Publication Notice: Predictability of Horizontal Water Vapor Transport Relative to Precipitation

CW3E Publication Notice

Predictability of Horizontal Water Vapor Transport Relative to Precipitation: Enhancing Situational Awareness for Forecasting Western U.S. Extreme Precipitation and Flooding

March, 2016

Lavers, D.A., D.E. Waliser, F.M. Ralph and M.D. Dettinger, 2016: Predictability of horizontal water vapor transport relative to precipitation: Enhancing situational awareness for forecasting western U.S. extreme precipitation and flooding. Geophysical Research Letters, 43, doi:10.1002/2016GL067765 (Please click here for personal use pdf file)

The following paper has just appeared in Geophysical Research Letters. It was motivated by the critical role of horizontal vapor transport in determining the strength and distribution of extreme precipitation in the Western U.S., and by the fact that this transport is the defining characteristic of atmospheric rivers, which are key to many extreme events in the region. The work was carried out primarily at CW3E in response to interest from State and local water agencies to explore new methods to predict extreme precipitation events. While the findings are based on U.S. West Coast domains, the results are applicable to other west coasts of mid latitude continents where cool season orographic precipitation is a key process. The results support the use of water vapor transport as a variable to monitor for earlier awareness of extreme hydrometeorological events.

(e) The average interannual predictability (r2) across the 30°N–50°N, 125°W–120°W region. (f) The predictability throughout the forecast horizon calculated using all winter forecasts (n = 2796) at 38°N, 122°W. From Lavers et al. (2016).

Contacts: David Lavers ( and F. Martin Ralph (


The western United States is vulnerable to socioeconomic disruption due to extreme winter precipitation and floods. Traditionally, forecasts of precipitation and river discharge provide the basis for preparations. Herein we show that earlier event awareness may be possible through use of horizontal water vapor transport (integrated vapor transport (IVT)) forecasts. Applying the potential predictability concept to the National Centers for Environmental Prediction global ensemble reforecasts, across 31 winters, IVT is found to be more predictable than precipitation. IVT ensemble forecasts with the smallest spreads (least forecast uncertainty) are associated with initiation states with anomalously high geopotential heights south of Alaska, a setup conducive for anticyclonic conditions and weak IVT into the western United States. IVT ensemble forecasts with the greatest spreads (most forecast uncertainty) have initiation states with anomalously low geopotential heights south of Alaska and correspond to atmospheric rivers. The greater IVT predictability could provide warnings of impending storminess with additional lead times for hydrometeorological applications.

CW3E R-Cat Alerts

CW3E R-Cat Alerts: Automated Notification of Heavy Precipitation Events

March 10, 2016

The Center for Western Weather and Water Extremes (CW3E) has set up an automated e-mail notification service that informs users of heavy precipitation events in near real-time. Along the West Coast, such events are often the result of land-falling atmospheric rivers, which transport substantial moisture into the area. Tracking, analyzing, improving the forecasting, and anticipating the impacts of such extreme events are a core element of CW3E’s mission.

The “Rainfall Category” or “R-Cat” 3-day precipitation classification of Ralph and Dettinger (2012) is a simple, effective measure of strong precipitation events, which can have a large impact on the Western U.S.:

R-Cat 1: 200-299 mm (roughly 8-12 inches) / 3 days

R-Cat 2: 300-399 mm (roughly 12-16 inches) / 3 days

R-Cat 3: 400-499 mm (roughly 16-20 inches) / 3 days

R-Cat 4: more than 500 mm (more than roughly 20 inches) / 3 days

An R-cat email alert includes a short summary of the 3-day total precipitation, location of the event (based on sources such as the Global Historical Climatology Network [GHCN] and the National Weather Service’s co-op precipitation stations), and a map showing the location of the event(s). Here is a recent example:

Station: BRUSH CREEK RS USC00041130
Location: (39.695, -121.345)
Date: 2016/03/07 (third day of event)
Event strength: R-Cat 1
3-day total precip (mm): 206.3
Individual days precip (mm): 76.5 102.1 27.7
Station: QUINCY USC00047195
Location: (39.937, -120.948)
Date: 2016/03/07 (third day of event)
Event strength: R-Cat 1
3-day total precip (mm): 202.7
Individual days precip (mm): 59.7 119.9 23.1

The email R-Cat alert service can be subscribed to by sending a message with the subject line “subscribe” to

For more information, please contact David Pierce,

Setting the Stage for a Global Science of Atmospheric Rivers

Setting the Stage for a Global Science of Atmospheric Rivers

January 25, 2016

The above image, from the EOS article, is a depiction of an atmospheric river, interacting with West Coast mountains and a midlatitude cyclone over the northeast Pacific on 5 February 2015. This image provides an example of approximate locations of associated tropical moisture exports and a warm conveyor belt (WCB). Credit: Adapted from NOAA/ESRL Physical Sciences Division

An EOS article from CW3E scientists Dettinger, Ralph and Lavers summarizes key outcomes from a unique workshop held at Scripps Institution of Oceanography in June 2015. The Workshop brought together leading scientists and users of scientific outputs to discuss emerging advances in Atmospheric River (AR) science and applications. A “Meeting Report” about the workshop appeared in print in the EOS issue published on 1 January 2016: Click here to acccess the report.

Points of contact: F.M. Ralph (, M.D. Dettinger (, and D. Lavers ( at the Scripps Institution of Oceanography, UCSD.

Climate change intensification of horizontal water vapor transport in CMIP5

CW3E Publication Notice

Climate change intensification of horizontal water vapor transport in CMIP5

June 25, 2015

Lavers, D.A., F.M. Ralph, D.E. Waliser, A. Gershunov, and M.D. Dettinger, 2015: Climate change intensification of horizontal water vapor transport in CMIP5. Geophys. Res. Lett., 42, doi:10.1002/2015GL064672.

Projected multimodel mean changes in the mean, standard deviation, and 95th percentile of winter water vapor transport (over 2073-2099) in the RCP4.5 and RCP8.5 scenarios. The multimodel mean percentage changes are shown in the right column. From Lavers et al. (2015).

Research over the last decade has shown that the majority of heavy precipitation and flood events on the western edges of mid-latitude land masses are connected to intense water vapor transport. This vapor transport is found within the atmospheric river region of extratropical cyclones. As climate change is expected to create a warmer atmosphere capable of supporting more water vapor, it is also thought that the global water cycle will intensify leading to more vapor flux and hydrological extremes, such as floods and droughts. Any changes to the water vapor transport by the atmosphere are likely to have hydrological ramifications of great significance to hydrometeorological applications.

The research presented in Lavers et al. (2015) investigates the historical and projected changes to water vapor transport in the latest Climate Model Intercomparison Project Phase 5 archive. Using output from 22 models, robust increases in vapor flux by the end of the 21st Century are found, which suggests the likelihood for larger precipitation and floods in the future. This research was a collaborative effort led by CW3E, with the aim of ascertaining the projected global water cycle changes that may need to be considered in the future.


Global warming of the Earth’s atmosphere is hypothesized to lead to an intensification of the global water cycle. To determine associated hydrological changes, most previous research has used precipitation. This study, however, investigates projected changes to global atmospheric water vapor transport (integrated vapor transport (IVT)), the key link between water source and sink regions. Using 22 global circulation models from the Climate Model Intercomparison Project Phase 5, we evaluate, globally, the mean, standard deviation, and the 95th percentiles of IVT from the historical simulations (1979–2005) and two emissions scenarios (2073–2099). Considering the more extreme emissions, multimodel mean IVT increases by 30–40% in the North Pacific and North Atlantic storm tracks and in the equatorial Pacific Ocean trade winds. An acceleration of the high-latitude IVT is also shown. Analysis of low-altitude moisture and winds suggests that these changes are mainly due to higher atmospheric water vapor content.

The Inland Penetration of Atmospheric Rivers

CW3E Publication Notice

The Inland Penetration of Atmospheric Rivers over Western North America: A Lagrangian Analysis

June 15, 2015

Jonathan J. Rutz, W. James Steenburgh, and F. Martin Ralph, 2015: The Inland Penetration of Atmospheric Rivers over Western North America: A Lagrangian Analysis. Mon. Wea. Rev., 143, 1924–1944. (Click here for personal-use pdf file of the article)

Schematic from Rutz et al. (2015) showing the primary pathways for the penetration of 950-hPa AR-related trajectories into the interior of western North America. Pathways associated with regimes 1–3 are represented by green, orange, and purple arrows, respectively. Regions associated with frequent AR decay are shaded in red. Topography is shaded in grayscale. Note that while this schematic highlights common regimes and pathways, individual trajectories follow many different paths.

Although atmospheric rivers (ARs) are typically regarded as coastal events, their impacts can be felt further inland as well. Recent work by Rutz et al. (2015) uses a forward trajectory analysis and AR thresholding criteria to examine the inland penetration of ARs over western North America, and identifies geographic corridors where inland-penetrating ARs are most likely. This paper builds on the earlier work led by Jon Rutz (Rutz and Steenburgh 2012 – Atmos. Sci. Lett., and Rutz et al. 2014 – Mon. Wea. Rev.) as part of his PhD dissertation with Jim Steenburgh at Univ. of Utah. Combined with recent results from Alexander et al. (2015, J. Hydrometeor) that used backward trajectories to examine inland penetration of ARs, as well as earlier studies on Arizona AR events (Neiman et al. 2013, Hughes et al. 2014; both in J. Hydrometeor.) and across the west (Ralph et al. 2014; J. Contemporary Water Resources Research and Education), it is now clear that ARs play a critical role in Western U.S. extreme precipitation, even well inland from the coastal areas where they were first studied. These results improve our understanding of water vapor transport and precipitation over the interior western U.S., and hence contribute to ongoing research interests and efforts at CW3E regarding the causes and prediction of extreme weather and water events across the Western U.S.


Although atmospheric rivers (ARs) typically weaken following landfall, those that penetrate inland can contribute to heavy precipitation and high-impact weather within the interior of western North America. In this paper, the authors examine the evolution of ARs over western North America using trajectories released at 950 and 700 hPa within cool-season ARs along the Pacific coast. These trajectories are classified as coastal decaying, inland penetrating, or interior penetrating based on whether they remain within an AR upon reaching selected transects over western North America. Interior-penetrating AR trajectories most frequently make landfall along the Oregon coast, but the greatest fraction of landfalling AR trajectories that eventually penetrate into the interior within an AR is found along the Baja Peninsula. In contrast, interior-penetrating AR trajectories rarely traverse the southern “high” Sierra. At landfall, interior-penetrating AR trajectories are associated with a more amplified flow pattern, more southwesterly (vs westerly) flow along the Pacific coast, and larger water vapor transport (qυ). The larger initial qυ of interior-penetrating AR trajectories is due primarily to larger initial water vapor q and wind speed υ for those initiated at 950 and 700 hPa, respectively.

Inland- and interior-penetrating AR trajectories maintain large qυ over the interior partially due to increases in υ that offset decreases in q, particularly in the vicinity of topographical barriers. Therefore, synoptic conditions and trajectory pathways favoring larger initial qυ at the coast, limited water vapor depletion by orographic precipitation, and increases in υ over the interior are keys to differentiating interior-penetrating from coastal-decaying ARs.

Climatology of extreme daily precipitation in Colorado

CW3E Publication Notice

Climatology of extreme daily precipitation in Colorado and its diverse spatial and seasonal variability

May 5, 2015

Seasonality of the top 10 daily precipitation events measured at Colorado COOP stations that have at least 30 years of data since 1950. Circles represent totals of 10 events. Seasons shaded as winter (DJF; blue), spring (MAM; yellow), summer (JJA; red), and fall (SON; green). Terrain elevation (m; gray shading) as in legend at left; Continental Divide shown by dashed black line.

The origins of extreme precipitation events in the Western U.S. range from landfalling atmospheric rivers, to the summer monsoon, upslope storms on the Rocky Mountain Front Range, and deep convection of the Great Plains variety. This was shown by an analysis across the west of the seasonality of the top 10 wettest days for each of thousands of COOP observer sites (Ralph et al. 2014**). Each of these sites had at least ~10,000 data points, so these top 10 days represent roughly the top 0.1% of days. Some areas were universally dominated by events in one season, or two. A couple of areas stood out in the diversity of their seasonality of extreme daily precipitation, including Colorado.

The study presented in Mahoney et al. 2015* explores this local variability more deeply, explores how the devastating flood of September 2013 in Colorado’s northern Front Range is related, and describes some of the implications of the findings for flood control and other sensitive sectors. The co-authors represent a diverse group themselves, including climate, weather, hydrology, hydrometeorology expertise from several organizations, (CIRES, NOAA/PSD, Scripps/CW3E and CSU). The paper is highlighted here as it represents an example of work on extreme events in the Western U.S. that the Center for Western Weather and Water Extremes is contributing to.

Abstract of Mahoney et al. 2015*: The climatology of Colorado’s historical extreme precipitation events shows a remarkable degree of seasonal and regional variability. Analysis of the largest historical daily-precipitation totals at COOP stations across Colorado by season indicates that the largest recorded daily precipitation totals have ranged from less than 60 mm/day in some areas to greater than 250 mm/day in others. East of the Continental Divide winter events are rarely among the top 10 events at a given site, but spring events dominate in and near the foothills; summer events are most common across the lower-elevation eastern plains, while fall events are most typical for the lower elevations west of the Divide. The seasonal signal in Colorado’s central mountains is complex; high-elevation intense precipitation events have occurred in all months of the year, including summer when precipitation is more likely to be liquid (as opposed to snow) which poses more of an instantaneous flood risk.

*Mahoney, K., F.M. Ralph, K. Wolter, N. Doesken, M. Dettinger, D. Gottas, T. Coleman, and A. White, 2015: Climatology of extreme daily precipitation in Colorado and its diverse spatial and seasonal variability. J. Hydrometeor. 16, 781-792.

** Ralph, F. M., M. Dettinger, A. White, D. Reynolds, D. Cayan, T. Schneider, R. Cifelli, K. Redmond, M. Anderson, F. Gherke, J. Jones, K. Mahoney, L. Johnson, S. Gutman, V. Chandrasekar, J. Lundquist, N.P. Molotch, L. Brekke, R. Pulwarty, J. Horel, L. Schick, A. Edman, P. Mote, J. Abatzoglou, R. Pierce and G. Wick, 2014: A vision for future observations for Western U.S. extreme precipitation and flooding– Special Issue of J. Contemporary Water Resources Research and Education, Universities Council for Water Resources, Issue 153, pp. 16-32.

California Precipitation: summary handout

California Precipitation: summary handout

February 8, 2015

Coefficient of variation (the standard deviation divided by the average) of total precipitation based on water year data from 1951-2008.

Please click here for the summary handout

CW3E and partners from the California Department of Water Resources, CNAP and the Southwest Climate Science Center have released a summary handout describing California precipitation. The seasonality and variability of precipitation for the state are examined in this summary. Special emphasis is on the link between large storms (AR storms) and the total precipitation for a season. The figure above (Dettinger et al., 2011) illustrates that how much variability there is from year to year in precipitation. The green and blue circles over California indicate the largest year-to-year variability is over this state at an order of about half the annual average precipitation.

Antarctic Atmospheric Rivers

CW3E Publication Notice

The role of atmospheric rivers in anomalous snow accumulation in East Antarctica

December 4, 2014

Gorodetskaya, I.V., M. Tsukernik, K. Claes, F.M. Ralph, W.D. Neff and N.P.M. Van Lipzig, 2014: The role of atmospheric rivers in anomalous snow accumulation in East Antarctica. Geophysical Research Letters, 41, 6199-6206.

(Please click here for a personal use copy of the article)

Integrated water vapor (colors) at 00Z on 15 February 2011. Red arrows indicated vertically integrated total moisture transport within the atmospheric river as identified using the definition adapted for Antarctica. Black contours are 500 hPa geopotential heights, where L shows a closed trough at 500 hPa influencing Dronning Maud Land and H shows the blocking high-pressure ridge downstream of the low. White square shows Princess Elisabeth station location. Based on the ERA-Interimm reanalysis.

Understanding changes in the Antarctic ice sheet mass are important for predicting global sea level rise. Recently, East Antarctica gained substantial mass, counterbalancing the increasing ice discharge from West Antarctica in these years. Occasional large snowfall events explained this increased mass load, which has been especially high in 2009 and 2011. Ground-based measurements at the Belgian Antarctic station Princess Elisabeth, established at the ascent to the East Antarctic plateau, have well captured these occasional intense snowfalls and associated snow accumulation responsible for 2009 and 2011 mass anomalies. The question is what has been causing this high accumulation?

Most of the water vapor transforming into the Antarctic snowfall arrives from lower latitudes. We have established that atmospheric rivers explain all extremely high snow accumulation events leading to the mass anomaly at Princess Elisabeth station in 2009 and 2011. These narrow bands of high moisture content have been more known in mid latitudes for their, sometimes catastrophic, impacts, such as heavy precipitation resulting in floods. The atmospheric rivers reaching the Antarctic ice sheet bring a lot of moisture from as far as subtropics and result in intense snowfall when reaching the steep ascent to the Antarctic plateau.
In addition, the work represents a significant advance in the understanding of how the global water cycle is affected by atmospheric rivers by

  • diagnosing their role in recent Antarctica extreme snowfall events,
  • developing an AR-detection methodology to track ARs into Polar Regions and
  • exploring their role in cryospheric processes of importance to global sea level in a changing climate.


Recent, heavy snow accumulation events over Dronning Maud Land (DML), East Antarctica, contributed significantly to the Antarctic ice sheet surface mass balance (SMB). Here we combine in situ accumulation measurements and radar-derived snowfall rates from Princess Elisabeth station (PE), located in the DML escarpment zone, along with the European Centre for Medium-range Weather Forecasts Interim reanalysis to investigate moisture transport patterns responsible for these events. In particular, two high-accumulation events in May 2009 and February 2011 showed an atmospheric river (AR) signature with enhanced integrated water vapor (IWV), concentrated in narrow long bands stretching from subtropical latitudes to the East Antarctic coast. Adapting IWV-based AR threshold criteria for Antarctica (by accounting for the much colder and drier environment), we find that it was four and five ARs reaching the coastal DML that contributed 74–80% of the outstanding SMB during 2009 and 2011 at PE. Therefore, accounting for ARs is crucial for understanding East Antarctic SMB.

Publication Notice: Climatological Characteristics of Atmospheric Rivers and Their Inland Penetration over the Western United States

CW3E Publication Notice

Climatological Characteristics of Atmospheric Rivers and Their Inland Penetration over the Western United States


Mean duration (h) of AR conditions based on IVT250. Histograms of IVT250 ARduration at selected coastal (left) and interior locations (right).

This paper quantifies the climatological frequency and duration of atmospheric rivers (ARs) over the western U.S., as well as the contribution of ARs to heavy precipitation events and cool-season hydroclimate over this region. ARs are objectively identified within reanalysis data based on integrated water vapor transport, which is not only shown to be well-correlated with cool-season precipitation over the West, but also useful for tracking AR penetration from the coast to the interior. Hence, this study lays the groundwork for the development of forecasting tools that will enhance the predictability of ARs and their impacts on the western U.S. This paper presents key findings from a dissertation completed by Jon Rutz at the University of Utah, and is coauthored by his advisor, Jim Steenburgh, and by the CW3E director, F. Martin Ralph. It has also become one of the 10 most-read articles in Monthly Weather Review for the year.


Narrow corridors of water vapor transport known as atmospheric rivers (ARs) contribute to extreme precipitation and flooding along the West Coast of the United States, but knowledge of their influence over the interior is limited. Here, the authors use Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) data, Climate Prediction Center (CPC) precipitation analyses, and Snowpack Telemetry (SNOTEL) observations to describe the characteristics of cool-season (November–April) ARs over the western United States. It is shown that AR frequency and duration exhibit a maximum along the Oregon–Washington coast, a strong transition zone upwind (west) of and over the Cascade–Sierra ranges, and a broad minimum that extends from the “high” Sierra south of Lake Tahoe eastward across the central Great Basin and into the deep interior. East of the Cascade–Sierra ranges, AR frequency and duration are largest over the interior northwest, while AR duration is large compared to AR frequency over the interior southwest. The fractions of cool-season precipitation and top-decile 24-h precipitation events attributable to ARs are largest over and west of the Cascade–Sierra ranges. Farther east, these fractions are largest over the northwest and southwest interior, with distinctly different large-scale patterns and AR orientations enabling AR penetration into each of these regions. In contrast, AR-related precipitation over the Great Basin east of the high Sierra is rare. These results indicate that water vapor depletion over major topographic barriers is a key contributor to AR decay, with ARs playing a more prominent role in the inland precipitation climatology where lower or less continuous topography facilitates the inland penetration of ARs.

A personal use copy of the article is available here.

El Nino Impacts and Outlook: Western Region

El Nino Impacts and Outlook: Western Region

October 29, 2014

The Western Regional Climate Center (Kelly Redmond and Nina Oakley) along with NOAA, NIDIS and other western region partners have released a summary discussing El Nino impacts and outlook (October 2014). These brief easy-to-read stories provide a convenient 2-page look at our chances for an El Nino winter and other issues of importance to the western region. Please click here or on the image below to see more.


Front page of the October 2014 El Nino Outlook and Impacts