Synoptic conditions associated with cool season post-fire debris flows in the Transverse Ranges of southern California

Abstract

The Transverse Ranges of southern California often experience fire followed by flood. This sequence sometimes causes post-fire debris flows (PFDFs) that threaten life and property situated on alluvial fans. The combination of steep topography, highly erodible rock and soil, and wildfire, coupled with intense rainfall, can initiate PFDFs even in cases of relatively small storm rainfall totals. This study identifies common atmospheric conditions during which damaging PFDFs occur in the Transverse Ranges during the cool season, defined here as November-March. A compilation of 93 PFDF events during 1980-2014 triggered by 19 precipitation events is compared against previous studies of the events, reanalysis, precipitation, and radar data to estimate PFDF trigger times. Each event was analyzed to determine common atmospheric features and their range of values present at and preceding the trigger time. Results show atmospheric rivers are a dominant feature, observed in 13 of the 19 events. Other common features include low-level winds orthogonal to the Transverse Ranges and other conditions favorable for orographic forcing, a strong upper level jet south of the region, and moist-neutral static stability. Several events included closed low-pressure systems or narrow cold frontal rain bands. These findings can help forecasters identify more precisely the synoptic-scale atmospheric conditions required to produce PFDF-triggering rainfall and thus reduce uncertainty when issuing warnings.

Simulating and Evaluating Atmospheric River‐Induced Precipitation Extremes Along the U.S. Pacific Coast: Case Studies From 1980–2017

“…During this event, simulated CAPE locally exceeds 200 J/kg, which is low in absolute terms but relatively high by local standards. Indeed, Oakley et al () find that the median CAPE during historical California ARs that produced extreme precipitation leading to debris flows was only on the order of 20–40 J/kg in cyclonic warm sector preceding cold frontal passage. Moreover, Monteverdi et al () find that low CAPE environments (200–400 J/kg) were typical even during tornadic thunderstorm events in this region—whereas much higher values (2,000 J/kg or more) are more typical of severe convective events elsewhere in North America.…”

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“…Other recent Santa Ana Wind driven fire events, such as those that occurred in October 2003 and 2007, directly caused more deaths and destruction. However, the flooding and post-fire debris flow triggered by a landfalling atmospheric river [40] over the Thomas Fire burn scar that occurred in Montecito and Santa Barbara claimed at least another 21 lives and destroyed more than 100 homes [41].…”

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“…Over the past several years, interest in atmospheric river (AR) science and applications has increased rapidly. Beyond the now well‐known impacts of heavy rain and flooding (e.g., Lamjiri et al, ; Neiman et al, ; Ralph et al, ), ARs have been shown to have applications in areas as diverse as avalanche hazard (Hatchett et al, ), dust transport (Ault et al, ), and postfire debris flows (Oakley et al, ). Furthermore, the study of ARs has become global in scope, and international in terms of participation, as evidenced by the well‐attended 2018 International Atmospheric Rivers Conference (Ramos et al, ).…”