cw3e-web - Center for Western Weather and Water Extremes

CW3E AR Update: 21 September 2020 Outlook

September 21, 2020

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Update on potentially strong atmospheric river forecast to impact the Pacific Northwest

Forecast agreement between models and ensemble members has increased in the magnitude, duration, and location of AR conditions in association with the potentially strong AR forecast to make landfall this week
Forecasts of the strongest IVT magnitudes have shifted further south where AR 5 conditions are now forecast to impact Northern Oregon
Although AR 5s are often hazardous later in the season, given early season conditions (dry soils, low streamflow) this AR is unlikely to produce hazardous impacts
Ensemble probabilities of strong AR conditions (IVT magnitudes >750 kg m–1 s–1) have also increased over portions of the PNW
Model forecasts of precipitation accumulations have increased for portions of the Cascade, Olympic, and Coastal Mountains, though there is still some model-to-model disagreement in exact accumulation

Click images to see loops of GFS IVT/IWV analyses and forecasts Valid 0000 UTC 21 September – 0000 UTC 01 October 2020

Summary provided by C. Hecht, C. Castellano, B. Kawzenuk, N. Oakley, J. Kalansky, and F. M. Ralph; 21 September 2020

*Outlook products are considered experimental

CW3E AR Update: 18 September 2020 Outlook

September 18, 2020

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A Potentially Strong Atmospheric River Could Douse the Pacific Northwest Next Week

Model forecasts are currently indicating the potential for a strong AR to make landfall over the PNW next week, but large ensemble spread in IVT forecasts is leading to considerable uncertainty in AR Scale (ranging from no AR to AR 5 in some locations)
Forecasts of maximum IVT magnitude over the Washington Coast range from < 400 kg m^–1 s^–1 to > 1200 kg m^–1 s^–1
While forecast uncertainty is currently high, more than 4 inches of precipitation are possible over the Olympic Peninsula during the next 7 days (Note: the 7-day forecast may not encompass the entirety of this event)

Click images to see loops of GFS IVT/IWV analyses and forecasts Valid 1200 UTC 18 September – 1200 UTC 28 September 2020

Summary provided by C. Hecht, C. Castellano, B. Kawzenuk, J. Kalansky, and F. M. Ralph; 18 September 2020

*Outlook products are considered experimental

CW3E To Launch Second Year of Experimental Seasonal Forecasts of Precipitation

September 15, 2020

CW3E researchers Alexander Gershunov and Tamara Shulgina, along with collaborators Kristen Guirguis, Julie Kalansky, Brian Kawzenuk, Mike DeFlorio, and Luca Delle Monache, are set to launch the second year of an experimental seasonal precipitation prediction system based on canonical correlation analysis (CCA) this fall on the CW3E website. The prediction system utilizes a statistical approach that is based on relationships between Pacific Ocean sea surface temperature and precipitation. This effort, sponsored by the U.S. Bureau of Reclamation, California Department of Water Resources, and the U.S. Geological Survey via the Southwest Climate Adaptation Science Center, is part of a broad effort at CW3E to improve subseasonal-to-seasonal (S2S) prediction of precipitation, atmospheric rivers (ARs), and ridging events to benefit western U.S. water resource management. A 2-pager document describing this effort in detail can be found here.

CW3E Explores Shifting Seasons and Precipitation in the Yampa River Basin Upcoming Webinar

CW3E Explores Shifting Seasons and Precipitation in the Yampa River Basin Upcoming Webinar:

Thursday, September 17, 11-12:30 Mountain Time

September 15, 2020

The Yampa River is one of the wildest remaining major tributaries of the Colorado River, and provides crucial water supplies to local stakeholders and to locations as far removed as Arizona and Southern California. A multitude of environmental and societal factors are expected to be affected by climate change in the Yampa River Basin, and are pertinent to other watersheds around the American West.

This summer, CW3E and our partners at Colorado Mountain College, Friends of the Yampa, Yampa Valley Sustainability Council, Steamboat Ski and Resort Corporation, and Vacasa, among others, have virtually come together for the third annual Yampa Basin Rendezvous (YBR). YBR 2020 is a series of four interactive webinars examining the Yampa River Basin through the lens of climate change and seasonal variability. The webinars include talks by regional experts and lively discussions.

The first webinar was held on June 4, 2020, introducing the series and providing an overview of the past year in the Yampa Basin with an eye to this year’s theme of Seasonal Variability. The panelists included Marty Ralph, CW3E Director; Kent Vertrees, with Friends of the Yampa and Steamboat Powdercats; and Nathan Stewart, Associate Professor of Sustainability Studies at Colorado Mountain College.

The second webinar of YBR 2020 was held on July 9, 2020. Webinar 2 was a panel discussion on Changes in Measurement with a Changing Climate, addressing what our measurement data are currently showing and ways we can adapt our strategies to be more effective. The panelists were Mike Dettinger, Visiting Researcher at Scripps Institution of Oceanography; Jeff Deems, Research Scientist with National Snow and Ice Data Center; and Gannet Hallar, Associate Professor of Atmospheric Science at University of Utah. Both the first and second webinars are now available to view online.

This week, on Thursday, September 17th, 11-12:30 (Mountain Time), we will have the third webinar of YBR 2020, hosted virtually on Zoom. The third webinar will delve into the changes we are seeing from shifting seasons and precipitation and how these changes are impacting our local and statewide watershed and forest health. Our panelists will include Russ Schumacher, Associate Professor of Atmospheric Science at Colorado State University, Director of the Colorado Climate Center, and Colorado’s State Climatologist; David Stahle, Distinguished Professor of Geosciences at University of Arkansas; and Courtney Peterson, Adaptive Silviculture for Climate Change (ASCC) Coordinator for the Northern Institute of Applied Climate Science. Register here!

This event is part of a larger effort to connect graduate students, post-doctoral scholars, researchers, staff, and faculty from CW3E to the local communities of river basins throughout the west, to share knowledge regarding climate variability and change that has impacts on the environment, people and the economy.

The second webinar of Yampa Basin Rendezvous 2020 was held on 7 July, 2020.

CW3E AR Update: 14 September 2020 Outlook

September 14, 2020

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Update on the Atmospheric River forecast to make landfall over the Pacific Northwest

An AR that is forecast to make landfall early this week could bring much needed precipitation to parts of Washington, Oregon, and potentially Northern California.
As time has progressed closer to verification, ensemble agreement associated with AR landfall timing and initial IVT magnitude has increased.
The parent low pressure system is forecast to cut off from the upper-level flow and become quasi-stationary off the PNW coast.
Forecast duration of the event is currently exhibiting larger uncertainties as the cut-off low remains off the coast and moves higher IVT magnitudes onshore.
Forecast precipitation accumulations from the Weather Prediction Center have primarily decreased since our last outlook, though the ECMWF has trended towards a larger event (max. precip. accumulations of 1.5–2.4 in. over Coastal OR and WA).
Forecasts continue to suggest that a majority of CA will remain dry with lighter precipitation rates over far Northern CA.

Click IVT image to see loop of GFS analyses/forecasts Valid 0000 UTC 14 September – 0000 UTC 24 September 2020

Summary provided by C. Hecht, B. Kawzenuk, C. Castellano, J. Kalansky, and F. M. Ralph; 14 September 2020

*Outlook products are considered experimental

CW3E AR Update: 10 September 2020 Outlook

September 10, 2020

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A Landfalling Atmospheric River is Forecast to Potentially Bring Wildfire Relief to the Pacific Northwest

An AR that is forecast to make landfall early next week could bring much needed precipitation to parts of Washington, Oregon, and potentially Northern California.
Due to large lead times in the forecasts, there is currently very large ensemble and model-to model uncertainties in AR conditions, magnitude, duration, and location.
The large forecast uncertainties make it difficult to pinpoint where beneficial and wildfire relieving rains could fall
The NOAA Weather Prediction Center is currently predicting as much as 2 inches over Coastal Washington/Northern Oregon, and ~1 inch over far Northern California.

Click IVT or IWV image to see loop of ECMWF analyses/forecasts Valid 0000 UTC 10 September – 0000 UTC 18 September 2020

Summary provided by C. Hecht, B. Kawzenuk, C. Castellano, J. Kalansky, and F. M. Ralph; 10 September 2020

*Outlook products are considered experimental

CW3E Publication Notice: Forecast Informed Reservoir Operations Using Ensemble Streamflow Predictions for a Multi‐Purpose Reservoir in Northern California

CW3E Publication Notice

Forecast Informed Reservoir Operations Using Ensemble Streamflow Predictions for a Multi‐Purpose Reservoir in Northern California

August 17, 2020

Sonoma Water Engineer Chris Delaney led the development of a forecast informed reservoir operations (FIRO) decision support system, Ensemble Forecast Operations (EFO), for Lake Mendocino with contributions from researchers at CW3E, U.S. Army Corps of Engineers (USACE), NOAA, and independent researchers. This project directly supports the FIRO priority area of CW3E’s 2019-2024 Strategic Plan by describing a crucial tool used in reservoir operations for Lake Mendocino, the first FIRO reservoir. A proof-of-concept EFO model was originally developed by Chris in 2015 as a personal research project, and has been refined over the past 5-years with research and real-time testing and evaluation using major deviation requests made to the USACE. Additionally, the significant evaluation required by the FIRO Preliminary Viability and the Full Viability Assessments, demonstrated that the EFO is a viable alternative for flood control operations at Lake Mendocino. An article documenting the computational processes of EFO has been published in the Water Resources Research scientific journal (Delaney et al., 2020).

EFO is a risk-based approach to reservoir flood control operations that incorporates ensemble streamflow predictions (ESPs) made by the California-Nevada River Forecast Center (CNRFC). Each member of an ESP is individually modeled to forecast system conditions and calculated risk of reaching critical operational thresholds (Fig. 1). Reservoir release decisions are simulated to manage forecasted risk with respect to established risk tolerance levels. EFO was developed for Lake Mendocino, a 111,000 acre-foot reservoir near Ukiah, California, to evaluate its viability to improve reservoir storage reliability without increasing downstream flood risk. Lake Mendocino is a dual use reservoir, owned and operated for flood control by the USACE and operated for water supply by Sonoma Water. EFO was simulated using a 26-year (1985-2010) ESP hindcast generated by the CNRFC, which provides 61-member ensembles of 15-day flow forecasts. EFO simulations yield generally higher storage levels during the flood management season while maintaining needed flood storage capacity by strategically pre-releasing water in advance of forecasted storms. Model results demonstrate a 33% increase in median storage at the end of the flood management season (May 10) over existing operations without marked changes in flood frequency for locations downstream from Lake Mendocino. EFO may be a viable alternative for managing flood control operations at Lake Mendocino that provides multiple benefits (water supply, flood mitigation and ecosystems) and provides a management framework that could be adapted and applied to other flood control reservoirs.

Figure 1. EFO forecasted storage (top panel) and risk (bottom panel) for February 8, 1986.

Delaney, C. J., Hartman, R. K., Mendoza, J., Dettinger, M., Monache, L. D., Jasperse, J., Ralph, F. M., Talbot, C., Brown, J., & Reynolds, D. (2020). Forecast Informed Reservoir Operations Using Ensemble Streamflow Predictions for a Multi‐Purpose Reservoir in Northern California. Water Resources Research, 56, e2019WR026604, https://doi.org/10.1029/2019WR026604.

CW3E Visiting Researcher Honored at California Extreme Precipitation Symposium

July 21, 2020

The 26^th Annual California Extreme Precipitation Symposium (CEPSYM), a Floodplain Management Association project, was held this year on June 30, 2020, virtually due to the ongoing pandemic. Since 2018, CW3E has partnered with CEPSYM to organize the Southwest Extreme Precipitation Symposium, whose purpose is to advance understanding of the causes of and processes within extreme precipitation in the southwest region of North America. This year’s CEPSYM theme was Connecting Rain-on-Snow Events, Atmospheric Rivers, and Floods. Slides from all presentations are available at: https://cepsym.org/proceedings-2020.php

Each year since 2004, a Special Recognition Award is presented to “highlight and honor the outstanding contributions of individuals and institutions that have supported and advanced the professions of meteorology, flood hydrology, and flood risk management. These professions share the public safety goal of protecting life and property from the ravages of flooding. Together these professionals provide the tools, information, and knowledge essential for forecasting flood events before they occur, estimating potential flood magnitudes and impacts used in designing flood risk reduction projects, and responding to floods when they occur. Each honoree contributed to making the people of California safer from flood events over their many years of service.” This year, that award was given to CW3E Visiting Researcher Dr. Michael Dettinger. More details are available on the California Extreme Precipitation Symposium website: https://cepsym.org/awards/dettinger.php

The award presented to Mike Dettinger at the 2020 California Extreme Precipitation Symposium.

CW3E Publication Notice: Floods due to atmospheric rivers along the U.S. West Coast: The role of antecedent soil moisture in a warming climate

CW3E Publication Notice

Floods due to atmospheric rivers along the U.S. West Coast: The role of antecedent soil moisture in a warming climate

July 20, 2020

Graduate student and now UCLA post-doc, Qian Cao, with her advisor, Professor Dennis Lettenmaier and his group, collaborated with CW3E director F. Martin Ralph, and CW3E researchers Alexander Gershunov and Tamara Shulgina on a recently published a paper in the Journal of Hydrometeorology titled “Floods due to atmospheric rivers along the U.S. West Coast: The role of antecedent soil moisture in a warming climate” (Cao et al., 2020). The research aligns with the Monitoring and Projections of Climate Variability and Change Priority Area within CW3E’s 2019-2024 Strategic Plan because it examines atmospheric rivers roles in flooding along the U.S. West Coast.

Precipitation extremes are projected to become more frequent along the U.S. West Coast due to increased atmospheric river (AR) activity, but the frequency of less intense precipitation events may decrease. Antecedent soil moisture (ASM) conditions can have a large impact on flood responses especially if pre-storm precipitation decreases. Taken together with increased antecedent evaporative demand due to warming, this would result in reduced soil moisture at the onset of extreme precipitation events. This article examines the impact of ASM on AR-related floods in a warming climate in three basins that form a transect along the U.S. Pacific Coast: the Chehalis River basin in Washington, the Russian River basin in Northern California, and the Santa Margarita River basin in Southern California (see Figure 1).

They ran the Distributed Hydrology Soil Vegetation Model (DHSVM) over the three river basins using forcings downscaled from 10 Global Climate Models (GCMs). They examined the dynamic role of ASM by comparing the changes in the largest 50, 100 and 150 extreme events in two periods, 1951-2000 and 2050-2099. They used the AR catalogs of Gershunov et al. (2019) in which they applied an automated AR detection scheme to daily GCM output. They found that in the Chehalis (northern-most) basin, the projected fraction of AR-related extreme discharge events slightly decreases. In the Russian basin, this fraction increases, however, and more substantially so in the Santa Margarita basin (see Table 1). This is due to increases in AR-related extreme precipitation events, as well as the fact that the relationship of extreme precipitation to extreme discharge is strengthened by projected increases in year-to-year volatility of annual precipitation in California, which increases the likelihood of concurrent occurrence of large storms and wet ASM conditions.

The research supported herein was funded by the CW3E at the Scripps Institution of Oceanography UC San Diego via AR Program Phase II. Understanding the projected roles of ARs in future flooding is important to building resilience in California.

Figure 1. Map of study region including a) the Chehalis River basin in Washington State, b) the Russian River basin in Northern California, and c) the Santa Margarita River basin in Southern California. The Kling-Gupta efficiency (KGE) during the calibration period (1986-2000) is displayed for each stream gauge in a)-c). d) Location of 60 unregulated coastal watersheds and their hydroclimatic conditions, including e) annual precipitation, f) seasonality of precipitation, and g) max flood ratio. The LOWESS (locally weighted scatterplot smoothing) curve is displayed in e)-g).

Table 1. Fraction [%] of AR-related Peak-Over-Threshold (POT) extreme precipitation events, including POT_N1P, POT_N2P, and POT_N3P events (i.e. extreme precipitation events with thresholds set to 1, 2 and 3 events per year on average), POT extreme discharge events, and annual maximum flow (AMF) events in three river basins based on the ensemble average of 10 GCMs.

Cao, Q., A. Gershunov, T. Shulgina, F.M. Ralph, N. Sun, and D.P. Lettenmaier, 2020: Floods due to atmospheric rivers along the U.S. West Coast: The role of antecedent soil moisture in a warming climate. J. Hydrometeor., https://doi.org/10.1175/JHM-D-19-0242.1 (early view online).

Gershunov, A., T. Shulgina, R.E.S. Clemesha, K. Guirguis, D.W. Pierce, M.D. Dettinger, D.A. Lavers, D.R. Cayan, S.D. Polade, J.F. Kalansky and F.M. Ralph, 2019: Precipitation regime change in Western North America: The role of Atmospheric Rivers. Sci. Rep., 9, 9944, https://doi.org/10.1038/s41598-019-46169-w.

CW3E Publication Notice: Freezing Level Forecast Error Can Consume Reservoir Flood Control Storage: Potentials for Lake Oroville and New Bullards Bar Reservoirs in California

CW3E Publication Notice

Freezing Level Forecast Error Can Consume Reservoir Flood Control Storage: Potentials for Lake Oroville and New Bullards Bar Reservoirs in California

July 20, 2020

CW3E hydrologist, Edwin Sumargo, CW3E researchers, F. Martin Ralph, Forest Cannon and Brian Henn (CW3E alumnus) published a paper in Water Resources Research, titled “Freezing Level Forecast Error Can Consume Reservoir Flood Control Storage: Potentials for Lake Oroville and New Bullards Bar Reservoirs in California” (Sumargo et al., 2020). As part of CW3E’s 2019-2024 Strategic Plan to support Forecast Informed Reservoir Operations, CW3E researches the impacts of atmospheric rivers (ARs) on water management and public safety and to improve the prediction capability. In particular, this study assesses the sensitivities reservoirs in the Yuba-Feather watershed, Lake Oroville and New Bullards Bar reservoirs, to freezing-level (Z_FL) forecast uncertainty. Specifically, it quantifies what percentages of the two reservoirs’ flood pools would be consumed by the prescribed Z_FL forecast error, with varying Z_FL altitudes and precipitation event magnitudes. This study offers a “guide curve” on the reservoir sensitivity to Z_FL forecast uncertainty for reservoir operations in the Yuba-Feather watershed. Ultimately, this work supports the ongoing collaborations involving CW3E, Yuba Water Agency, California Department of Water Resources, NOAA, and U.S. Army Corps of Engineers.

The atmospheric Z_FL determines the rain‐snow transition zone at the surface, how much rainfall is available for runoff, and the flood risk during a precipitation event (Figure 1). An accurate Z_FL forecast is thus critical for reservoir operations, especially in mountain watersheds with narrow elevation bands like the Feather and North Fork Yuba in Northern California, where a 500‐m elevation gain can amount to >50% of the watershed area. Using a ±350‐m Z_FL forecast error, we find inflow volume uncertainties of <10% to >50% of the flood pool storages at Lake Oroville and New Bullards Bar reservoirs, depending on the Z_FL, antecedent moisture, and the precipitation magnitude (Figure 2). In other words, the uncertainties can increase by up to >3% per inch (25.4 mm) of precipitation, depending on the Z_FL and antecedent moisture condition. This result substantiates the significant impact of Z_FL forecast error and the critical need of Z_FL forecast accuracy to support reservoir flood control operations in the two watersheds.

Figure 1. (Figure 4 in the manuscript) Schematic description of the impact of Z_FL forecast uncertainty on (a) storm runoff from the watershed and (b) the associated inflow to reservoir flood pool. The ±350-m ambient Z_FL forecast uncertainty in (a) is based on Henn et al. (2020) finding for up to 72-hour forecast lead time, which is used throughout this paper. The 0-500-m downward bending of Z_FL over the mountain topography in (a) is based on Minder & Kingsmill (2013) estimate.

Figure 2. (Figure 3 in the manuscript) Feather River (a-c) and North Fork Yuba River (d-f) Watersheds runoff uncertainties associated with a Z_FL forecast error of ±350 m in percent of the reservoir flood pool capacities as functions of Z_FL and event return periods based on the 1981-2018 daily PRISM precipitation (colors). The top-to-bottom panels represent the runoff uncertainties corresponding to (a and d) dry, (b and e) average, and (c and f) wet antecedent moisture conditions (AMCs). The gray horizontal lines denote the mean, the -1*standard deviation (?) from the mean, and the minimum CNRFC Z_FL of the top 10th percentile precipitation events since 2010 (see Appendix A in the manuscript)..

Sumargo, E., Cannon, F., Ralph, F. M., & Henn, B. (2020). Freezing Level Forecast Error Can Consume Reservoir Flood Control Storage: Potentials for Lake Oroville and New Bullards Bar Reservoirs in California. Water Resources Research, 56, e2020WR027072. https://doi.org/10.1029/2020WR027072