WATER SUPPLY INDEX: For April 1, 2018

From the Department of Water Resources:

The April 1, 2018 Water Supply Index (WSI) and Bulletin 120 (B120) forecasts. are finished; the forecasts include observed conditions through the end of March.

The forecasts are posted at:

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Forecast Summary:

The projected median April-July (AJ) runoff ranges from 37 percent on the Tule River to 79 percent for the Inflow to Lake Shasta and Tuolumne River. The statewide seasonal (April-July) median forecast is 9.62 MAF which puts it at 69 percent of the historic average. Compared to March 1, April-July forecasts have more than doubled for several watersheds (American, Cosumnes, Mokelumne, Stanislaus, Merced, Kings, and Kern Rivers) while others were just less than double the March 1 forecast [Feather (179%), Yuba (170%), Tuolumne (192%), San Joaquin (191%) , and Tule (192%) Rivers].

The projected median Water Year (WY) runoff ranges from 39 percent on the Trinity River to 83 percent for the American and Tuolumne Rivers. The projected statewide median WY runoff is 69 percent of the historic average.

This forecast does not include the storms that have hit California since the Beginning of April. The storms have not influenced the forecast. This is a true April 1, 2018 forecast, using median conditions after April 1.

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

   Sacramento River Unimpaired Runoff Water Year Forecast

(50 percent exceedance)

12.1 MAF

(68 percent of average)

Sacramento Valley Index (SVI)

(50 percent exceedance)

6.9

(Below Normal)

San Joaquin Valley Index (SJI)

(75 percent exceedance)

2.7

(Below Normal)

The Sacramento Valley Index increased from Dry to Below Normal compared to last month. The San Joaquin Valley Index increased from Critical to Below Normal compared to last month completely skipping over the Dry category.

Runoff:

March statewide runoff was 95 percent of average and the water year to date was 59 percent of average. During March, the Feather through Kings Rivers all flowed above average with the Feather at 118 percent of average and the Tuolumne River at 167 percent of average. Compared to last month when all rivers on the western side of the Sierra Nevada were all flowing less than 50 percent of average for the month.

With the atmospheric river event during the first week of April, rivers are flowing near or well above average to start the month. The Yuba southward through the Kings rivers are all flowing above 200 percent of average. The Feather River is flowing near 170 percent of average.

Unimpaired flows for the 2017-2018 water year:

Region October-March Runoff (%) March Runoff (%)
Sacramento Valley Index (4 rivers) 65 99
San Joaquin Valley Index (6 rivers) 84 151
Tulare Lake Basin (4 rivers) 67 101

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 March 31, 2018

Sacramento River 82
San Joaquin River 79
Tulare Lake 68
Statewide 70

 

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

April 9, 2018

Month-to-date precipitation

as a percent of month total (inches) through April 9, 2018

Northern Sierra 8-Station Index 83 (37.1 inches) 84 (3.2 inches)
San Joaquin 5-Station Index 83 (28.5 inches) 80 (2.8 inches)
Tulare Basin 6-Station Index 67 (16.7 inches) 68 (1.7 inches)

The 12.6 inches of precipitation measured during March in the Northern Sierra 8-Station Index ranks as the 13th wettest total over the entire record of the 8-Station Index dating back to 1921. The 15.3 inches of precipitation measured during March in the San Joaquin 5-Station Index ranks as the 4th wettest total over the entire record of the 5-Station Index dating back to 1913. The 9.8 inches of precipitation measured during March in the Tulare Basin 6-Station Index ranks as the 8th wettest total over the entire record of the 6-Station Index dating back to 1922.

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.

March’s prolific storm activities significantly boosted the statewide snowpack.  Whereas on March 1 the Statewide snow pack was at just 17 percent of the expected April 1 total, the April 1 snow surveys now show a statewide snowpack at 58 percent of average – a significant boost during the month.  While not a “March Miracle” as widely publicized in the media, the snowpack, albeit still well below average, received a welcome boost during the month.

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

  Region No. Courses

Measured

Average WC (inches) % Average
April 1
North Coast 18 9.9 35
Sacramento 79 15.3 56
San Joaquin Valley 71 20.7 66
Tulare Lake 41 13.9 54
North Lahontan 17 18.7 72
South Lahontan 19 15.0 68
Statewide Average (weighted) 58

This month, the snow course readings from the automated snow sensor network do not quite match with the manual snow course readings because most snow courses were measured a few days prior to April 1 and melted out by the time April 1 occurred. Also, the snow course network includes data from a greater range in elevations compared to the snow sensors. 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.

The snowpack as of the morning of April 9, 2018 stands at the following (based on snow sensors):

Region Snow Water Equivalent (inches) % of Average (Apr 1) % of Average (Apr 9)
Northern 9.4 34 36
Central 15.3 52 53
Southern 10.3 40 41
Statewide 12.1 44 45

Comparing April 1 to March 1, the Statewide snow pack increased by 9.4 inches of snow water content, which accounts for about 34 percent increase in the April 1 average.

Weather and Climate Outlooks:

The 6-day weather outlook from the CNRFC predicts precipitation over the northern 2/3rds of the state, with a gradient north to south. The north coast will see some precipitation today through Thursday, totaling about 2 inches.   The mountains in the Sacramento River region will see as much as 1.2 inches of precipitation Tuesday through Thursday.  The San Joaquin and Tulare regions may receive up to 0.7 inches of precipitation mostly on Wednesday.  Freezing elevations will start out quite high today (above 10,000 feet) but drop to about 4,000 feet by Thursday in the northern mountains and 5,000-6,000 feet in the central and southern regions.  Freezing levels will increase again after the storm passes on Thursday.

The NWS Climate Prediction Center (CPC) one-month outlook for April, issued on March 31, indicates increased chances of above normal precipitation for the northern third of the State, increased chances of below normal precipitation for the southern third of the State, and equal chances of above or below normal precipitation elsewhere. The outlook also indicates increased chances of above normal temperatures for Southern California and equal chances of above or below normal temperatures elsewhere.

The NWS CPC three-month (April-May-June) outlook, issued March 15, indicates increased chances of above normal temperatures statewide. The same outlook indicates increased chances of below normal precipitation for the entire state.

According to the latest ENSO discussion from the CPC, issued on April 9, 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 March-May season, with neutral conditions likely to continue into the second half of the year.

Next Update:

The May 1, 2018 Bulletin 120 and Water Supply Index forecasts will be available on Tuesday, May 8, 2018. If you have any questions regarding this forecast, please contact a member of the Snow Surveys staff.

Important Links:

Full Natural Flow Data :

Precipitation Data:

Snow Data:

Extended Regional Forecasts:

 

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