Southern California’s Thomas Fire and the Deadly Montecito Debris Flow: NWS Successes and Challenges in Providing Impact-Based Decision Support Services for Preparedness, Response, and Recovery

The Thomas Fire erupted on December 4, 2017 and burned 281,893 acres within Ventura and Santa Barbara Counties in southern California – becoming at the time what was California’s largest wildfire on record. The fire destroyed just over 1000 structures, but with only 2 injured and 2 fatalities. The National Weather Service (NWS) office in Oxnard provided aggressive and timely Impact-Based Decision Support Services (IDSS) for NWS partners leading up to and during the strong Santa Ana wind event that fueled to the fire. Partner response included nearly unprecedented pre-positioning and augmented staffing actions – employing staffing levels typically reserved for after a wildfire starts. NWS support continued during the fire by providing an Incident Meteorologist as well as conducting elevated IDSS prior to a strong "sundowner" wind event that threatened to spread the fire into populated areas of the Santa Barbara coast. The exceptional pre-positioning of fire agency resources helped saved hundreds of homes. The threat of post-fire debris flows from the Thomas burn scar and other burn scars in the area began almost immediately given their occurrence during southern California's rainy season. Numerous communities lie just beneath the Thomas burn scar, including the community of Montecito located along coastal Santa Barbara County. As part of the process to alert officials of this threat, the NWS office in Oxnard has partnered with the U.S. Geological Survey (USGS) to employ empirical rainfall rate thresholds determined as conducive to producing post-fire debris flows. Anticipating the impacts from a storm system forecast to move through the area in early morning hours of January 9, 2018, the Oxnard NWS office began providing IDSS as much as six days in advance. This included conducting special calls and providing daily email briefings and webinars for critical partners preparing for the storm. Partner actions included activating the Santa Barbara County Incident Command Post (ICP) and issuing evacuation orders just over of a day prior to when the storm was expected to arrive. The storm ultimately produced exceptionally high intensity rainfall over the Thomas burn scar just above Montecito. This torrential rain tragically produced one of the more catastrophic post-fire debris-flow events on California record, resulting in 21 fatalities and 2 residents still missing and presumed deceased, a total of 128 single family residences destroyed and 307 damaged, and 6 commercial properties destroyed and 17 damaged. U.S. Highway 101 - a major route for commuters between Ventura and Santa Barbara - was closed for 12 days due to extensive flooding and debris. Despite an event that truly changed Montecito’s physical landscape – in some areas, permanently – pre-positioned and specialized resources helped prevent even more loss of life as first responders were able to conduct 1000 rescues in the first 24 hours. Officials estimate the number of fatalities would have been closer to 100 had it not been for these resources in place for immediate response. Local NWS IDSS continued after the debris flow to support further rescue, recovery, and debris removal – most notably in providing weather briefings on over 100 joint agency conference calls. The threat of post-fire debris flows from the Thomas burn scar continued well into March and again through the winter of 2018-19. NWS Oxnard support continued with participation in Santa Barbara County Storm Risk Decision Team meetings and providing onsite support at their ICP for multiple storms posing additional debris-flow threats. Most significant of the contributions is providing weather intelligence for the ICP’s Unified Coordination Group decisions to repopulate evacuated areas that help achieve their core objectives of evacuation only when necessary, and only when safe. This presentation will describe IDSS provided to partners by NWS Oxnard and highlight resulting preparedness actions intended to mitigate property damage and loss of life from wildfires and post-fire debris flows. Also covered will be the challenges and lessons learned in communicating post-fire debris-flow threats to the public in a way that helps ensure this rare but deadly natural hazard is understood, and protective action is taken in a timely manner. Results of a post-event public survey intended to better understand the public's response to the Montecito debris-flow threat and instructions from local officials are presented to reinforce these challenges and help shape future communication strategies. Finally, preliminary results will be presented on ongoing work with the USGS to categorize debris flow magnitudes in order to identify events that are a true threat to life and property. This work is needed to help reduce warning and evacuation false alarms.