Observations and Predictability of a High-Impact Narrow Cold-Frontal Rainband over Southern California on 2 February 2019

Abstract

Short-duration, high-intensity rainfall in Southern California, often associated with narrow cold-frontal rainbands (NCFR), threaten life and property. While the mechanisms that drive NCFRs are relatively well understood, their regional characteristics, specific contribution to precipitation hazards, and their predictability in the Western U.S. have received little research attention relative to their impact. This manuscript presents observations of NCFR physical processes made during the Atmospheric River Reconnaissance field campaign on 2 February 2019 and investigates the predictability of the observed NCFR across spatiotemporal scales and forecast lead time. Dropsonde data collected along transects of an atmospheric river (AR) and its attendant cyclone during rapid cyclogenesis, and radiosonde observations during landfall 24 hours later, are used to demonstrate that a configuration of the Weather Research and Forecasting model (WRF) skillfully reproduces the physical processes responsible for the development and maintenance of the impactful NCFR. Ensemble simulations provide quantitative uncertainty information on the representation of these features in numerical weather prediction and instill confidence in the utility of WRF as a forecast guidance tool for short-to-medium range prediction of mesoscale precipitation processes in landfalling ARs. This research incorporates novel data and methodologies to improve forecast guidance for NCFRs impacting Southern California. While this study focuses on a single event, the outlined approach to observing and predicting high-impact weather across a range of spatial and temporal scales will support regional water management and hazard mitigation, in general.

Representation of Dropsonde‐Observed Atmospheric River Conditions in Reanalyses

“…Despite having similar IVT, there is larger CRMSE in the cold sector than warm sector. These sectors represent different atmospheric conditions, the former likely behind the cold front in a cold, dry, dynamically complex environment (Cannon et al., 2020) and the latter ahead of the cold front, in warm moist air with comparatively little forcing and weaker gradients.…”

Forecasting the Frequency and Magnitude of Postfire Debris Flows Across Southern California

“…We estimate this footprint using the length scale (100 km) and travel distance (100 km) of a narrow cold‐frontal rain band, a several kilometer‐wide band of intense rainfall associated with a cold front that has triggered many debris flows in southern California, including the 2018 Montecito debris flows (Oakley et al., 2018). Although narrow cold‐frontal rain bands can propagate across the entire Bight of southern California (Cannon et al., 2020), we use a shorter 100 km length scale to represent the travel distance because they often do not hold together that far. Using 100 km by 100 km subregions, we estimate P ( DF region ) = 0.7, suggesting that, at a minimum, small postfire debris flows can be expected almost every year in southern California.…”

A soil moisture monitoring network to assess controls on runoff generation during atmospheric river events

“…Correlations are weakest in winter, as soils are relatively wet throughout the basin, but have different VWC FC and saturation levels. Consequently, the soil at one site may reach saturation more quickly than soil at another site during a storm event, especially since AR rainfall can vary across the basin (Cannon et al, 2020). Despite the varying correlation magnitudes, similar seasonal patterns are observed when the cross‐correlations are performed at the other five CW3E stations (not shown), so the choice of site is trivial in this seasonal correlation analysis.…”