Apr 22, 2017

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

by
Nina S. Oakley, Jeremy T. Lancaster, Michael L. Kaplan, F. Martin Ralph
,
Natural Hazards
  • 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