PROJECT ALERT NOTICE (CA) California experiencing another wave of AR storms—Continued: Day 3

Date: Sat, 21 Jan 2017 15:09:10 EST

Summary: Flooding is expected as multiple atmospheric rivers continue to bring heavy rainfall and snow to parts of California today and into next week.

Storms during the past 3 days have produced major snow and rainfall in northern California. Precipitation reports from the Center for Western Weather and Water Extremes show areas of northern California received as much as 8 to 12 inches in the past 72-hours, with Vendao, CA in Sonoma County receiving as much as 13 to 16 inches. High-elevation areas of the Sierra Nevada received 3 to 5 feet of snow in the past 72-hours, and are predicted to get another 1-3 feet of snow by Monday.

Yesterday. storms brought heavy rainfall to parts of central and southern California, with some locations receiving over 3 inches of rain in a 24-hour period. Tomorrow, a large storm is expected to bring heavy rainfall and snow to the entire state, with areas of central and southern California expected to receive another 1 to 4 inches of rainfall. Major and wide-spread flooding is expected. Currently, streams are receding, but many are predicted to exceed monitor or flood stage tomorrow or Monday.

The persistent and heavy rainfall yesterday caused extensive flash flooding in central and southern California. Heavy debris carried by rushing flood waters choked channels, which increased water pressure and caused some levees and floodwalls to fail. For example, yesterday USGS crews measured the second highest streamflow at Pacheco Creek (11153000) just before the creek breached the levee for the second time this month causing major flooding in Hollister, CA. A debris flow from the 2016 Sherpa Fire burn scar dammed up water and caused flooding at a downstream campground, which floated several cars and cabins and required evacuations. The USGS does not operate a gage in this area, but does operate gages (11124500 and 1112350) in the nearby 2016 Rey Fire burn scar area.

USGS crews worked late into the evening yesterday to measure flow in flashy central and southern California streams and washes, many of which have not had substantial flow in several years. Road closures made access difficult, but the biggest challenge for crews was high turbidity and heavy sediment loads which caused measurement difficulties. Where appropriate, crews relied on mechanical meters or made surface velocity measurements and flagged high-water marks for surveying.

The California Water Science Center (CAWSC) has 15 crews responding to this event today from the following service areas: Santa Maria, CA (6); Redlands, CA (4); and Poway, CA (5). Most crews are performing recessional measurements, but some are performing service repairs or maintenance.

Although only a few streamgages remain offline due to flood-related damage, maintenance work has been a big issue. Stormflow and debris have damaged or destroyed many staff plates and crest stage gages. Many orifice lines that had been extended to low-flow levels due to the drought are now prone to sedimentation and need to be raised. Several water-quality probes or their deployment tubes have also sustained damages.

More crews are planned to deploy tomorrow and into next week in response to these events. No safety issues have occurred during this series of storms and all staff have followed USGS field reporting and safety protocols. Updates will continue to be provided during the course of this event.