cw3e-web - Center for Western Weather and Water Extremes

CW3E Welcomes Emily Slinskey

October 12, 2023

Dr. Emily Slinskey joined CW3E as a Precipitation Scientist in October 2023. Emily earned her B.S. (2013) in Geography at SUNY Geneseo, M.S. (2018) in Geography at Portland State University, and Ph.D. (2021) in the Earth, Environment, and Society program at Portland State University.

As a graduate student at the Portland State Climate Science Lab, under the direction of Dr. Paul Loikith and in collaboration with the Regional Climate Model Evaluation System (RCMES) group at NASA’s Jet Propulsion Laboratory (JPL), Emily applied an extreme precipitation categorization scheme as a target for a dataset intercomparison over the contiguous United States (CONUS). This work further motivated her doctoral research identifying and characterizing ARs and associated precipitation over the seven USNational Climate Assessment (NCA) regions involving an observational analysis, climate model evaluation, and assessment of change under warming. During this time, Emily also contributed to the Forecast Informed Reservoir Operations (FIRO) effort ledby CW3E and the US Army Corps of Engineers. Her research involved the development and application of an approach to quantify the contribution of non-orographic lift during AR-driven precipitation along the US West Coast.

More recently, Emily completed a postdoc at the UCLA Center for Climate Science under the supervision of Dr. Alex Hall. Supported by the HyperFACETS effort, she developed and applied a statistical framework for quantifying temporal clustering amongARs across the Western US. Following a thorough investigation of the climatological characteristics of clustered ARs, she linked these events to the occurrence and distribution of precipitation using a dynamically downscaled reanalysis product driven by a regional climate model.

At CW3E, Emily will contribute to AR research and operations partnerships to gain an improved process-based understanding of extreme hydrometeorological events and associated impacts to support their predictability over the Western US and beyond.

CW3E AR Update: 11 October 2023 Outlook

October 11, 2023

Click here for a pdf of this information.

Several ARs to Move Across the Pacific Northwest During the Next Week

Several Atmospheric Rivers (ARs) will impinge on the Washington, Oregon, and Northern California coasts beginning Friday (Oct 13) and continuing into early next week
Weak AR1-2 conditions (based on the Ralph et al. 2019 AR Scale) are likely Friday afternoon through Sunday (Oct 15) along the WA/OR coastal areas with the 1st and 2nd ARs
The jet stream will remain very active across the North Pacific Ocean early next week, providing optimal conditions for continued AR activity in the Monday (Oct 16) to Wednesday (Oct 18) timeframe
However, there is considerable model uncertainty surrounding the details of the 3rd AR
In total, AR1 or stronger conditions are likely (90%) from Point Reyes, CA and northward around Monday. There is a moderate chance (70%) of up to AR3 conditions along the WA and OR coasts during this time and a low (20%) chance for AR5 conditions along the OR coast
Through the next seven days, 2-5” of rain are expected along the coastal mountains and Cascades with localized higher amounts possible
Snow levels will remain elevated and mostly above 8,000’ through the event
While notable rain is forecast and area rivers will rise, currently no rivers are forecast to exceed flood stage
With much more uncertainty, there is potential for a 4th AR later next week

Click images to see loops of GFS IVT and IWV forecasts Valid 0000 UTC 11 October – 1200 UTC 17 October 2023

Summary provided by P. Iniguez, C. Castellano, M. Steen, and S. Bartlett; 11 October 2023

To sign up for email alerts when CW3E post new AR updates click here.

*Outlook products are considered experimental

CW3E AR Update: 6 October 2023 Outlook

October 6, 2023

Click here for a pdf of this information.

Pair of Atmospheric Rivers Forecast to Impact Pacific Northwest and Northern California

A pair of atmospheric rivers (AR) are forecast to make landfall in the Pacific Northwest, the first early on Mon 9 Oct and the second on Tues 10 Oct
AR1 conditions (based on Ralph et al. scale) are forecast during the first AR, with a ~24 hour period of IVT >500 kg m-1 s-1 forecast for Washington to Northern California
AR2 conditions are forecast during the second AR for a more southerly latitude range along the coast of Central Oregon into Northern California, with a ~42 hour period of IVT >250 kg m-1 s-1 forecast in this region
The 00Z GFS and 00Z ECMWF are forecasting 1 to 3 inches of precipitation over much of western Washington and Oregon over the next 10 days. Some of the precipitation is forecast to fall after the ARs
The NWS Weather Prediction Center (WPC) is forecasting 7-day precipitation totals >3 inches over the Olympic Peninsula and 1-2 inches along the Oregon and Washington coasts and the windward (west) side of the Cascade Range
Precipitation associated with these ARs are forecast to be primarily beneficial to the Pacific Northwest where widespread drought conditions are present, with no river levels forecast to rise above action stage within the boundaries of the NWS Northwest River Forecast center

Click images to see loops of GFS IVT and IWV forecasts Valid 1800 UTC 08 October – 0600 UTC 11 October 2023

Summary provided by M. Steen, P. Iniguez and S. Bartlett; 6 October 2023

To sign up for email alerts when CW3E post new AR updates click here.

*Outlook products are considered experimental

CW3E Welcomes Kevin Lupo

October 6, 2023

Kevin Lupo joined CW3E in October 2023 as an Atmospheric Data Scientist. Prior to joining CW3E, Kevin earned his B.S. in Meteorology and M.S. in Applied Meteorology at Plymouth State University (2014, 2016), and his Ph.D. in Atmospheric Science at SUNY Albany (2021). At SUNY Albany, Kevin’s research focused on the use of stochastic model error schemes in ensemble forecasts of heavy rainfall events, with a particular focus on the physical processes by which these schemes introduce precipitation variability in ensemble forecasts using the Weather Research and Forecasting (WRF) model. This research was supported by an NSF Partnership for International Research and Education (PIRE) grant, which allowed Kevin to collaborate with students and faculty with diverse research interests from universities both in the U.S. and in Taiwan. More recently, Kevin was a postdoc at NCAR in the Mesoscale and Microscale Meteorology (MMM) Lab, where his work focused on objectively evaluating a progressive, or “too fast” bias in GFS forecasts of cutoff lows, with the goal of identifying corresponding model deficiencies and potential improvements to the Unified Forecast System – Medium Range Weather Application (UFS-MRW) which may improve this bias.

At CW3E, Kevin will contribute to operational and research aspects of West-WRF. Specifically, his work will support funded projects with SDG&E and contribute to the development and evaluation of the West-WRF Ensemble.

CW3E Participates in the 2nd Annual Yampa Youth Water Festival

September 27, 2023

El Knappe (left) and Jeri Wilcox (right) explain radiosonde launch heights to 5th graders during the grandstands opening ceremony of the event

On the 27th of September, the Upper Yampa Water Conservancy District of Steamboat, Colorado, hosted the 2nd Annual Yampa Youth Water Festival at the Routt County Fairgrounds in Hayden, Colorado. Researchers from CW3E participated in this event as guest speakers during the grandstands opening event as well as by hosting a booth during the rotational part of the event. This festival brought together nearly 400 5th grade students from the surrounding Routt and Moffat counties. The goal of this festival is to introduce students to the importance of our water resources, particularly in northwest Colorado, through interactive and hands-on activities with the kids about the Yampa River Basin.

At this event, the Land-Based Field Operations Manager, El Knappe, and Field Researcher Jeri Wilcox joined the fun by doing a demonstration of launching a weather balloon at the opening event in front of the grandstands packed with cheering 5th graders. During this presentation, the researchers walked the students through what radiosondes are and the data that the balloon will collect. Following this, CW3E hosted a booth during the rotational segment of the event. These students got the unique opportunity to spend the rest of day rotating through 22 stations, learning about topics including meteorology, ecology, hydrology, and water quality, among other topics. These booths were hosted by a diverse group of organizations, ranging from the Colorado Water Conservation Board to the United States Forest Service. At the CW3E booth hosted by Knappe and Wilcox, students learned about the importance of taking observations and data collection for the scientific process, particularly when making weather forecasts that can have major implications for daily life. Students got to play the role of being a meteorologist by collecting their own data on weather conditions and synthesizing their results into a weather report for effective science communication. This activity was followed up by sending a lesson plan to the teachers that participated in the event which utilized their results from the booth activity as well as the data collected from the weather balloon launch done with the students.

The event was covered by local news outlets including the Steamboat Pilot & Today and Steamboat Radio

CW3E hosts a table for rotating groups of 5th graders where students collect weather observations and create a weather report for their peers based on real-time data

Jeri Wilcox (left) describes the balloon launch as Holly Kirkpatrick of the Upper Yampa Water Conservancy District (middle) and El Knappe (right) prepare to release the balloon

CW3E Publication Notice: Large-Scale Circulation Context for North American West Coast Atmospheric Rivers: Influence of the Subseasonal NPO/WP Teleconnection

CW3E Publication Notice

Large-Scale Circulation Context for North American West Coast Atmospheric Rivers: Influence of the Subseasonal NPO/WP Teleconnection

September 27, 2023

A new paper titled “Large-Scale Circulation Context for North American West Coast Atmospheric Rivers: Influence of the Subseasonal NPO/WP Teleconnection” by Justin Hicks (University of Maryland), Bin Guan (UCLA, NASA JPL), Sumant Nigam (University of Maryland), and Alfredo Ruiz-Barradas (University of Maryland) was recently published in the American Geophysical Union’s Journal of Geophysical Research. This study identifies a subseasonal weather pattern that is particularly influential on AR activity in the western US. The North Pacific Oscillation/West Pacific teleconnection pattern (NPO/WP) is more influential than other subseasonal patterns previously linked to AR activity, both in terms of landfall occurrence and inland reach. The results advance prospects of subseasonal prediction of winter AR activity over the western US, from the incipient-phase knowledge of the impactful subseasonal teleconnections operating over the Pacific-North American region, especially the NPO/WP. This prospect of improved long-term forecasts of high-impact precipitation events (i.e. ARs) is of particular interest to those in the water resources community in the western US.

ABSTRACT

Understanding the variability of atmospheric rivers (ARs) on subseasonal time scales is pivotal for efficient water resource management along the west coast of North America. ARs during 1980–2018 based on the Modern-Era Retrospective analysis for Research and Applications, version 2 are analyzed to quantify the modulation of winter (December–February) landfalling ARs in the western US by leading subseasonal teleconnections, focusing on pentad evolution rather than seasonal-mean patterns. The growth phase of the North Pacific Oscillation/West Pacific (NPO/WP) teleconnection—the second leading pattern in 200-hPa geopotential heights in boreal winter—is found to be particularly influential in modulating the number of landfalling ARs in this region. In the positive phase of NPO/WP growth, the presence of anomalous low pressure centered just south of Alaska (i.e., a strengthening of the Aleutian Low) and anomalous high pressure around Hawaii results in moisture convergence in the central and eastern Pacific, bringing southwesterly moisture fluxes to the coast and inland. The modulation by NPO/WP is stronger than by commonly-considered climate variability modes, such as the Pacific/North American (PNA) pattern. Although southwesterly fluxes are stronger over the Pacific Ocean during the positive phase of PNA, they tend to transition to southerly fluxes before extending inland, resulting in smaller overland impacts in the western US. The analysis of temporal evolutions indicates AR activity peaks 5 days after the mature phase of NPO/WP growth, as in the case of PNA. Overall, the study suggests potential subseasonal predictability of US West Coast ARs from incipient-phase knowledge of the leading teleconnection patterns, especially the NPO/WP.

KEY POINTS

The modulation of atmospheric rivers (ARs) in the western US by climate variability modes is analyzed, focusing on pentad evolution

The North Pacific Oscillation/West Pacific (NPO/WP) pattern is more influential on western US AR activity than commonly-considered modes

The finding advances prospects of subseasonal prediction of ARs over the western US from the NPO/WP’s incipient-phase knowledge

Figure 1: Figure 3 from Hicks et al. (2023): Left Panels: Characteristic pentad atmospheric river (AR) counts and 850-hPa height anomalies associated with the leading subseasonal teleconnection patterns and El Niño variability from Niño3.4 sea surface temperature (SST) index during the extended winter season (November–March), obtained from linear regressions during 1980–2018 and 1982–2018, respectively. From top to bottom: the North Atlantic Oscillation (NAO), the growth phase of North Pacific Oscillation/West Pacific (NPO/WP) pattern, the decay phase of NPO/WP, the Pacific North American (PNA) pattern, the Scandinavian (SCAND) pattern, and El Niño. Positive/negative AR regressions are shaded red/blue at 0.2 AR/pentad interval; 850-hPa height anomalies are contoured at 10 m interval with continuous/dashed lines for positive/negative anomalies. Right Panels: AR regressions are superposed on the AR climatology in the Pacific–North American region, where climatological AR counts are contoured at 0.4 AR/pentad; the 2.0 ARs/pentad anomaly contour is thickened for emphasis. A 9-point spatial smoother is applied twice to the AR regressions and climatology.

Figure 1: Figure 4 from Hicks et al. (2023): Inter-reanalysis comparison of climatological atmospheric river (AR) counts and AR-count regressions on winter teleconnections during 1980–2018: Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2) (right column) and ERA-Interim (left column). From the top: regressions on the North Atlantic Oscillation (NAO), the growth phase of North Pacific Oscillation/West Pacific (NPO/WP) pattern, the decay phase of NPO/WP, the Pacific North American (PNA) pattern, and the Scandinavian (SCAND) pattern. AR regressions are shaded red/blue for positive/negative values at 0.2 AR/pentad/index interval (see color bar). The climatological AR count per pentad is contoured in black at 0.4 intervals, with the 2.0 ARs/pentad isoline thickened for emphasis. A 9-point spatial smoother is applied twice to AR regressions and climatology.

Hicks, J., Guan, B., Nigam, S., & Ruiz-Barradas, A. (2023). Large-scale circulation context for North American west coast atmospheric rivers: Influence of the subseasonal NPO/WP teleconnection. Journal of Geophysical Research: Atmospheres, 128, e2023JD038693. https://doi.org/10.1029/2023JD038693

CW3E Publication Notice: Advances in Precipitation Retrieval and Applications from Low Earth Orbiting Satellite Information

CW3E Publication Notice

Advances in Precipitation Retrieval and Applications from Low Earth Orbiting Satellite Information

September 27, 2023

CW3E postdoctoral researcher Vesta Afzali Gorooh, in collaboration with the UCI Center for Hydrometeorology and Remote Sensing, NOAA-NESDIS, NASA-JPL, and UMD System Science Interdisciplinary Center recently published a paper titled “Advances in Precipitation Retrieval and Applications from Low Earth Orbiting Satellite Information” in the Bulletin of the American Meteorological Society. This article summarizes a virtual NOAA workshop on “Precipitation Estimation from Low Earth Orbit Satellites: Retrieval and Applications,” organized with support from the Office of Low Earth Orbit Observations at NOAA-NESDIS.

The workshop brought together experts in satellite precipitation retrieval and operational data users for the two-day event (1-2 March 2023). It covered the state of the science and users’ needs for operational precipitation algorithms and products from current and future meteorological satellites (Fig.1). The work contributes to the goals of CW3E’s 2019-2024 Strategic Plan by sharing insights from the hydrologist, atmospheric scientists, and stakeholders, who provided essential insights for enhancing satellite-based precipitation sensing and addressing the associated requirements and considerations, resulting in the following overarching recommendations:

Observation system and data delivery requirements: NOAA should actively collaborate with national and international partners to enhance precipitation-sensing microwave sensors for frequent updates (hourly to sub-hourly). It is crucial to prioritize timely access (within one hour) to satellite data for nowcasting and short-term forecasting by expanding data acquisition capacities. Maintaining a joint satellite precipitation radar and passive microwave (PMW) radiometer reference systems is crucial for ensuring accurate intercalibration of constellation radiometers used in Level-3 global precipitation datasets.
Value/impact studies: NOAA should consistently invest in performing impact studies on precipitation products when new observation capabilities are introduced, involving domestic, international, and private sector partners, and these studies should also precede any decisions regarding decommissioning existing observation capabilities. NOAA should maintain its active involvement in International Precipitation Working Group activities, serving as a crucial platform for scientific collaboration in precipitation retrievals, facilitating the improvement of existing techniques, the development of innovative methodologies, and the resolution of pertinent challenges.
Measurement requirements: NOAA should use satellite/sensor impact study results to define and specify desired channel requirements, including resolution and polarization for PMW-based rainfall and snowfall retrieval, including window channels near 6, 10, 19, 37, and 89 GHz, together with temperature and water vapor sounding bands near 23, 50, 118, 166, and 183 GHz and higher.
Algorithm development and applications: The meeting showcased the enhancements of satellite precipitation estimation through diverse probabilistic and machine learning methods alongside global radar and ground observations for uncertainty assessment. Collaboration with the user community is essential for establishing a robust climate data record using precipitation radar reference, particularly given the long duration of operational PMW satellite missions dating back to 1987 with the Special Sensor Microwave Imager (SSMI) series. Moreover, incorporating spaceborne radars to calibrate passive sensors and integrating ground-based observations are vital for identifying errors and enhancing precipitation retrievals. Incorporating precipitation parameterizations and prior ground observation data can reduce uncertainties in spaceborne radar measurements, leading to more accurate and dependable multi-satellite precipitation retrievals. It is also recommended to develop location-specific error and uncertainty models for satellite products to optimize their use in hydrological modeling, water resource management, and climate studies, considering regional variations, timeframes, and specific applications.

Figure 1: Precipitation-related missions in Japan (Source: Misako Kachi and Takuji Kubota Presentations)

Afzali Gorooh, V., and Coauthors, 2023: Advances in Precipitation Retrieval and Applications from Low Earth Orbiting Satellite Information. Bull. Amer. Meteor. Soc., https://doi.org/10.1175/BAMS-D-23-0229.1, in press

CW3E AR Update: 22 September 2023 Outlook

September 22, 2023

Click here for a pdf of this information.

Atmospheric River Forecast to Impact Pacific Northwest and Northern California

An atmospheric river (AR) is forecast to make landfall in the Pacific Northwest early Sun 24 Sep with the greatest IVT making landfall along the Oregon-California Border late Sun 24 Sep. IVT values above 250 kg m-1 s-1 are forecast to persist in the Pacific Northwest through Wed 27 Sep
AR 4 conditions (based on the Ralph et al. 2019 AR Scale) are forecast along the Oregon coast while AR3 conditions are forecast along the Washington and northern California coasts in both the GFS and ECMWF
The 00Z GFS is forecasting 5.28 inches of precipitation over the next 10 days in the Chetco Watershed, located in SW Oregon along the Oregon-California border, while the 00Z ECMWF is forecasting 2.97 inches over the same period. Some of the precipitation is forecast to fall after this AR
The NWS Weather Prediction Center (WPC) is forecasting 5-day precipitation totals >3 inches over the Olympic Peninsula and the Oregon-California border with >1.5 inches along the Oregon and Washington coasts. WPC excessive rainfall outlooks have a marginal risk for rainfall exceeding flash flooding guidance along the Oregon-California Border for 12Z Mon 25 Sep -12Z Tues 26 Sep
Despite higher amounts of precipitation along much of the Pacific Northwest Coast, both the CNRFC and NWRFC do not forecast any river stage locations to pass above action stage. This is largely tied to much of the Pacific Northwest currently experiencing Severe Drought conditions or worse

Click images to see loops of GFS IVT and IWV forecasts Valid 0000 UTC 24 September – 1800 UTC 27 September 2023

Summary provided by M. Steen, S. Roj, P. Iniguez and S. Bartlett; 22 September 2023

To sign up for email alerts when CW3E post new AR updates click here.

*Outlook products are considered experimental

CW3E Publication Notice: West-WRF 34-year reforecast: Description and Validation

CW3E Publication Notice

West-WRF 34-year reforecast: Description and Validation

September 19, 2023

A new paper titled “West-WRF 34-year reforecast: Description and Validation” by Alison Cobb, Daniel Steinhoff (CW3E), Rachel Weihs (CW3E), Luca Delle Monache (Deputy Director, CW3E), Laurel DeHaan (CW3E), David Reynolds (U Colorado), Forest Cannon (Tomorrow.IO), Brian Kawzenuk (CW3E), Caroline Papadopolous (CW3E), and Marty Ralph (Director, CW3E) was recently accepted to the Journal of Hydrometeorology. This paper describes the development and evaluation of a novel high-resolution long-term regional forecast product, based on CW3E’s near-real time forecast model, West-WRF. The West-WRF 34-year reforecast was generated by dynamically downscaling the control member of the GEFSv10 reforecast over the Western U.S. and northeastern Pacific Ocean during the cool season months (1 December through 31 March) between 1986-2019. The forecasts are available at a 9-km resolution over much of the Eastern Pacific and Western North America out to 7 days and at a 3-km resolution primarily over California. This valuable dataset is being used to further study extreme precipitation and atmospheric river activity through physical process studies, training for post-processing and machine learning techniques, and climatological analysis. It supports CW3E’s 2019-2024 Strategic Plan by providing an evaluation of West-WRF with the goal to identify benefits and added value of the forecasting system as well as and areas to further target model development.

This paper shows the added value of the dynamical downscaling of GEFS into the West-WRF reforecast on scales important for both basin/reservoir water management and for climatological understanding of extreme events across the West. Verification of near-surface temperature, wind, and humidity highlight the added value in the reforecast compared to GEFSv10. The West-WRF reforecast also shows clear improvement in atmospheric river characteristics (intensity and landfall) over GEFS. Figure 1 shows the intensity error, landfall position error, and distance from perfect MoE averaged across all categorized ARs for both the West-WRF 9-km reforecast and GEFS, as well as the differences between the two. For all three metrics, the reforecast has smaller errors across all forecast lead times than GEFS. The improvements are statistically significant at the 95% confidence level for three lead times for landfall position error, four lead times for AR intensity error, and all seven lead times for MoE (when the error bars for the difference are below 0). The largest improvement in intensity error occurs at the 48-hour forecast, improving the intensity forecast by almost 4 kg m^-1 s^-1, which is an average error reduction of over 5%, while the largest improvement in landfall position error occurs at the 144-hour forecast, with an improvement of over 30 km. The reduction in MoE error with the reforecast corresponds to an average improvement in location of the whole AR of 1% of the area of the AR. This 1% can range from an area of 20,000 km² for a small AR to over 500,000 km² for a large AR that is correctly forecasted as an AR. These results demonstrate a consistent, clear improvement in AR intensity, landfall, and location characteristics using the West-WRF reforecast compared to GEFS.

Analysis of mean areal precipitation (MAP) shows that at the basin-scale, the reforecast can improve MAP compared to GEFSv10 and reveals a consistent low bias in the reforecast for a coastal watershed (Russian) and a high bias observed in a Northern Sierra watershed (Yuba). The reforecast has a dry bias in seasonal precipitation in the northern Central Valley and Coastal Mountain ranges, and a wet bias in the Northern Sierra Nevada, which is consistent with other operational high resolution (< 25 km) regional models.

Overall, the paper serves to introduce this reforecast dataset to the scientific community as a resource for quality, high resolution atmospheric forecasts of extreme precipitation events largely from ARs. It can be used to further quantify ARs, their characteristics, uncertainties and impacts, and the representation of those characteristics and impacts in the West-WRF reforecast beyond that presented in the paper. The positive verification results of the reforecast show that it can be leveraged for scientific studies of ARs and extreme precipitation, machine learning, and further model evaluation.

Figure 1: Figure 7 from Cobb et al. (2023) a) AR intensity error using a threshold of 500 kg m^-1 s^-1, where intensity is defined as the 90th percentile value within any AR object. Upper panel shows full values, and the lower panel shows the difference (Reforecast minus GEFS). b) AR landfall position error using a threshold of 500 kg m^-1 s^-1, where landfall position is defined as the latitude of maximum IVT at the coastline. c) distance from perfect MoE for a threshold of 500 kg m^-1 s^-1. In all cases, the error bars are the 95 % confidence interval computed with bootstrapping.

Cobb, A., Steinhoff, D., Weihs, R., Delle Monache, L., DeHaan, L., Reynolds, D., Cannon, F., Kawzenuk, B., Papadopolous, C., & Ralph, F.M. (2023). West-WRF 34-Year Reforecast: Description and Validation. Journal of Hydrometeorology (published online ahead of print 2023). https://doi.org/10.1175/JHM-D-22-0235.1

Heavy Rainfall in Arizona

CW3E Event Summary: Heavy Rainfall in Arizona

September 19, 2023

Click here for a pdf of this information.

A Fall “Transition” Monsoon Event Brings Heavy Rain to Parts of Arizona

On September 12-13, 2023, a deep southwesterly flow of moist air was directed into Arizona, ahead of an advancing upper level trough.
This setup was a classic “Transition” monsoon event, signified by the transition from the hot, humid active summer into quieter fall months. Synoptic conditions for transition events are distinctly different from typical monsoon events when high pressure is overhead. While precipitation trends down in September, the combination of summer moisture and fall dynamics can result in significant rainfall events. The pattern resulted in persistent convection near and just east of the Phoenix area.
The Southwest Monsoon runs from June 15 through September 30.
Heavy precipitation fell over the Salt River watershed in central Arizona, with some locations receiving more than 20% of the normal total water year precipitation in a 24-hour period.
In excess of 6” of rain fell near Roosevelt, AZ. The Return Interval (Annual Probability) of 5.5”/6 hr is over 1000 years (<0.1%). For a nearby COOP station (Roosevelt 1 S, 1905+), the all-time 1-day record is 4.14" (1978-03-02). For comparison the average annual precipitation total is 15.91”.
The heavy rain resulted in flooding and rock slides, causing at least one highway to close.
IVT tools (based on the GEFS) indicated the potential for this notable moisture flux days in advance.