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 AR Update: 8-10 January 2018 Post Event Summary

CW3E AR Update: 8-10 January 2018 Post Event Summary

January 8, 2018

Click here for a pdf of this information.

Atmospheric river conditions brought widespread precipitation throughout California

  • A low pressure system developed off the CA coast on 7 January and interacted with tropical moisture to produce heavy precipitation over nearly all of CA
  • Nearly all of CA experienced AR conditions (IVT >250 kg m-1 s-1 and IWV >20 mm) for ~24 hours
  • The highest precipitation amounts were observed over the Coastal and Transverse Ranges, with some locations receiving over 200 mm of precipitation, making this and R-Cat 1 event

Click IVT or IWV image to see loop of GFS analysis

Valid 0000 UTC 5 January – 0000 UTC 10 January 2018

SSMI/SSMIS/ARMSR2-Derived Integrated Water Vapor (IWV)

Valid 0000 UTC 7 January – 1200 UTC 10 January 2018

NEXRAD Radar Reflectivity

Valid 0000 UTC 8 January – 1200 UTC 10 January 2018

Precipitation began over California around 400 UTC 8 January and lasted until early 10 January









Summary provided by B. Kawzenuk and F.M. Ralph; 12 PM PT Thursday 11 January 2018

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.

Odds of Reaching 100% Water Year Precipitation – Jan Update

Odds of Reaching 100% of Normal Precipitation for Water Year 2018 (January Update)

January 8, 2018

Contribution from Dr. M.D. Dettinger, USGS

The odds shown here are the odds of precipitation in the rest of the water year (after December 2017) totaling a large enough amount to bring the water-year total to equal or exceed the percentage of normal listed. “All Yrs” odds based on monthly divisional precipitation totals from water year 1896-2015. Numbers in parenthesis are the corresponding odds if precipitation through December had been precisely normal (1981-2010 baseline).

Click here for a pdf file of this information.




How these probabilities were estimated:

At the end of a given month, if we know how much precipitation has fallen to date (in the water year), the amount of precipitation that will be required to close out the water year (on Sept 30) with a water-year total equal to the long-term normal is just that normal amount minus the amount received to date. Thus the odds of reaching normal by the end of the water year are just the odds of precipitation during the remaining of the year equaling or exceeding that remaining amount.

To arrive at the probabilities shown, the precipitation totals for the remaining months of the water year were tabulated in the long-term historical record and the number of years in which that precipitation total equaled or exceeded the amount still needed to reach normal were counted. The fraction of years that at least reached that threshold is the probability estimate. This simple calculation was performed for a full range of possible starting months (from November thru September) and for a wide range of initial (year-to-date) precipitation conditions. The calculation was also made for the probabilities of reaching 75% of normal by end of water year, 125%, and 150%, to ensure that the resulting tables of probabilities cover almost the full range of situations that will come up in the future.

[One key simplifying assumption goes into estimating the probabilities this way: The assumption that the amount of precipitation that will fall in the remainder of a water year does not depend on the amount that has already fallen in that water year to date. This assumption was tested for each month of the year by correlating historical year-to-date amounts with the remainder-of-the-year amounts, and the resulting correlations were never statistically significantly different from zero, except possibly when the beginning month is March, for which there is a small positive correlation between Oct-Mar and Apr-Sept precipitation historically.]

Contact: Michael Dettinger (USGS)

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 Update: 3 January 2018 Outlook

CW3E AR Update: 3 January 2018 Outlook

January 3, 2018

Click here for a pdf of this information.

Two systems expected to produce precipitation over the U.S. West Coast in the next week

  • AR conditions (IVT >250 kg m-1 s-1 and IWV >20 mm) are expected over most of the U.S. West Coast over the next two days
  • While AR conditions are forecast for some locations of the USWC, this event is not necessarily an AR due to geometric and spatial structure, but could produce up to 5 inches of precipitation over the Sierra Nevada
  • A second period of AR conditions is expected to make landfall over CA, OR, and WA on 9 January 2018
  • Both periods of AR conditions are currently expected to have southerly oriented IVT which will result in less extreme precipitation

Click IVT or IWV image to see loop of 0-180 hour GFS forecast

Valid 0600 UTC 3 January – 1800 UTC 10 January 2018








Summary provided by B. Kawzenuk, J. Kalansky, and F.M. Ralph; 3 PM PT Wednesday 3 January 2018

*Outlook products are considered experimental

CW3E Welcomes Dr. Ali Hamidi

CW3E Welcomes Dr. Ali Hamidi

December 20, 2017

Dr. Ali Hamidi has joined CW3E at the Scripps Institution of Oceanography as a Postdoctoral Scholar in December 2017. Ali earned his Ph.D. in Civil Engineering at the City University of New York under the supervision of Dr. Reza Khanbilvardi. His dissertation focused on the spatial-temporal variation of extreme rainfall and its effects on urban infrastructure systems. For this work he analyzed high-resolution radar rainfall data along with the other remote sensing and observational climate data. Ali has experience in many areas including: hydrological modeling, machine learning, GIS, and remote sensing. His Ph.D. research provided seasonal classification of extreme rainfall patterns and employed them in improving the uncertainty estimation in urban hydrological modeling of New York City. To help support agricultural planning, he also developed a seasonal forecasting model for the extreme rainfall in the northeast United States that relied on the teleconnection patterns.

As a post-doc at CW3E, Ali plans to improve hydrologic model performance associated with extreme events based on data assimilation of in-situ soil moisture observations and remotely sensed observations, as well as examining atmospheric forcing for hydrologic model applications. The research is part of the Forecast Informed Reservoir Operations (FIRO) project at CW3E and will support the development of tools and information for operational decision making.

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