Analysis of Multiple Atmospheric River Impacts on California in Early 2017

The year 2017 began with an impactful series of atmospheric rivers (AR) along the California coast. We present a case study of these events that took place within the span of just over a week, as three distinct ARs hit the coast leading to widespread rain and mountain snow. While these were not the only ARs to impact the region, they were perhaps the most impactful in terms of precipitation accumulations, and setting the stage for numerous infrastructure problems in the weeks and months to follow (e.g., Oroville Dam). A closer look at these events allows us to better understand the elements that led to the brimming California watershed, saturated soils and deep concerns for infrastructure.

We first summarize the synoptic evolution of the AR events. Later, we will examine how well various models (GFS, HRRR etc.) were able to forecast the timing, duration, and location of the heaviest rain/snow. On Tuesday, January 3^rd, the first of the three ARs hit the coast of California. This was a fairly wide AR, targeting much of the state from Santa Barbara north to the King Range, producing heavy rain accumulations, particularly over the western slopes of both the coastal range and the Sierra Nevada. GFS analysis shows that the deepest moisture over the central and southern Sierras was collocated with the strongest 700mb vertical velocities. The foothills received 2-6” of rain from this AR event (the upper end is quite large by California standards!)

A second and more powerful AR event developed on the following Saturday night. The positioning of this AR was more challenging for forecasters. It was initially over south-central California, but then the plume of deep moisture swiftly moved northward. Precipitable water values over the immediate coast exceeded 1.5”. the accompanying synoptics pushed snow levels higher, leading to heavy rain on top of existing snow at higher elevations of the Sierras, and causing hydrological problems. During this, event the moisture “hose” shifted northward and again back south, making predictions of timing of the heaviest precipitation difficult. Ultimately, this AR produced largest accumulations in the Plumas and Lassen National Forests, with some locations receiving over 10” of rain. This was the most impactful of the three ARs in the period.

The third and final event took place on Tuesday, January 10^th. This AR developed and persisted over central California. Initially wider, the AR feature strengthened and became focused between Santa Cruz and Mendocino. Snow levels remained more seasonal, producing snow on the Sierra passes. The stability of the AR positioning createed more flooding and erosion problems. For the first time in over a decade, state controllers were forced to open Yolo Bypass, an area designed to flood in such events. Accumulations exceeded 4” in much of the Sierra foothills, and snowfall was measured in feet along the Sierra Crest.

This case study uses various sources of information. The Global Forecasting System (GFS) analysis was examined to capture the synoptic-scale features, including the positioning of the ARs as well as fronts, moisture, and winds at various levels. Satellite data was referenced for confirmation of some of these features. Station data gives us information on total rainfall and the timing of the rain events. Area Forecast Discussions (AFDs) also proved helpful in understanding the major impacts attributable to these systems as well as noting how well models did or did not perform.

The three strong ARs in California at the start of 2017 created a number of hydrological and infrastructure problems. While residents experienced the discomfort of closures, outages and some property damage, the rainy season was far from over. Following a historic drought, planners faced difficult choices such as capturing runoff versus leaving capacity for future rains in the season. This set the stage for more serious concerns in the following months as more heavy rain led to concerns about the integrity of reservoir structures.