and the three watersheds above Lake Shasta (Upper Sacramento, McCloud, and Pit Rivers).

The series of atmospheric rivers that arrived in California between late December and mid-January bolstered the Sierra Nevada snowpack to levels that many areas have not seen in over 50 years. What has become apparent during the current severe drought is that snowpack accumulation may not occur similar to the way that snow accumulated in the past. While snow course and snow sensor data remain crucial to seasonal and water year forecasting, data gathered from ASO and processed through the iSnoBal model provides DWR and its partners with a full watershed level understanding of where and how snow accumulated and what the physical status (i.e., snowpack density, cold content, and albedo) of the snowpack currently is.

The ASO data gathered to date is summarized in the table below. Based on the ASO data and the iSnoBal model results, these flights validate the snow course and snow sensor measurements and reveal a spatially distributed measurement of snow water content (SWE) in watersheds where ASO flights occur that substantiates corresponding February 1 Bulletin 120 runoff forecasts. In addition to the estimates of the volume of SWE shown below, the iSnoBal model results also reveal that in general the snow has a good cold content and snowpack density for a February 1 condition. This confirms that for now there is no immediate threat for a large-scale melt event for the snowpack above 6000 ft. In contrast, the density and the cold content of the snow that lies below 5000 ft. Is likely to melt or already has begun melting as would be expected during dryer and warmer periods in the winter. Future ASO flights and iSnoBal reports will provide DWR and its partners with updated information on these physical aspects of the snowpack and help determine when the snow may be more likely to begin to melt and at what elevations. DWR partners with the Center for Western Weather and Water Extremes (CW3E) in developing subseasonal weather and climate forecasts. These forecasts, while still considered experimental, indicate that the month of February has a good chance of having below average snow accumulation and unseasonably warm, sunny weather. The data and iSnoBal model results from the March 1 ASO flights will help DWR and its partners understand in more depth the impacts on the snowpack if indeed February is predominantly dry.

ASO data from these flights, as shown below, were considered in this forecast and the updated forecasts throughout February as additional data becomes available. SWE volumes are based on ASO data assimilated into iSnoBal models unless otherwise noted.

	ASO Flights Conducted		iSnoBal Model Estimates
Watershed	Flight Date	Raw Data SWE (TAF) Estimate	Model Date	Modeled SWE (TAF) Estimate
Lake Shasta	2/9 – 2/12	In progress	In progress	In progress
Feather	2/6 – 2/7	In progress	In progress	In progress
Yuba	1/28	761	2/6	778
American	1/31 – 2/1	958	In progress	In progress
Truckee*	—	—	—	—
Carson*	—	—	—	—
Tuolumne	1/22 – 1/25	1,442	2/2	1,452
Merced	1/31	845	1/31	849
San Joaquin	1/21 – 1/24	2,259	2/1	2,277
Kings	1/24 – 1/25	2,184	2/3	2,197
Kaweah	1/27	424	2/6	424
Kern	2/4	In progress	In progress	In progress

* Truckee River and Carson River ASO flights will begin in the spring

During the months of March, April, and May more ASO flights are planned for all the watersheds listed above. Data from all ASO flights will be available on the Snow Product Comparison Dashboard and will be scrutinized to aid in future updates of the Bulletin 120 forecasts.

Weather and Climate Outlooks:

According to CNRFC 6-day forecast, precipitation is forecast across the northern third of the state, the Sierra Nevada mountains and parts of the central and southern coasts. The largest expected accumulations are less than a quarter inch on the north coast and parts of the Sierra Nevada mountains. Most other locations are expected to see less than 0.1 inches of accumulation. Precipitation will start on the North Coast on Thursday and work its way south through the rest of the weekend. Freezing levels are currently between 7,000 and 10,000 feet for most of the state. As the storm progresses across the state, freezing levels will fall to 3,000 to 5,000 feet.

The NWS Climate Prediction Center (CPC) one‐month revised outlook for February 2023 issued on January 19, 2023, suggests equal chances of above or below normal precipitation across the entire state. The same outlook suggests increased chances of below normal temperatures for the northern third of the state with the rest seeing equal chances of above or below normal temperatures.

The CPC three-month (February-March-April) outlook, issued on January 19, 2023, suggests increased chances of below normal temperatures for the entire state. The outlook also shows equal chances of above or below normal precipitation the entire state as well reflecting the lack of predictability in the climate system for California at this time.

According to the latest El Niño/Southern Oscillation (ENSO) discussion issued by the Climate Prediction Center on February 6, 2023, La Niña conditions are present while a transition to neutral conditions is expected over the next few months. Equatorial sea surface temperatures are below average across most of the Pacific Ocean but are beginning to approach average conditions. ENSO neutral conditions are favored to appear (82% chance) over the next three months. ENSO neutral conditions are expected to persist through the summer.

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

The January 27, 2023 CW3E S2S Outlook says: Week 2 forecasts (Feb 3-9): Models generally agree on low likelihood (< 30% probability) of atmospheric river (AR) activity over most of California during Week 2.

NCEP and ECMWF are forecasting strong Madden-Julian oscillation (MJO) activity over the Indian Ocean during Week 1, which is climatologically unfavorable for AR activity in California during Weeks 1-2.

Models disagree on predicted ridging activity near the U.S> West Coast during Weeks 1-2, with NCEP showing higher likelihood of persistent ridging activity west of California.

Week 3 forecasts (Feb 10-16): All models are predicting below-normal AR activity over California with high confidence.

Both NCEP and ECMWF both show potential for persistent ridging activity west of California during Weeks 3-4, but forecast confidence in this outcome is currently low.

Next Update:

A Bulletin 120 update forecast for conditions as of February 14 will be available by Thursday, February 16. The March 1, 2023 Bulletin 120 and Water Supply Index forecasts will be available by Wednesday, March 8. If you have any questions regarding this forecast or need additional help, please contact a member of the Snow Surveys and Water Supply Forecasting Unit.

Interpreting Bulletin 120 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.