Science Source

Nina S. Oakley, Jeremy T. Lancaster, Michael L. Kaplan, F. Martin Ralph

Natural Hazards

Published date April 22, 2017

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

States that the Transverse Ranges of southern California often experience fire followed by flood and that this sequence sometimes causes post-fire debris flows (PFDFs) that threaten life and property
Identifies common atmospheric conditions during which damaging PFDFs occur in the Transverse Ranges during the cool season, defined here as November–March
Compares a compilation of 93 PFDF events during 1980–2014 triggered by 19 precipitation events against previous studies of the events, reanalysis, precipitation, and radar data to estimate PFDF trigger times
Analyzes each event 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
Finds that all events had radar returns of at a minimum 50 dBZ (approximately >48 mm per hour) indicative of heavy precipitation
Finds that 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