Weather-radar inferred intensity and duration of rainfall that triggered the January 9, 2018, Montecito, California, disaster

Abstract

Slopes above Montecito burned by the Thomas Fire in December 2017 produced disastrous debris flows in response to a short period of intense precipitation on January 9, 2018, killing 23 people, injuring many others, destroying and damaging residential buildings, and community infrastructure. The intense precipitation was in a narrow cold frontal rainband which obviously exceeded the intensity and duration threshold for post-wildfire debris flows. Rain gauges with self-activating radio transmitters reported by County of Santa Barbara Department of Public Works documented the precipitation in the Montecito area as it occurred, which allowed short-duration intensities to be calculated. Data from the rain gauge on Montecito Creek was used in this paper and showed that the rainband that produced the precipitation that generated the debris flows passed over Montecito in about one-half hour. Two weather radar stations operated by the National Weather Service are located within about 100 km of Montecito. Both stations were operational and recorded radar reflectivity on a frequency of about five minutes during the entire storm; data from the KVTX station located east of Montecito was used for this paper. Montecito is located on a coastal plain south of the Santa Ynez Mountains, which shield the lower elevations in the Montecito area from direct view of the radar stations. Composite radar reflectivity represents the amount of water droplets in the atmosphere in line-of-sight above the ground. The weather radar shows patterns similar to the precipitation documented by the rain gauges. Radar reflectivity at the coordinates of the rain gauge on Montecito Creek and at the coordinates of a point in the Santa Ynez Mountains on the west side of the Santa Ynez Creek watershed was extracted and converted to an approximate rainfall depth using a general National Weather Service relationship. The results are used to demonstrate the value of weather radar reflectivity for visualization and for developing approximate rainfall intensity and duration estimates at positions of interest remote from rain gauges for comparison with post-wildfire debris-flow thresholds.. The analysis in this paper was developed as part of the Geotechnical Extreme Event Reconnaissance (GEER) Association response to the Montecito disaster.