Historical and National Perspectives on Extreme West Coast Precipitation Associated with Atmospheric Rivers during December 2010

Abstract A large-scale analysis of landfalling atmospheric rivers (ARs) along the west coast of North America and their association with the upper-tropospheric flow is performed for the extended winter (November–March) for the years 1979–2011 using Modern-Era Retrospective Analysis for Research and Applications (MERRA) reanalysis data. The climatology, relationship to the El Niño–Southern Oscillation and the Madden–Julian oscillation, and upper-level characteristics of approximately 750 landfalling ARs are presented based on the 85th percentile of peak daily moisture flux. AR occurrence along the West Coast is dominated by early season events. In composites of upper-level fields during AR occurrences, certain characteristics stand out irrespective of the tropical climate indices. This suggests that extratropical dynamical processes play a key role in AR dynamics. The influence of the large-scale circulation on AR intensity prior to landfall is examined by objectively selecting an extreme subset of 112 landfalling AR dates representing the 95th percentile of strongest cases. Each landfalling AR date that is identified is traced backward in time using a novel semiautomated tracking algorithm based on spatially and temporally connected organized features in integrated moisture transport. Composites of dynamical fields following the eastward progression of ARs show a close relationship of the location of the jet, Rossby wave propagation, and anticyclonic Rossby wave breaking in the upper troposphere of the eastern Pacific and moisture transport in the lower troposphere. Comparison between the strongest and the weakest ARs within the most extreme subset shows differences in both the intensity of moisture transport and the scale and development of anticyclonic Rossby wave breaking in the eastern Pacific.

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Evaluation of the subseasonal forecast skill of floods associated with atmospheric rivers in coastal Western U.S. watersheds

Journal of Hydrometeorology ◽

10.1175/jhm-d-20-0219.1 ◽

2021 ◽

Author(s):

Qian Cao ◽

Shraddhanand Shukla ◽

Michael J. DeFlorio ◽

F. Martin Ralph ◽

Dennis P. Lettenmaier

Keyword(s):

River Basin ◽

West Coast ◽

Forecast Skill ◽

Probabilistic Forecast ◽

Lead Times ◽

Flood Events ◽

Streamflow Prediction ◽

Antecedent Soil Moisture ◽

Atmospheric Rivers ◽

The U.S

AbstractAtmospheric rivers (ARs) are responsible for up to 90% of major flood events along the U.S. West Coast. The timescale of subseasonal forecasting (two weeks to one month) is a critical lead time for proactive mitigation of flood disasters. The NOAA/Climate Testbed Subseasonal Experiment (SubX) is a research-to-operations project with almost immediate availability of forecasts. It has produced a reforecast database that facilitates evaluation of flood forecasts at these subseasonal lead times. Here, we examine the SubX driven forecast skill of AR-related flooding out to 4-week lead using the Distributed Hydrology Soil Vegetation Model (DHSVM), with particular attention to the role of antecedent soil moisture (ASM), which modulates the relationship between meteorological and hydrological forecast skill. We study three watersheds along a transect of the U.S. West Coast: the Chehalis River basin in Washington, the Russian River basin in Northern California, and the Santa Margarita River basin in Southern California. We find that the SubX driven flood forecast skill drops quickly after week 1, during which there is relatively high deterministic forecast skill. We find some probabilistic forecast skill relative to climatology as well as ensemble streamflow prediction (ESP) in week 2, but minimal skill in weeks 3-4, especially for annual maximum floods, notwithstanding some probabilistic skill for smaller floods in week 3. Using ESP and reverse-ESP experiments to consider the relative influence of ASM and SubX reforecast skill, we find that ASM dominates probabilistic forecast skill only for small flood events at week 1, while SubX reforecast skill dominates for large flood events at all lead times.

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Forecasting Atmospheric Rivers during CalWater 2015

Bulletin of the American Meteorological Society ◽

10.1175/bams-d-15-00245.1 ◽

2017 ◽

Vol 98 (3) ◽

pp. 449-459 ◽

Cited By ~ 23

Author(s):

Jason M. Cordeira ◽

F. Martin Ralph ◽

Andrew Martin ◽

Natalie Gaggini ◽

J. Ryan Spackman ◽

...

Keyword(s):

Water Vapor ◽

West Coast ◽

Heavy Precipitation ◽

Field Studies ◽

Field Phase ◽

Forecast System ◽

Field Campaign ◽

Atmospheric Rivers ◽

Vapor Flux ◽

The U.S

Abstract Atmospheric rivers (ARs) are long and narrow corridors of enhanced vertically integrated water vapor (IWV) and IWV transport (IVT) within the warm sector of extra tropical cyclones that can produce heavy precipitation and flooding in regions of complex terrain, especially along the U.S. West Coast. Several field campaigns have investigated ARs under the CalWater program of field studies. The first field phase of CalWater during 2009–11 increased the number of observations of precipitation and aerosols, among other parameters, across California and sampled ARs in the coastal and near-coastal environment, whereas the second field phase of CalWater during 2014–15 observed the structure and intensity of ARs and aerosols in the coastal and offshore environment over the northeast Pacific. This manuscript highlights the forecasts that were prepared for the CalWater field campaign in 2015, and the development and use of an “AR portal” that was used to inform these forecasts. The AR portal contains archived and real-time deterministic and probabilistic gridded forecast tools related to ARs that emphasize water vapor concentrations and water vapor flux distributions over the eastern North Pacific, among other parameters, in a variety of formats derived from the National Centers for Environmental Prediction (NCEP) Global Forecast System and Global Ensemble Forecast System. The tools created for the CalWater 2015 field campaign provided valuable guidance for flight planning and field activity purposes, and they may prove useful in forecasting ARs and better anticipating hydrometeorological extremes along the U.S. West Coast.

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The role of nearshore air-sea interactions for landfalling atmospheric rivers on the U.S. West Coast

10.1002/essoar.10506135.1 ◽

2021 ◽

Author(s):

Samuel T Bartusek ◽

Hyodae Seo ◽

Caroline C Ummenhofer ◽

John Steffen

Keyword(s):

West Coast ◽

Atmospheric Rivers ◽

The U.S

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Hourly storm characteristics along the U.S. West Coast: Role of atmospheric rivers in extreme precipitation

Geophysical Research Letters ◽

10.1002/2017gl074193 ◽

2017 ◽

Vol 44 (13) ◽

pp. 7020-7028 ◽

Cited By ~ 48

Author(s):

M. A. Lamjiri ◽

M. D. Dettinger ◽

F. M. Ralph ◽

Bin Guan

Keyword(s):

West Coast ◽

Extreme Precipitation ◽

Atmospheric Rivers ◽

Storm Characteristics ◽

The U.S

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Impacts of Atmospheric Rivers on Precipitation in Southern South America

Journal of Hydrometeorology ◽

10.1175/jhm-d-18-0006.1 ◽

2018 ◽

Vol 19 (10) ◽

pp. 1671-1687 ◽

Cited By ~ 37

Author(s):

Maximiliano Viale ◽

Raúl Valenzuela ◽

René D. Garreaud ◽

F. Martin Ralph

Keyword(s):

South America ◽

West Coast ◽

Detection Algorithm ◽

Water Vapor Transport ◽

The South ◽

Southern South America ◽

Atmospheric Rivers ◽

The Andes ◽

Hourly Data ◽

The Impact

Abstract This study quantifies the impact of atmospheric rivers (ARs) on precipitation in southern South America. An AR detection algorithm was developed based on integrated water vapor transport (IVT) from 6-hourly CFSR reanalysis data over a 16-yr period (2001–16). AR landfalls were linked to precipitation using a comprehensive observing network that spanned large variations in terrain along and across the Andes from 27° to 55°S, including some sites with hourly data. Along the Pacific (west) coast, AR landfalls are most frequent between 38° and 50°S, averaging 35–40 days yr−1. This decreases rapidly to the south and north of this maximum, as well as to the east of the Andes. Landfalling ARs are more frequent in winter/spring (summer/fall) to the north (south) of ~43°S. ARs contribute 45%–60% of the annual precipitation in subtropical Chile (37°–32°S) and 40%–55% along the midlatitude west coast (37°–47°S). These values significantly exceed those in western North America, likely due to the Andes being taller. In subtropical and midlatitude regions, roughly half of all events with top-quartile precipitation rates occur under AR conditions. Median daily and hourly precipitation in ARs is 2–3 times that of other storms. The results of this study extend knowledge of the key roles of ARs on precipitation, weather, and climate in the South American region. They enable comparisons with other areas globally, provide context for specific events, and support local nowcasting and forecasting.

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