A morning view of the Truckee River in Truckee, California after a winter storm moved across the Sierra Nevada. Photo taken December 10, 2021. Kelly M. Grow / California Department of Water Resources

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:

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)

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.

Important Links: 

Full Natural Flow Data:  
Daily FNF Monthly FNF
Seasonal FNF
Tableau Dashboard – Historical FNF Comparison (Interactive Data Visualization)  
Tableau Dashboard – Daily FNF (Interactive Data Visualization)

Precipitation Data:  
Latest Northern Sierra 8-Station Precipitation Index Tabular Data
Latest San Joaquin 5-Station Precipitation Index Tabular Data

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