(208 percent of average) Sacramento Valley Index (SVI) (50 percent exceedance) 13.9 (Wet) San Joaquin Valley Index (75 percent exceedance) 5.8 (Wet) Forecasting Record Water Year Runoff Volumes: Due in large part to the high runoff volumes in January and February, the projected median Water Year forecasts for the Feather, Yuba, American, Mokelumne, Stanislaus, Truckee, and East Carson are predicting records.
For other watersheds, record Water Year volumes are forecast in the 25 and 10 percent exceedance levels, and in the case of the Yuba and American Rivers the 99, 90, and 75 percent exceedance levels as well.
Eight River Index (8RI)
Runoff: After February, when the flow rate was near 400 percent of average, March data indicates a statewide flow of about 155 percent of average.
During March, all rivers in the Sierra north of the Mokelumne flowed at a rate less than 170 percent of average.
Unimpaired flows for the 2016-2017 water year: Region October-March Runoff (%) March Runoff (%) Sacramento Valley Index (4 rivers) 246 147 San Joaquin Valley Index (6 rivers) 350 193 Tulare Lake Basin (4 rivers) 282 222 Precipitation: The 83.5 inches of precipitation measured during October-March in the Northern Sierra 8-Station Index ranks as the highest total during that six-month span in the entire record of the 8-Station Index dating back to 1921.
8-Station Index 5-Station Index 6-Station Index 2006 2011 2017 2006 2011 2017 2006 2011 2017 April 12.04 3.40 4.30 10.78 1.87 3.09 7.27 1.56 1.80 May 1.48 4.88 — 1.56 3.52 — 0.87 2.53 — June 0.32 2.95 — 0.08 2.77 — 0.07 0.88 — July 0.00 0.02 — 0.61 0.04 — 0.64 0.58 — Total 13.84 11.25 — 13.03 8.20 — 8.85 5.55 — Precipitation for the 2016-2017 water year accumulated at the following rates of average: Hydrologic Region October-March precipitation (%) Sacramento River 185 San Joaquin River 187 Tulare Lake 185 Statewide 175 Precipitation Index Percent of Seasonal Average to Date through April 10, 2017 Northern Sierra 8-Station Index 204 (87.7 inches) San Joaquin 5-Station Index 195 (68.0 inches) Tulare Basin 6-Station Index 179 (45.0 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.
The snowpack as of the morning of April 10, 2017 stands at the following (based on snow sensors): Region Snow Water Equivalent (inches) % of Average (Apr 1) % of Average (Apr 10) Northern 42.0 149 157 Central 51.1 177 180 Southern 43.7 166 170 Statewide 46.4 166 171 Weather and Climate Outlooks: The 6-day weather forecast indicates precipitation Statewide, with the majority of precipitation falling on day 3 (Wednesday) and day 4 (Thursday).
ENSO-neutral conditions are favored to continue through at least the Northern Hemisphere spring 2017, with increasing chances for El Niño development into the fall.