Publication Notice: CalWater Field Studies Designed to Quantify the Roles of Atmospheric Rivers and Aerosols in Modulating U.S. West Coast Precipitation in a Changing Climate

CW3E Publication Notice

CalWater Field Studies Designed to Quantify the Roles of Atmospheric Rivers and Aerosols in Modulating U.S. West Coast Precipitation in a Changing Climate

November 28, 2016

Ralph F.M., K. A. Prather, D. Cayan, J.R. Spackman, P. DeMott, M. Dettinger, C. Fairall, R. Leung, D. Rosenfeld, S. Rutledge, D. Waliser, A. B. White, J. Cordeira, A. Martin, J. Helly, and J. Intrieri, 2016: CalWater Field Studies Designed to Quantify the Roles of Atmospheric Rivers and Aerosols in Modulating U.S. West Coast Precipitation in a Changing Climate. Bull. Amer. Meteor. Soc. 97, yyy-zzz. doi: 10.1175/BAMS-D-14-00043.1.

This paper summarizes the 8-year-long CalWater program of field studies, from planning to field operations and analysis efforts. It also summarizes the major motivations for the program as well as science gaps addressed, and serves as the standard reference for future CalWater analysis papers.

Contact: F. Martin Ralph (


Quantifying the roles of atmospheric rivers and aerosols in modulating U.S. West Coast precipitation, water supply, flood risks and drought in a changing climate.

The variability of precipitation and water supply along the U.S. West Coast creates major challenges to the region’s economy and environment, as evidenced by the recent California drought. This variability is strongly influenced by atmospheric rivers (AR), which deliver much of the precipitation along the U.S. West Coast and can cause flooding, and by aerosols (from local sources and transported from remote continents and oceans) that modulate clouds and precipitation. A better understanding of these processes is needed to reduce uncertainties in weather predictions and climate projections of droughts and floods, both now and under changing climate conditions.

To address these gaps a group of meteorologists, hydrologists, climate scientists, atmospheric chemists, and oceanographers have created an interdisciplinary research effort, with support from multiple agencies. From 2009-2011 a series of field campaigns (CalWater 1) collected atmospheric chemistry, cloud microphysics and meteorological measurements in California and associated modeling and diagnostic studies were carried out. Based on remaining gaps, a vision was developed to extend these studies offshore over the Eastern North Pacific and to enhance land-based measurements from 2014-2018 (CalWater 2). The data set and selected results from CalWater 1 are summarized here. The goals of CalWater-2, and measurements to date, are then described.

CalWater is producing new findings and exploring new technologies to evaluate and improve global climate models and their regional performance and to develop tools supporting water and hydropower management. These advances also have potential to enhance hazard mitigation by improving near-term weather prediction and subseasonal and seasonal outlooks.

Publication Notice: Forecasting Atmospheric Rivers during CalWater 2015

CW3E Publication Notice

Forecasting Atmospheric Rivers during CalWater 2015

November 22, 2016

Cordeira, J., F. Ralph, A. Martin, N. Gaggini, R. Spackman, P. Neiman, J. Rutz, and R. Pierce, 0: Forecasting Atmospheric Rivers during CalWater 2015. Bull. Amer. Meteor. Soc., 0, doi: 10.1175/BAMS-D-15-00245.1.

As part of CW3E’s mission and goals a new set of atmospheric river (AR)-focused diagnostic and prediction tools have been created, in close partnership with Plymouth State University’s Prof. Jason Cordeira, and building upon work done earlier at NOAA under the HMT Program (see Ralph et al. 2013 BAMS, Wick et al. 2013 Wea. Forecasting). These developments were accelerated and focused by the needs for specialized AR forecast displays to support the CalWater field campaigns in 2014 and 2015 (see Ralph et al. 2016, BAMS). CalWater used research aircraft to observe atmospheric rivers and carried out aerosol science. These developments are summarized in a paper on the forecasting tools that were used in the CalWater field campaign by CW3E researchers and collaborators (Cordeira et al.) that was recently published in Bulletin of the American Meteorological Society (BAMS). The paper details some of the new AR forecasting tools developed using NCEP Global Forecast System and Global Ensemble Forecast System. A novel AR landfall detection forecast tool illustrates the probability of AR conditions at different locations along the western coast of the US. Another new forecast tool that used the various ensemble members illustrates the possible range of integrated water vapor transport (IVT) at a specific location using each of the ensemble members. In addition, the high quality plots of forecasted IVT and observed integrated water vapor supported the CalWater field campaign. Beyond supporting the CalWater Field Campaign, these new forecasting tools will likely improve AR forecasting throughout the West Coast. All these and more of the new forecasting tools can be found on the CW3E website under “Atmospheric River Resources.”

84-h NCEP GFS gridded forecast of IVT magnitude (kg m-1s-1 and direction; initialized at 1200 UTC on 3 February 2015; (b) as in (a), except for the verifying analysis of IVT magnitude and direction at 0000 UTC 7 February 2015 with overlaid draft flight track of the NOAA G-IV aircraft (c) GPS-derived IWV (mm) at 0015 UTC 7 February 2015.


Atmospheric Rivers (ARs) are long and narrow corridors of enhanced vertically integrated water vapor (IWV) and IWV transport (IVT) within the warm sector of extratropical 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 2009–2011 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 2014–2015 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 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 may prove useful in forecasting ARs and better anticipating hydrometeorological extremes along the U.S. West Coast.

Click here for personal use PDF file

Points of contact: Jason Cordeira, F. Martin Ralph, Brian Kawzenuk

Publication Notice: Extreme Daily Precipitation in the Northern California Upper Sacramento River Watershed Requires a Combination of a Landfalling Atmospheric River and a Sierra Barrier Jet

CW3E Publication Notice

Extreme Daily Precipitation in the Northern California Upper Sacramento River Watershed Requires a Combination of a Landfalling Atmospheric River and a Sierra Barrier Jet

July 18, 2016

Ralph, F.M., J.M. Cordeira, P.J. Neiman and M. Hughes, 2016: Extreme Daily Precipitation in the Northern California Upper Sacramento River Watershed Requires a Combination of a Landfalling Atmospheric River and a Sierra Barrier Jet. J. Hydrometeor., 17, 1904-1915.

The top 0.3% most extreme daily precipitation events in the key Sacramento River watershed all involved both a landfalling atmospheric river and a Sierra Barrier Jet. Thus, forecasts of extreme precipitation are related to the skill of forecasts of each of these key phenomena, and can be enhanced by evaluation of, and enhancement of, skill in predicting each of these key processes. This study was led by the CW3E Director, was supported by the California Department of Water Resources, used data from NOAA’s Hydrometeorology Testbed collected over a decade, and epitomizes the focus of the “Center for Western Weather and Water Extremes,” and its partnership with NOAA Research’s Physical Sciences Division and Plymouth State University.

Contact: F. Martin Ralph (


The upper Sacramento River watershed is vital to California’s water supply and is susceptible to major floods. Orographic precipitation in this complex terrain involves both atmospheric rivers (ARs) and the Sierra barrier jet (SBJ). The south-southeasterly SBJ induces orographic precipitation along south-facing slopes in the Mt. Shasta–Trinity Alps, whereas landfalling ARs ascend up and over the statically stable SBJ and induce orographic precipitation along west-facing slopes in the northern Sierra Nevada. This paper explores the occurrence of extreme daily precipitation (EDP) in this region in association with landfalling ARs and the SBJ. The 50 wettest days (i.e., days with EDP) for water years (WYs) 2002–11 based on the average of daily precipitation from eight rain gauges known as the Northern Sierra 8-Station Index (NS8I) are compared to dates from an SSM/I satellite-based landfalling AR-detection method and dates with SBJ events identified from nearby wind profiler data. These 50 days with EDP accounted for 20% of all precipitation during the 10-WY period, or 5 days with EDP per year on average account for one-fifth of WY precipitation. In summary, 46 of 50 (92%) days with EDP are associated with landfalling ARs on either the day before or the day of precipitation, whereas 45 of 50 (90%) days with EDP are associated with SBJ conditions on the day of EDP. Forty-one of 50 (82%) days with EDP are associated with both a landfalling AR and an SBJ. The top 10 days with EDP were all associated with both a landfalling AR and an SBJ.

Mesoscale Frontal Wave AR during CalWater-2014

CW3E Publication Notice

An Airborne and Ground-Based Study of a Long-Lived and Intense Atmospheric River with Mesoscale Frontal Waves Impacting California during CalWater-2014

May 10, 2016

Neiman, P.J., B.J. Moore, A.B. White, G.A. Wick, J. Aikins, D.L. Jackson, J.R Spackman, and F.M. Ralph, 2016: An Airborne and Ground-Based Study of a Long-Lived and Intense Atmospheric River with Mesoscale Frontal Waves Impacting California during CalWater-2014. Mon. Wea. Rev., 144, 1115-1144.

This study provides the most comprehensive observations to date of a mesoscale frontal wave associated with an atmospheric river, including its structure offshore, landfall characteristics and impacts on precipitation. It utilizes research aircraft, a unique array of coastal hydrometeorological measurements and inland data. This paper reflects the broader scientific collaboration between CW3E and NOAA/PSD, and adds to the knowledge of phenomena that are critical to creating extreme precipitation on the U.S. West Coast – a major thrust of CW3E. Dr. Ralph contributed to this paper by proposing the experiment (Ralph et al. 2016 BAMS), identifying the science objective for the flights (i.e., mapping out the structure of a mesoscale frontal wave with dropsondes and airborne radar), laying out the flight tracks, guiding the mission onboard, having been the PI of the major projects that created the unique land-based observing network (NOAA HMT- Ralph et al. 2013 BAMS, and the DWR-sponsored EFREP mesonet – White et al. 2013 JTech) used in the study and contributing to the analysis and interpretation of the measurements in this paper.

Contacts: Paul Neiman ( and F. Martin Ralph (


The wettest period during the CalWater-2014 winter field campaign occurred with a long-lived, intense atmospheric river (AR) that impacted California on 7–10 February. The AR was maintained in conjunction with the development and propagation of three successive mesoscale frontal waves. Based on Lagrangian trajectory analysis, moist air of tropical origin was tapped by the AR and was subsequently transported into California. Widespread heavy precipitation (200–400 mm) fell across the coastal mountain ranges northwest of San Francisco and across the northern Sierra Nevada, although only modest flooding ensued due to anomalously dry antecedent conditions. A NOAA G-IV aircraft flew through two of the frontal waves in the AR environment offshore during a ;24-h period. Parallel dropsonde curtains documented key three dimensional thermodynamic and kinematic characteristics across the AR and the frontal waves prior to landfall. The AR characteristics varied, depending on the location of the cross section through the frontal waves. A newly implemented tail-mounted Doppler radar on the G-IV simultaneously captured coherent precipitation features. Along the coast, a 449-MHz wind profiler and collocated global positioning system (GPS) receiver documented prolonged AR conditions linked to the propagation of the three frontal waves and highlighted the orographic character of the coastal-mountain rainfall with the waves’ landfall. Avertically pointing S-PROF radar in the coastal mountains provided detailed information on the bulk microphysical characteristics of the rainfall. Farther inland, a pair of 915-MHz wind profilers and GPS receivers quantified the orographic precipitation forcing as the AR ascended the Sierra Nevada, and as the terrain-induced Sierra barrier jet ascended the northern terminus of California’s Central Valley.

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.