Theme for 2017:
2017 Water Year: What Happened and Historical Context

Historical and Future Relations Between Large Storms and Droughts in California

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Abstract:

California precipitation varies more dramatically from year to year than elsewhere in the conterminous United States. This paper analyzes the extent to which contributions of the wettest days to overall precipitation dictate the state's precipitation seasonality and frequent multiyear periods of drought (as precipitation deficit) and plenty is analyzed, historically and in projections of future climates.

The wettest 5% of wet days in California contribute about a third of precipitation but about two-thirds of the variance of water-year precipitation. Year-to-year fluctuations in precipitation strongly reflect year-to-year fluctuations of contributions from the largest storms, with the large-storm contributions explaining about twice as much precipitation fluctuation as do contributions from all remaining storms combined. This extreme dominance of large storms is largely unique to California within the United States.

In climate-change projections, eight of ten climate models considered here yield increases in precipitation from the largest storms, and when the increases are large, total precipitation follows suit. All of the models project declines in contributions from the smaller storms and models projecting total-precipitation declines reflect this decline. Projected changes in variance of water-year precipitation reflect changes in variance of large-storm contributions.

The disproportionately large overall contributions from California's largest storms, and their outsized year-to-year variability, ensure that the state's largest storms dictate the state's regimes of wet and dry spells, historically and in climate-change projections.

Perspectives on California's Wet and Wild 2016-17

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Drought, severe drought, extreme drought, atmospheric rivers, more atmospheric rivers, floods, snow pack, full reservoirs, soggy soils, infrastructure damage, full allocations — all words and phrases permeating our most recent California water conversations. If anything, these words, all used this winter, show how quickly California's precipitation regime staggers and lurches from dry to wet and back again.

Mired in drought for half a decade, storm after storm brought much needed relief to all of California this winter. Now, surrounded by lush landscapes, the transformation seems truly remarkable as drought memories fade from short term memory. How quickly we forget.

But, we shouldn't forget. If fact we should consider how this year fits in historical context and how this year might compare to what's in store for California.

This presentation takes a look at precipitation in California where we are fortunate enough to have a few records dating back a century and half. We peer further back into our precipitation history with the help of 1100 years of tree ring data. And, we read some tea leaves to infer what we might expect from a future California climate.

Putting the Record-Breaking Wet 2017 Water Year into Historical Context

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The 2017 water year has been an extremely wet season, but how does it stack up against previous wet years? To put the 2017 water year into historical context, 60-day precipitation totals from the period of the greatest rainfall were compared against the greatest 60-day precipitation totals from years past. Further, the mechanisms in which the precipitation was produced (e.g., one large atmospheric river, trains of multiple relatively small events, etc.) were explored. For 2017, it appears as though multiple relatively small storm events occurring in rapid succession were responsible for the large amounts of precipitation and resultant flooding. A selection of these storm events were reviewed in greater detail.

This presentation will also present an alternate perspective on the 2017 water year by showing the annual exceedance probability of the maximum 60-day precipitation total. The 60-day period was considered in its entirety; and, in order to more completely convey the combined effects of the back-to-back storm events, some of the individual storms were also considered in frequency space. Historical 60-day periods were also translated into annual exceedance probability for comparison to 2017 along with a few of the most extreme individual storms.

The 2016/2017 California Pluvial

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Pluvials are classically defined as spans of time characterized by increased rainfall over centurial to millennial timescales. However, dramatic changes on human and natural landscapes can be produced during singular extremely wet winters, especially in mid-latitude semiarid regions such as California.

The record-setting precipitation during the cool season of water year (WY) 2017 produced widespread environmental impacts, from floods and mass wasting to drought termination and the filling of pluvial lake basins east of the Sierra Nevada. Based upon these impacts, we argue that WY 2017 satisfies the minimum requirements of a pluvial period.

At longer timescales, hydrologic modeling of pluvial lakes in the eastern Sierra Nevada and Great Basin indicates that a water year like 2017 would be required to persist for >100 years in order to attain deglacial (~15,000 years ago) lake highstands consistent with the geologic definition of a pluvial. WY 2017 provided ample opportunities to utilize surface-based observations from networks supported by the California Department of Water Resources and NOAA Hydrometeorological Testbed to evaluate hydrometeorological processes associated with atmospheric rivers in California's mountain regions.

This presentation will provide a chronological discussion of notable atmospheric rivers and their properties including snow levels, precipitation characteristics, and responses of snowpack and streamflow to the resultant extreme precipitation. Throughout the presentation, WY 2017 will be examined through perspectives that span recent (<20 year), historical (20-150 year), and paleoclimatic (> 150 year) timescales.

Numerous rain-on-snow floods in WY 2017 on both windward and leeward slopes of the Sierra Nevada and their associated snow levels will be highlighted and compared to historic events such as 1956 and 1997. Some of the records broken by WY 2017 will be provided. For example, in the North Fork of the American River basin, cumulative water year discharge from 1 October 2016-1 April 2017 exceeded the previous record discharge (WY 1997) by over 20% and the 1 April snow survey in the Mount Rose region of the Lake Tahoe basin recorded the highest measured snow water equivalent since record keeping began in 1910.

Finally, results demonstrating robust recent snow level rises and precipitation phase shifts towards increased rainfall in regions near the climatological snow-rain transition elevation will be presented. These changes have been ongoing for a decade and have resulted in a 10-25% reduction in the total precipitation falling as snow in the northern Sierra Nevada. Potential implications of these ongoing hydrometeorological changes for long-term water resource management will be discussed.

Operational Perspective on Forecasting and Communicating Extreme Precipitation Events in Northern CA During the 2016-2017 Winter Season

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NWS has always been in the business of forecasting weather and issuing warnings for the protection of life and property and enhancement of the national economy. However, over the past several years, our mission has grown to include increased partner relations to improve communication to the public and partners on impacts and preparedness when hazardous weather threatens. This presentation will focus on the challenges NWS Sacramento faced as successive atmospheric river storms pummeled interior Northern California, and ensuring a clear and consistent message was being conveyed to keep the public and infrastructure aware and prepared.

Real-Time Modeling During Flood Control Operations

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At 3 PM on Feburary 20, 2017, TID opened the Don Pedro spillway gates for the first time since 1997. The gates were opened to pre-release spill, initially at 18,000 cfs for 3 hours and then releases were held at 16,000 cfs for the remainder of the storm. This early action minimized flooding downstream during the storm event, which if not had taken place, on February 21 at 6 PM an estimated inflow of 30,000 cfs would have been released to the river. The reservoir elevation reached a maximum elevation of 829.73 feet at midnight on February 22, only 3" below the spillway crest.

This presentation will describe the HFAM hydrologic model and how the HFAM inflow forecasts and those provided by the California Nevada River Forecast Center were used to analyze reservoir flood operations and downstream effects. The presentation will include long-term probabilistic HFAM model results based on historic weather data and will discuss analysis of climate variability and expected climate change in the Tuolumne.

FIRO Field Campaign: Description and Preliminary Results from a Unique Observing Period in the Russian River Watershed in Northern California during Jan-Mar 2017

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Forecast Informed Reservoir Operations (FIRO) is a proposed management strategy that uses data from state of the art watershed monitoring and weather and water forecasting to support water management operations, with the goals of improving water supply, maintaining reduction in flood risk, and achieving ecosystem sustainability.

FIRO development and testing is a collaborative effort between the Center for Western Weather and Water Extremes (CW3E), the U.S. Army Corps of Engineers, the California Department of Water Resources, the Sonoma County Water Agency, and others. The first testbed for this strategy is Lake Mendocino, in the Russian River Watershed in northern California. An essential part of FIRO at Lake Mendocino is to understand and better predict Atmospheric Rivers (ARs), which provide ~50% of the annual precipitation, and cause most of the heavy rain and flood events in this region.

To support this effort, a field campaign was held during January-March 2017 in the Russian River Watershed with the science objectives of investigating AR evolution as the AR interacts with terrain, and to form a unique database for model verification. Field sites equipped with Vaisala radiosonde systems, MicroRain Radars (MRRs), optical disdrometers, surface meteorology, and GPS Trimbles were deployed at the coast (Bodega Bay, also the site of an Atmospheric River Observatory (ARO) that has been recording data continuously since 2004) and inland (Ukiah and Potter Valley, near Lake Mendocino) to study water vapor transport by ARs as they move through the watershed.

The 2017 water year has been among the wettest recorded in California. During the January-March 2017 period, the coastal/inland pair of radiosonde systems captured 13 storms with maximum IVT values nearing 1200 kg/m/s. 275 balloons were released during this data collection effort, and data were continuously recorded by the MRRs and other ground instrumentation. Radiosonde profiles were provided to local NWS offices in near real time.

This presentation will provide an overview of the field campaign as well as preliminary results, including analysis of the most extreme precipitation events.

Ensemble Flow Forecasts for Risk Based Reservoir Operations of Lake Mendocino in Mendocino County, CA

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Forecast informed reservoir operations (FIRO) is a methodology that incorporates short to mid-range precipitation or flow forecasts to inform the flood operations of reservoirs. The Ensemble Forecast Operations (EFO) alternative is a probabilistic approach of FIRO that incorporates flow forecasts made by NOAA's California-Nevada River Forecast Center (CNRFC) to model and assess risk of meeting or exceeding identified management targets or thresholds. Forecasted storage probabilities are evaluated against set probability tolerances to set reservoir flood releases.

A water management model was developed for Lake Mendocino, a 111,000 acre-foot reservoir located near Ukiah, California. Lake Mendocino is a dual use reservoir, which is owned and operated for flood control by the United State Army Corps of Engineers and is operated by the Sonoma County Water Agency for water supply. Due to recent changes in the operations of an upstream hydroelectric facility, this reservoir has been plagued with water supply reliability issues since 2007.

FIRO is applied to Lake Mendocino by simulating daily hydrologic conditions from 1985 to 2010 in the Upper Russian River from Lake Mendocino to the City of Healdsburg approximately 50 miles downstream. The EFO alternative is simulated using a 15-day, 61 member streamflow hindcast generated by the CNRFC. Model simulation results of the EFO alternative demonstrate a 36% increase in median end of water year (September 30) storage levels over existing operations. Model results show no increase in occurrence of flows above flood stage for points downstream of Lake Mendocino.

In addition to the retrospective analysis of historical conditions, the EFO alternative was also evaluated for 2017 in a virtual sense by incorporating actual daily ensemble flow forecasts from the CNRFC and observed hydrologic conditions. Results to date of the 2017 virtual test trial are consistent with the historical analysis. These investigations demonstrate that the EFO alternative may be a viable flood control operations approach for Lake Mendocino and warrants further investigation through additional modeling and analysis.

How the California Atmospheric River Events of Winter 2016/2017 Compare to Normal

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California experienced very wet conditions during Winter 2016/2017, as the Northern Sierra Eight-Station Index set an all-time record for precipitation, and drought conditions were eased or eliminated across much of the state. These wet conditions can be attributed to atmospheric river (AR) events, which were not only more frequent than normal, but also longer-lived and more intense than average.

This presentation will highlight the extent to which the frequency, duration, and intensity of AR events during Winter 2016/2017 differed from past years. This analysis is based on data from a publically available "AR catalog" designed to benefit users from a variety of backgrounds.

Reviewing Historical Context of Extreme Events through the Lens of Modern Understanding of Atmospheric Rivers

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Over the past decade a great deal of research has been done exploring the atmospheric dynamics and physical constructs of atmospheric rivers along with their ties to extreme events in California. Knowledge and experience continue to advance as new observations are taken and added to the catalog of extreme conditions. Given this new perspective on extremes, this talk will go back and explore some of the foundational documents on extremes like HMR 37 and offer an interpretation of historical states of knowledge through our modern understanding of atmospheric rivers. Opportunities to explore historical extremes from this perspective and how that can further contribute to our understanding of extremes and California will be discussed.

Panel Discussion: 2017 Water Year: What Happened and Historical Context

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Moderator:

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