BULLETIN 120 and Water Supply Index Forecast – May 1, 2022

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From the Department of Water Resources:

We’ve finished the May 1, 2022 Water Supply Index (WSI) and Bulletin 120 (B120) forecasts. These forecasts include observed conditions through the end of April.

The forecasts are posted at:

Forecast Summary:

The projected median April-July (AJ) runoff in the major Southern Cascades/Sacramento River basins ranges from 24% of average for the Trinity River basin to 62% of average for the Pit River, in the major San Joaquin River basins ranges from 40% of average for the Merced River basin to 47% of average for the Cosumnes and Stanislaus river basins, and in the major Tulare Lake river basins ranges from 16% of average for the Tule River basin to 41% of average in the Kings River basin. The statewide seasonal AJ median forecast is 6.5 MAF which is 46% of the historic average.

The projected median Water Year (WY) runoff in the major Sierra Nevada river basins ranges from 27% on the Tule River to 68% for the Yuba River. The projected statewide median WY runoff is 16.9 MAF which is 56% of the historic average.

The WSI forecast is based on precipitation, snow, and flows observed through March 2022 and can be summarized as follows:

Sacramento River Unimpaired Runoff Water Year Forecast

(50% exceedance)

 10.6 MAF

(60% of average)
Sacramento Valley Index (SVI)

(50% exceedance)

 4.5

(Critical)
San Joaquin Valley Index (SJI)

(75% exceedance)

 1.5

(Critical)

Runoff:

Water Year 2022 flows fell considerably after the October and December storms and were relatively steady until late March and early April. Snowmelt started in most basins but temporarily decreased with cooler temperatures and in northern basins saw large increases due to the rainfall that fell across the state. Snowmelt is starting to decrease for northern basins but continues to increase into May in the more southern basins. Despite the precipitation, average flow rates for all major basins are below average for May, ranging from 24% of average in the Tule to 78% of average in the American.

For unimpaired flows through April 2022, the Sacramento Valley (includes the Sacramento River at Bend Bridge, Feather River, Yuba River, and American River) accumulated unimpaired runoff of 8.1 MAF is 63% of average. For the San Joaquin Valley (includes the Stanislaus River, Tuolumne River, Merced River, and San Joaquin River), the accumulated unimpaired runoff of 2.2 MAF is 76% of average. For the Tulare Lake Basin (Kings River, Kaweah River, Tule River, and Kern River), the accumulated unimpaired runoff of 0.7 MAF is 56% of average.

Unimpaired flows in Percent of Average for Water Year 2022 are as follows:

River Oct Nov Dec Jan Feb Mar Apr Oct-Apr May (Month to Date)
Trinity 312 195 40 46 32 28 31 45 33
Shasta 104 104 72 67 29 25 39 51 46
Sacramento at Bend Bridge 125 115 76 54 25 23 39 49 46
Feather 348 137 116 73 44 46 66 75 57
Yuba 566 163 118 60 37 44 75 76 68
American 814 198 144 58 40 48 76 79 78
Sacramento Region 233 131 99 60 33 35 58 63
Cosumnes 1334 151 254 51 28 22 58 64 69
Mokelumne 732 234 135 58 49 58 67 81 63
Stanislaus 432 161 146 47 47 56 63 73 62
Tuolumne 384 176 143 45 39 49 68 72 53
Merced 277 167 140 42 36 51 68 66 58
San Joaquin 282 170 215 71 67 73 74 92 56
San Joaquin Region 402 176 165 52 45 53 68 76
Kings 96 100 126 54 49 59 70 68 53
Kaweah 70 49 99 38 34 42 51 48 45
Tule 65 29 95 35 22 26 27 34 24
Kern 51 58 72 45 37 34 35 42 27
Tulare Region 76 74 104 48 41 47 57 56

Precipitation:

After three consecutive months of well below average precipitation the state finally saw significant precipitation in April. The northern part of the state was the most impacted with the Northern Sierra 8-Station Precipitation Index (8SI) seeing a total of 6.1 inches in April, 141% of the month average. While the central and southern Sierras saw precipitation, it was still below average with the San Joaquin 5-Station Precipitation Index (5SI) and the Tulare Basin 6-Station Precipitation Index (6SI) measuring 2.2 and 1.1 inches, respectively, which is 62% and 44% of their April averages.

Precipitation for Water Year 2022 accumulated at the following rates of average:

Region % of Average Precipitation Index (inches)
Oct Nov Dec Jan Feb Mar Apr May (Month to Date) WY to Date May (Month to Date) WY to Date
Northern Sierra 8-Station Index 453 58 144 14 4 16 141 3 79 0.1 39.4
San Joaquin 5-Station Index 314 25 195 0 2 26 62 0 65 0.0 24.3
Tulare Basin 6-Station Index 216 7 221 0 9 31 44 0 60 0.0 16.0

Monthly Precipitation to date in Percent of Average for Water Year 2022 for Regional Precipitation Indices

Hydrologic Region Oct Nov Dec Jan Feb Mar Apr Oct-Apr
Sacramento River 480 51 145 10 4 19 121 78
San Joaquin River 349 28 204 2 5 29 88 70
Tulare Lake Basin 209 21 230 4 9 47 52 69
Statewide Weighted Average 330 40 164 10 5 29 92 69

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.

Snow course measurements were conducted during the May 10-day measurement period which ran from April 24 through May 3. Some snow courses that were scheduled for measurement were not measured due to limited staff or access issues.

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

Region No. Courses Avg WC (inches) % Average,

Apr 1

 % Average,

May 1
North Coast 9 2.3 8 12
Sacramento River 59 8.8 23 32
San Joaquin River 52 10.5 28 34
Tulare Lake 29 5.0 16 20
North Lahontan 4 6.4 21 25
South Lahontan 1 14.5 66 77
Statewide Average (weighted) 23 30

Based on the automated snow sensor network, the statewide snowpack was 6.4 inches on May 1 which is 23% of the April 1 average.

In April, the state saw the first significant snow events since December. Due to the weather system, there was minimal melt in the northern half of the state with some areas seeing net accumulation. There was less snowfall in the south.  With higher temperatures, snowmelt has increased across those basins.

The snowpack as of the morning of May 9, 2022 stands at the following (based on snow sensors):

Region No. of Stations Avg. SWC % of April 1 % of Average
Northern 30 4 14 24
Central 45 4.9 16 23
Southern 25 1.6 7 10
Statewide 100 3.8 14 21

Airborne Remote Sensing of Snow (ARSS):

During the month of April, the ARSS program conducted Airborne Snow Observatory (ASO) flights over the Feather, Yuba, Truckee, Carson, Tuolumne, Merced, San Joaquin, Kings, and Kaweah River basins with bi-weekly updates to the iSnoBal snow hydrology model provided in-between flights.

As was noted in the discussion above related to the snow course and snow sensor data, the April winter storms provided some mid-month accumulation in Northern Sierra/Lake Tahoe region watersheds or at the minimum slowed down the melt rate.  The ASO data also captured SWE accumulation in the Tuolumne River watershed, however for the watersheds from the Merced River southward to the Kaweah River, the combination of ASO data and iSnoBal model updates shows a continual melt of the snowpack.  Further evident in the ASO data is the more rapid melt in recent burn areas; particularly in the burn scars of the Dixie Fire (2021 Feather River) and Creek Fire (2020 San Joaquin River).

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

ASO Flights Conducted iSnoBal Bi-Weekly Model Updates
Watershed Flight Date Volume SWE (TAF) Model

Date

 Volume SWE (TAF)
Feather 4/2 182 4/19 180
5/1 125*
Yuba 4/24 295 4/19 255
5/1 231
Truckee 4/9 195 4/1 296
4/19 237
5/1 217
Carson 4/6 123 4/1 138
4/19 110
5/1 96
Tuolumne 4/5 338 4/1 288
4/29 316 4/19 313
Merced 4/12 135 4/1 183
4/28 119 4/19 129
San Joaquin 4/17 328 4/1 420
4/30 304
Kings 4/10 358 4/19 350
4/25 316 5/1 236
Kaweah 4/29 48 4/19 70

* SWE volume estimated from ASO data prior to assimilation into iSnoBal model

During the month of May, one more flight is currently planned for each of the Yuba, Truckee, Carson, Tuolumne, Merced, Kings, and Kaweah watersheds pending weather and hydrologic conditions. 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, there is precipitation forecasted across the north half of the state and the Sierras. Northern parts of the state and Sierra Nevada mountains will see totals over 0.5 inches to as much as 1.25 inches in a few locations. The Coast and Central Valley north of San Francisco are forecasted to receive less than 0.1 inch of precipitation. Freezing levels are currently between 3,000 to 5,000 feet for most of the state but will rise to 12,000-14,000 feet by the end of the forecast window.

The NWS Climate Prediction Center (CPC) one‐month revised outlook for May 2022 issued on April 30, 2022, suggests below normal chances of precipitation across the northern third of the state and equal chances of above or below normal precipitation for the rest of the state. The same outlook suggests increased chances of above normal temperatures for the southern third of the state with the rest seeing equal chances of above or below normal temperatures except for a small area along the Oregon border which has increased chances of below average temperatures.

The CPC three-month (May-June-July) outlook, issued on April 21, 2022, suggests increased chances of above normal temperatures for the entire state away from the coast. The coast will have equal chances of above or below normal temperatures. The outlook also shows increased chances of below normal precipitation for the northern third of the state with the southern two-thirds seeing equal chances of above or below normal precipitation except for the area along the Arizona border which will see above average chances of precipitation.

According to the latest El Niño/Southern Oscillation (ENSO) discussion issued by the Climate Prediction Center on May 9, 2022, La Niña conditions are present. Equatorial sea surface temperatures are below average across most of the Pacific Ocean. The tropical Pacific atmosphere is consistent with La Niña. La Niña is favored to continue into the Northern Hemisphere summer (59% chance during June-August 2022), with a 50-55% chance through the fall.

Next Update:

A Bulletin 120 update forecast for conditions as of May 10 will be available by Thursday, May 12. This current forecast is the last issuance of the WSI forecasts for Water Year 2022. The next WSI will be available in December 2022. 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.

Important Links:

Full Natural Flow Data:

Precipitation Data:

Snow Data:

Extended Regional Forecasts:

Bulletin 120:

Other Useful Links:

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