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

TitleSynoptic conditions associated with cool season post-fire debris flows in the Transverse Ranges of southern California
Publication TypeJournal Article
Year of Publication2017
AuthorsOakley N.S, Lancaster J.T, Kaplan M.L, Ralph FM
JournalNatural Hazards
Date Published2017/08
Type of ArticleArticle
ISBN Number0921-030X
Accession NumberWOS:000405328100015
KeywordsAlluvial fan flooding; atmospheric; Atmospheric River; caljet; Closed low; duration; empirical-models; floods; Geology; mesoscale; meteorology; Meteorology & Atmospheric Sciences; northeastern pacific-ocean; Post-fire debris flow; rainfall; rivers; Transverse Ranges; Water resources; west-coast precipitation; wildfire runoff

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.

Short TitleNat. Hazards
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