CW3E Publication Notice: An Inter-comparison Between Reanalysis and Dropsonde Observations of the Total Water Vapor Transport in Individual Atmospheric Rivers

CW3E Publication Notice

An Inter-comparison Between Reanalysis and Dropsonde Observations of the Total Water Vapor Transport in Individual Atmospheric Rivers

February 2, 2018

CW3E collaborators Bin Guan (UCLA), Duane Waliser (NASA/JPL), along with CW3E director Marty Ralph, recently published a paper in the Journal of Hydrometeorology, titled An Inter-comparison Between Reanalysis and Dropsonde Observations of the Total Water Vapor Transport in Individual Atmospheric Rivers ( The paper is included in the journal’s special collection on MERRA-2, Modern-Era Retrospective analysis for Research and Applications version 2.

Using airborne observations from various field campaigns over the northeastern Pacific along with two atmospheric reanalysis products (ERA-Interim and MERRA-2), the study validated key characteristics of atmospheric rivers (ARs) depicted by reanalyses against observations, as well as evaluating how well the 21 observed ARs represent the total of about 6000 ARs that occurred during the winters of 1979-2016 over the northeastern Pacific.

Results showed that the reanalysis products accurately depict the strength of the observed ARs in terms of the total water vapor flowing along an individual AR across its entire width, with a mean error of only +3% or -1% depending on the reanalysis product being evaluated. Additionally the 21 observed ARs well represent the mean strength of the total of about 6000 ARs identified in reanalysis products, with a mean difference of 5% or 14% depending on the reanalysis product being compared. Similar comparisons were also done for AR width, and for ARs in other regions and seasons. The study highlights the values of both dedicated observations of specific cases and spatiotemporally more complete global reanalysis products in understanding the characteristics and impacts of ARs.

Figure Caption: (left) Histogram of AR widths based on all ARs detected in ERA-Interim over the northeastern Pacific (AR centroids within 163.4–124.6°W, 23–46.4°N) during 15 January to 25 March of 1979–2016 (gray bars). Also shown are the mean AR width (km) based on all reanalysis ARs that contributed to the histogram (red solid), the subset of the reanalysis ARs that correspond to the 21 dropsonde transects (red dashed), and the observed value based on the 21 dropsonde transects as reported in Ralph et al. (2017b) (blue dashed for the mean, and blue circles for individual transects). The mean AR width value is also indicated in the figure legend for each sample. Red shading indicates the 95% confidence interval of the mean reanalysis AR width for a random 21-member sample drawn from the pool of reanalysis ARs based on 10,000 iterations. The error bar centered on the blue dashed line indicates the 95% confidence interval of the difference between the blue and red dashed lines based on a two-tailed, paired t-test. (right) As in the left but for total integrated water vapor transport (108 kg s−1) across AR widths.

CW3E Represented at the National Council for Science and the Environment National Conference and Global Forum

CW3E Represented at the National Council for Science and the Environment National Conference and Global Forum

January 29, 2018

On January 23-24, 2018, the National Council for Science and the Environment (NCSE) held its 18th National Conference and Global Forum: The Science, Business, and Education of Sustainable Infrastructure: Building Resilience in a Changing World, at the Hyatt Regency Crystal City near Washington, DC. Shown above, Rob Hartman representing CW3E, describes Atmospheric Rivers, AR research, and FIRO for Lake Mendocino during a panel discussion entitled “Innovations and Success Stories in Sustainable Water Management at the Federal, State, and Local levels. The panel was co-chaired by Dr. Robert Wilkinson (UCSB, seated far left) and David Berry. Panelists included (right to left) Carl Morrison, Dale Roberts (SCWA), Shirley Zane (Sonoma County Commissioner), Robin Webb (NOAA/OAR), and Rob Hartman (representing CW3E, standing). NCSE attendees come primarily from research, education and policy sectors, with representatives from federal, state, and local agencies along with university researchers. Many in attendance had not been exposed to the concept and importance of ARs in the West or the research effort to demonstrate the value of FIRO in the Russian River Basin.

CW3E Launches Interactive AR Rain Versus Snow Forecast Maps and Watershed Plots

CW3E Launches Interactive AR Rain Versus Snow Forecast Maps and Watershed Plots

January 26, 2018

CW3E has launched a new forecast tool designed to visualize the impacts of the freezing level (and thus the rain versus snow partitioning in mountain watersheds) during atmospheric river storms over the U.S. West Coast. The interactive map shows whether the NOAA NCEP global forecasting ensemble predicts rain, snow, or uncertainty (some models predicting rain, some snow) downscaled to a 1 km resolution, and out to 7 days forecast lead times. For major watersheds in California, Oregon, Washington, Idaho, and Nevada time series of the forecasted freezing levels, precipitation amounts, and fractions falling as rain versus snow are also availalbe. The tool is designed to be of used for operational stakeholders for regions that are sensitive to the impacts of precipitation phase on hydrology, infrastructure, and public safety. This tool was developed by Jason Cordeira (Plymouth State University) and Brian Henn (CW3E) and is available on the CW3E Interactive Maps webpage.

Tracking Atmospheric River Activity over the Weekend

Tracking Atmospheric River Activity over the Weekend

January 24, 2018


CW3E enlisted the help of Dr. Philippe Papin, a Postdoc at Naval Research Laboratory, to create this annotated loop of annotated CW3E integrated vapor transport (IVT) graphics highlighting the current and forecast atmospheric river activity along the US West Coast over the next few days. NOAA Weather Prediction Center 1 to 7 day precipitation accumulations could reach more than 15 inches over the higher elevations of the Pacific Northwest.

Meteorological Conditions Associated with the Deadly 9 January 2018 Debris Flow on the Thomas Fire Burn Area Impacting Montecito, CA: A Preliminary Analysis

Meteorological Conditions Associated with the Deadly 9 January 2018 Debris Flow on the Thomas Fire Burn Area Impacting Montecito, CA: A Preliminary Analysis

January 16, 2018

Nina Oakley1, 2, 3 and Marty Ralph3

1 Western Regional Climate Center, Desert Research Institute, Reno, NV

2 California-Nevada Applications Program, a NOAA RISA Team

3 Center for Western Weather and Water Extremes at Scripps Institution of Oceanography


  • A period of very intense rainfall associated with a Narrow Cold Frontal Rainband (NCFR) appears to be the primary meteorological trigger for the deadly and destructive post-fire debris flow in and below the Thomas Fire burn area.
  • When a watershed experiences sufficient burn severity during a wildfire, water repellent soils can develop. Rainfall runoff is dramatically increased in these areas as compared to unburned areas. When intense rainfall occurs over the burned watershed, progressive bulking of sediment and debris (ash, rock, burned vegetation) occurs due to the increased runoff, and this debris is mobilized downstream.
  • No antecedent rainfall is necessary for post-fire debris flows. In contrast, landslides (like the deadly La Conchita event that struck nearby in 2005) require sufficient prior rainfall to saturate the soil.
  • The broader, ¬two-day long, storm, within which the NCFR occurred, included a weak-to-moderate atmospheric river and a closed low-pressure system. However, it appears that a narrow, localized band of heavy precipitation along the cold front that passed after the AR, played a primary role in triggering the debris flow.

Figure 1: Prior to the storm it had already been well established that debris flows were a serious potential hazard. This map shows the USGS’ debris flow hazard assessment: Thomas Fire.

Figure 1 shows the likelihood of debris flow occurrence with a design storm for the Thomas Fire. The area above Montecito was evaluated as having a high likelihood of debris flow with the design storm (peak 15 min intensity of 24 mm/h rate, or about 0.25 inches in 15 minutes).

Figure 2: Looking towards burn area in Montecito. Roads have become debris flow paths and houses destroyed. Photo: Ventura County Air Unit.

As of 15 January 2018, reports indicated 20 deaths, and more remained unaccounted for. Many homes, businesses, and vehicles were damaged or destroyed. Highway 101 is not anticipated to open until at least January 22, an additional week after the initial estimate of January 15. Debris from the debris flow traveled all the way from the burn scar in the mountains to the ocean.

Figure 3: Narrow Cold Frontal Rainband that produced high intensity rainfall.

A Narrow Cold Frontal Rainband (NCFR, Figure 3) is a narrow band of intense convection and heavy rainfall along a cold front. An NCFR formed offshore and made landfall at Pt. Conception just after 1 am PST on the 9th. As the NCFR interacted with the land surface and the terrain, it temporarily weakened, broke up, and strengthened again within the Southern California Bight. The NCFR brought high intensity rainfall to the westernmost part of the Thomas Fire burn area around 4 am PST.

Figure 4: Closed low and a double-banded atmospheric river at the time of event (12 UTC/4 am LST 9 Jan 2018).

This event featured a north-south oriented atmospheric river with two moisture bands interacting with a closed low-pressure system (Figure 4). The main AR had moved southeast by the time of the debris flow event. While the NCFR drove the high rain rates that produced the debris flow, the AR helped transport moisture into the area.

Figure 5: Total storm rainfall and precipitation intensity for stations near and within the burn area. Rainfall rates shown are for a 15-minute interval, and are distinct from the 15-minute maximum. Rainfall total image: CNRFC. Rainfall intensity data: SBCPWD (

Across the Santa Ynez and Topatopa Mountains, approximately 2-5+ inches of rain fell over a 2-day period. This is a moderate storm for the region in terms of precipitation totals. However, the NCFR produced periods of intense rain. The 15-minute rain rates observed at several locations correspond to a 25-50 year event according to NOAA Atlas 14. Carpinteria FS (not shown) reported a 15 min total of 0.86 in, which corresponds to a 100-year event.

Figure 6: Debris flow following Coyote Fire in 1964. Image shows corner of San Ysidro and East Valley Rd in Montecito. Photo credit: “John Bartholomay’s father” (via Facebook).

Post-fire debris flows are relatively common in the area; in 1964, Montecito experienced a damaging debris flow after the 1964 Coyote Fire (Figure 6). Post-fire debris flows and their driving atmospheric features have been catalogued in Oakley et al. (2017). A 10-year study by Young et al. (2017) identified that 50 of 57 cool-season (Oct-Mar) debris flows in California, inclusive of those occurring in both an unburned and post-fire setting, occurred on the day or day after a landfalling AR. Fifteen of 25 of these debris flows occurred over southern California on the day or day after a landfalling AR; data derived from their Figure 2. These studies were based on research supported by California’s Department of Water Resources and the Center for Western Weather and Water Extremes in collaboration with the USGS and California Geological Survey in an effort to better understand the role of atmospheric rivers and cut-off lows in creating debris flows and landslides. This storm event has similar characteristics to other storms that have impacted Santa Barbara County. One well documented event was on 3 February 1998 (Neiman et al. 2004), which also documented a landfalling atmospheric river followed by a convective line (NCFR) that produced roughly 0.5 inches of rain in 10 minutes.

Further Reading:

Oakley, N. S., Lancaster, J. T., Kaplan, M. L., & Ralph, F. M. (2017). Synoptic conditions associated with cool season post-fire debris flows in the Transverse Ranges of southern California. Natural Hazards, 88(1), 327-354.

Neiman, P. J., Martin Ralph, F., Persson, P. O. G., White, A. B., Jorgensen, D. P., & Kingsmill, D. E. (2004). Modification of fronts and precipitation by coastal blocking during an intense landfalling winter storm in southern California: Observations during CALJET. Monthly weather review, 132(1), 242-273.

Young, A.M., K.T. Skelly, and J. Cordeira, 2017: High-impact hydrologic events and atmospheric rivers in California: An investigation using the NCEI Storm Events Database. Geophysical Research Letters, 44, doi:10.1002/2017GL073077.

CW3E Fieldwork Season Begins

CW3E Fieldwork Season Begins

January 10, 2018

A team of CW3E postdocs, students, staff, and collaborators headed to Northern California on Sunday, 7 January to begin the winter 2018 fieldwork campaign. Throughout this winter season, CW3E plans to release radiosondes, conduct stream surveys, and collect isotope samples. The campaign aims to continue efforts in understanding atmospheric rivers (ARs) and their impacts on the Russian River Watershed. In support of the Forecast Informed Reservoir Operation (FIRO), hydrometeorological data from the campaign will be used to enhance water resources and flood control operations.

The team is launching from two sites: a coastal site, the UC Davis Bodega Bay Marine Laboratory and an inland site in Ukiah, CA, southwest of Lake Mendocino. These launches are being shared with National Weather Service Weather Forecast Offices in Eureka, Sacramento, and Monterey. Peak launches recorded 511 units integrated water vapor transport (IVT) at Bodega Bay (0000Z 9 January 2018) and 389 units IVT at Ukiah (2100Z 8 January 2018).

A radiosonde launch completed in Bodega Bay (0259Z 9 January 2018) shows a sounding with typical AR conditions.

Note: no orographic enhancement present (NOAA Earth System Research Laboratory)

The Regional and Mesoscale Meteorology Branch (RAMMB) of NOAA/NESDIS and Cooperative Institute for Research in the Atmosphere (CIRCA)

Leah Campbell and Anna Wilson, Postdocs, prepare to release radiosondes from Bodega Bay

Photograph taken at the mouth of the Russian River after the storm.

Other members of the team have been working on stream installations and measurements, along with isotope sampling. Working with the Sonoma County Water Agency (SCWA), CW3E has begun inventorying supplies to continue using the stream gauges that were installed during the previous fieldwork season. They have completed discharge measurements at five of the six streams where gauges are deployed, and will complete measurements at the remaining site today.

The team will continue collecting data and releasing radiosondes throughout this event with plans to return to sample ARs as they occur in the coming months. CW3E will also be partnering with NOAA and the U.S. Air Force, as part of the field campaign, for a series of Reconnaissance (Recon) flights into AR events. The AR Recon missions will start on 25 January, and continue through 28 February. In addition to the NOAA G-IV aircraft, flying out of Seattle for three storms, the campaign will also include two Air Force C-130s that will fly through a total of six storms, overlapping with the NOAA G-IV for three storms. These flights are a valuable method in improving the forecasting of AR conditions offshore and can provide enhanced prediction of AR landfall duration and intensity.

CW3E AR Update: 8 January 2018 Outlook

CW3E AR Update: 8 January 2018 Outlook

January 8, 2018

Click here for a pdf of this information.

AR conditions currently bringing precipitation to the U.S. West Coast

  • The majority of the U.S. West Coast is currently experiencing AR conditions (IVT >250 kg m-1 s-1 and IWV >20 mm) and precipitation associated with these conditions
  • These conditions could lead to precipitation over the majority of CA and southwest OR for the next 36 hours with accumulations up to 7 inches over CA
  • An AR is expected to make landfall over the Pacific Northwest on 10 January 2018 and could produce up to 6 inches of precipitation over the Cascade Mountains

Click IVT or IWV image to see loop of 0-120 hour GFS forecast

Valid 1200 UTC 8 January – 1200 UTC 13 January 2018

NEXRAD Radar Imagery

0000 UTC – 1800 UTC 8 January 2018

CNRFC Observed Precipitation

Raw Data: 0635 UTC – 1835 UTC 8 January 2018

  • Precipitation began over CA around 0400 UTC 8 January
  • As of 1835 UTC 8 January, up to 1.65 inches of precipitation has been observed over coastal CA







Summary provided by B. Kawzenuk, J. Kalansky, and F.M. Ralph; 11 AM PT Monday 8 January 2018

*Outlook products are considered experimental

CW3E AR Outlook: 14 December 2017 Ridge Update

CW3E AR Outlook: 14 December 2017 Ridge Update

December 14, 2017

Click here for a pdf of this information.

Dry Conditions Expected to Persist over CA for the Foreseeable Future

  • Persistent high pressure and ridging over the northeast Pacific and USWC is directing moisture transport towards AK and resulting in long periods of dry conditions over the USWC
  • The lack of precipitation over the southern USWC is increasing drought conditions and has resulted in the Northern Sierra 8-station index dropping below normal accumulations to date
  • While ridging is forecast to persist, AR conditions are currently forecast to impact the West Coast but the unfavorable north/northwesterly orientation of IVT will result in little or no precipitation over CA
  • Click IVT or IWV image to see loop of 0-180 hour GFS forecast

    Valid 1200 UTC 14 December – 0000 UTC 22 December 2017

    Click 500-hPa Geopotential Height & Vorticity image to see loop of 0-180 hour GFS forecast







    Summary provided by C. Hecht, J. Cordeira B. Kawzenuk, J. Kalansky, and F.M. Ralph; 1 PM PT Thursday 14 December 2017

    *Outlook products are considered experimental

CW3E Publication Notice: Flood runoff in relation to water vapor transport by atmospheric rivers over the western United States

CW3E Publication Notice

Flood runoff in relation to water vapor transport by atmospheric rivers over the western United States

December 1, 2017

CW3E long-time collaborator, Mike Dettinger, and USGS colleague, recently published a paper in Geophysical Research Letter titled: Flood runoff in relation to water vapor transport by atmospheric rivers over the western United States.

In the study they analyzed historical flood flows at over 5000 streamgages across the western US in relation to landfalling atmospheric-river storms. Specifically, they focused on the probabilities of floods flows occurring as conditioned by the presence of an atmospheric river and by the water vapor-transport rates in the atmospheric river. Through this analysis they were able to show that stronger the atmospheric river, the more likely are flood flows to develop.

Along the west coast, these peak flows coincide with atmospheric rivers about 80+% of the time, falling off to about 40-50% of the time in southern California, and falling off the farther inland the river basin (with notable regional anomalies, e.g., around Phoenix and in northern Idaho).