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

Summary

  • 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. https://landslides.usgs.gov/hazards/postfire_debrisflow/

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 (https://rain.cosbpw.net).

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. https://link.springer.com/article/10.1007/s11069-017-28676

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. http://journals.ametsoc.org/doi/full/10.1175/1520-0493(2004)132%3C0242%3AMOFAPB%3E2.0.CO%3B2

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. http://cw3e.ucsd.edu/wp-content/uploads/2017/04/YoungSkellyCordeira2017GRL.pdf

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 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).

CW3E AR Update: 20 November 2017 Outlook

CW3E AR Update: 20 November 2017 Outlook and Summary

November 20, 2017

Click here for a pdf of this information.

A Potentially Extreme and Persistent AR is forecast to Impact the Pacific Northwest

  • A potentially extreme AR is forecast to impact the Pacific Northwest over the next 4 days
  • AR conditions could last over 50 hours in portions of Oregon and Northern California
  • As much as 15 inches of precipitation is forecast to fall over the Olympic Mountains over the next 5 days
  • Several rivers in Washington are currently forecast to rise above flood stage

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

Valid 1200 UTC 20 November – 0000 UTC 28 November 2017

 

 

 

 

 

 

Summary provided by C. Hecht, B. Kawzenuk, J. Kalansky, and F.M. Ralph; 1 PM PT Friday 20 November 2017

*Outlook products are considered experimental

CW3E AR Update: 17 November 2017 Outlook and Summary

CW3E AR Update: 17 November 2017 Outlook and Summary

November 17, 2017

Click here for a pdf of this information.

Strong AR recently made landfall over northern California

  • The AR reached its strongest magnitude of ~750 kg m-1 s-1 at ~6 PM PST over northern California making this a strong AR
  • IWV values within the AR ranged from 34–40 mm during the event
  • A second AR is forecast to make landfall over the USWC between 19 and 21 November 2017
  • The AR is currently over Southern California bringing precipitation to the Los Angeles area
  • Another AR is forecast to impact the USWC in the next several days

SSMI/SSMIS/AMSR2-derived Integrated Water Vapor (IWV)

Valid 0000 UTC 15 November – 1600 UTC 17 November 2017

Images from CIMSS/Univ. of Wisconsin

Click IVT or IWV image to see loop of GFS Analysis

Valid 0000 UTC 15 November – 1200 UTC 17 November 2017


 

 

A potentially extreme AR is forecasted to make landfall over the U.S. West Coast next week

  • The current AR impacting Southern California is forecast to end by later tonight
  • Multiple systems are forecast to bring potentially strong to extreme and prolonged AR conditions to the USWC
  • As much as 19 inches of precipitation could fall over the Olympic Mountains over the next week
  • Multiple rivers in Washington are currently forecast to rise above flood stage

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

Valid 1200 UTC 17 November – 0000 UTC 25 November 2017

 

 

 

 

 

 

Summary provided by C. Hecht, B. Kawzenuk, J. Kalansky, and F.M. Ralph; 3 PM PT Friday 17 November 2017

*Outlook products are considered experimental

CW3E AR Update: 8 November 2017 Outlook

CW3E AR Update: 8 November 2017 Outlook

November 8, 2017

Click here for a pdf of this information.

Two ARs Forecasted to Make Landfall over the U.S. West Coast in the Next Week

  • A strong AR is currently making landfall over the U.S. West Coast
  • This AR is expected to produce up to 6 inches of precipitation over northern CA
  • The southerly orientation of this AR will result in the heaviest precipitation over the north Central Valley
  • A second, moderate strength, AR is forecasted to make landfall over northern CA, OR, and WA on 12 November 2017
  • The second AR is expected to make landfall further north resulting in the highest precipitation over the Olympic and Cascade

Click IVT or IWV image to see loop of 0-168 hour GFS forecast

Valid 1200 UTC 8 November – 1200 UTC 15 November 2017


 

 

 

 

 

Summary provided by B. Kawzenuk, J. Kalansky, and F.M. Ralph; 12 PM PT Wednesday 8 November 2017

*Outlook products are considered experimental

CW3E AR Update: 3 November 2017 Outlook

CW3E AR Update: 3 November 2017 Outlook

November 3, 2017

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 CA over the next four 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 and some snow over the Sierra Nevada
  • A potentially strong AR is expected to make landfall over CA, OR, and WA on 8 November 2017
  • The highest amounts of precipitation are expected over the coastal ranges of CA and OR
  • The AR is currently expected to have a southerly orientation which will result in less extreme precipitation

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

Valid 1200 UTC 3 November – 0000 UTC 11 November 2017


 

 

 

 

 

 

 

Summary provided by B. Kawzenuk, J. Kalansky, C. Hecht, and F.M. Ralph; 3 PM PT Friday 3 November 2017

*Outlook products are considered experimental

CW3E AR Update: 31 October 2017 Outlook

CW3E AR Update: 31 October 2017 Outlook

October 31, 2017

Click here for a pdf of this information.

Atmospheric River to potentially make landfall over California

  • A weak to moderate AR is predicted to make landfall over California during 3-8 November 2017
  • Current forecasts indicate the geometry of the AR conditions may not meet standard criteria to be considered and AR by AR conditions (IVT >250 kg m-1 s-1 and IWV >20 mm) are expected to impact the majority of CA
  • Precipitation amounts up to 5 inches are expected over the Sierra Nevada with the majority of CA receiving at least 0.25 inches of precipitation
  • Forecast certainty is currently low on timing and strength of AR conditions but confidence of at least weak AR conditions over central and southern CA is high

Click IVT or IWV image to see loop of 72-180 hour GFS forecast

Valid 1200 UTC 3 November – 0000 UTC 8 November 2017


 

 

 

 

 

Summary provided by B. Kawzenuk, J. Kalansky, C. Hecht, and F.M. Ralph; 3 PM PT Tuesday 31 October 2017

*Outlook products are considered experimental