WATER SUPPLY INDEX FORECAST for February 1, 2018

Snow along Highway 88 on January 26, 2018. Photo by Lois Elling.

From the Department of Resources:

Department of Resources staff has finished the February 1, 2018 Water Supply Index (WSI) and Bulletin 120 (B120) forecasts. The forecasts include observed conditions through the end of January.

The forecasts are posted at:

WSI:       http://cdec.water.ca.gov/cgi-progs/iodir/wsi

B120:     http://cdec.water.ca.gov/cgi-progs/iodir/b120

Forecast Summary:

The projected median April-July (AJ) runoff in the major Sierra river basins ranges from 29 percent on the Tule River to 82 percent for the Inflow to Lake Shasta.  There is a fairly significant drop in the forecast from Shasta at 82 percent to the Feather River at Oroville at 52 percent.  Much of this is from the geologic features in the two watersheds.  Whereas the Inflow to Shasta forecast is heavily influenced by the prior two water year’s runoff due to the volcanic soils (which were near average in 2016 and well above average in 2017), the Feather River forecast is more influenced by the current snow pack, which of course is quite dry.

The statewide seasonal (April-July) median forecast is 7.99 MAF which puts it at 57 percent of the historic average.  Natural comparisons will be made to the dry years of 2014 and 2015 given the snow pack as of February 1.  The current runoff forecast for this year exceeds both from those years due to 2017 being such a wet year.

The projected median Water Year (WY) runoff in the major Sierra river basins ranges from 31 percent on the Tule River to 72 percent for the Inflow to Lake Shasta.

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

   Sacramento River Unimpaired Runoff Water Year Forecast

(50 percent exceedance)

11.7 MAF

(66 percent of average)

Sacramento Valley Index (SVI)

(50 percent exceedance)

6.7

(Below Normal)

San Joaquin Valley Index

(75 percent exceedance)

2.3

(Dry)

The influence from the poor snow pack is evident in the WSI forecast when compared to last month.  The Sacramento River Runoff (SRR) median forecast dropped 4.2 MAF from the January 1 forecast – roughly 23 percent.  The Sacramento Valley Index (SVI) dropped into the Below Normal category, while the San Joaquin Index (SJI) remained in the Dry category.

Runoff:

What a difference a year makes.  A year ago we were discussing 200 percent water year to date runoff rates and near record breaking monthly FNF volumes.  This year we’re wondering what happened to winter.  Following the very dry December, statewide January runoff was 46 percent of average and the water year to date just 54 percent of average.  In comparison, many individual rivers last January were flowing above 400 percent of average.  The highest rates of runoff in the State this January are all in the Eastern Sierra Nevada.  The North Lahontan region runoff was 101 percent of average for January and 134 percent of the water year average to date.  The South Lahontan region runoff was 97 percent for January and 122 percent of the water year average to date.  The rain and snow from 2017 appears to still be draining from the Eastern Sierra,  however with a poor snow pack, those runoff rates won’t last.

With dry conditions dominating the first week of February, all major rivers in the Western Sierra Nevada are flowing below 50 percent of average so far in February.  Runoff rates during the first week of February range from 15 percent of average on the Cosumnes River to 44 percent of average on the Kern River.  With dry weather forecast over the next week or two, there does not appear to be much reason for the runoff to increase anytime soon.

Unimpaired flows for the 2017-2018 water year:

Region October-January Runoff (%) January Runoff (%)
Sacramento Valley Index (4 rivers) 62 51
San Joaquin Valley Index (6 rivers) 67 48
Tulare Lake Basin (4 rivers) 63 42

Precipitation:

Precipitation for the 2016-2017 water year accumulated at the following rates of average:

Region/Index WY-to-date precipitation (%) through January 31, 2018
Sacramento River 38
San Joaquin River 30
Tulare Lake 17
Statewide 34
Northern Sierra 8-Station Index 70 (19.6 inches )
San Joaquin 5-Station Index 44 (9.2 inches )
Tulare Basin 6-Station Index 31 (4.5 inches)

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.

A dry December followed by below average precipitation in January with few snow producing storms has resulted in a very paltry snow pack on February 1st.  Statewide the snow pack is at 24 percent of average to date and just 15 percent of the expected April 1 total.  The current snow pack represents the third driest February 1 snow pack on record.  Only the February 1 conditions in 2014 (9 percent of the February average and 6 percent of the April average) and 2015 (19 percent of the February average and 12 percent of the April average) were drier.  The snow on February 1, 1977 was at 25 percent of the February average and 16 percent of the April 1 average.

The results of the February 2018 statewide snow surveys are as follows:

  Region No. Courses

Measured

Avg WC % Average
April 1
% Average
February 1
North Coast 11 4.6″ 15% 24%
Sacramento 68 4.9″ 18% 28%
San Joaquin Valley 65 4.7″ 13% 21%
Tulare Lake 40 3.0″ 11% 18%
North Lahontan 11 4.0″ 17% 27%
South Lahontan 17 5.1″ 20% 32%
Statewide Average (weighted) 15% 24%

The automated snow sensor network shows similar results to those found in the manual snow course readings.  Since February 1, the unseasonably dry and warm weather has continued to exacerbate an already meager snow pack.

The snowpack as of the morning of February 8, 2018 stands at the following (based on snow sensors):

Region Snow Water Equivalent (inches) % of Average (Apr 1) % of Average (Feb 8)
Northern 3.8 14 20
Central 5.6 19 27
Southern 3.4 13 20
Statewide 4.4 16 23

Since February 1, the Statewide snow pack has dropped 0.5” of snow water content, which accounts for about 1 percent drop in the April 1 average.

Weather and Climate Outlooks:

The 6-day weather forecast only indicates precipitation over Southern California on Monday next week with totals up to 0.25 inch. Freezing levels are highest today near 11,000-12,000 feet and are expected to drop over the forecast period to its lowest levels on the sixth day of the forecast near 7,000-8,000 feet.

THE NWS Climate Prediction Center (CPC) one-month outlook for February, issued January 31, indicates increased chances of above normal temperatures statewide. The same outlook also indicates increased chances of below normal precipitation.

The CPC three-month (February-March-April) outlook, issued January 18, indicates increased chances of above normal temperatures for the southern half of the state and equal chances of above or below normal temperatures for the northern half of the state. The same outlook indicates increased chances of below normal precipitation for the southern half of the state and equal chances of above or below normal precipitation for the northern half of the state.

According to the latest ENSO discussion from the CPC, La Niña was evident in the equatorial Pacific during January 2018 with the latest value close to -1.0 degrees Celsius.  A transition from La Niña to ENSO-neutral is most likely during the Northern Hemisphere spring (~55% chance of ENSO-neutral during the March-May season).

A Bulletin 120 update for conditions as of February 13  will be available Thursday, February 15. The March 1, 2018 Bulletin 120 and Water Supply Index forecasts will be available on Thursday, March 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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