Description
Study region: Los Angeles and San Diego metropolitan area, Southern California, USA Study focus: The main objective of this research was to assess the extent of flooding in urban Southern California caused by narrow cold frontal rainbands (NCFRs), which bring brief but intense periods of rain during wintertime extratropical cyclones. Two methodological approaches were used for assessment. The first approach found the proportion of flooding in the Los Angeles (including Orange County) and San Diego metropolitan watersheds caused by NCFRs. Finding the proportion entailed identifying flood events via Watches, Warnings, and Advisories (WWAs) issued by the National Weather Service (NWS) and streamflow measurements in four urban watersheds (Sepulveda Dam, Whittier Narrows Dam, Santa Ana River, and San Diego River) and then linking the flooding with NCFRs via radar imagery and information provided by an “NCFR catalog.” The second approach sought to understand the hydrometeorological characteristics of NCFRs that caused flooding in urban Southern California. This approach entailed composite analyses of hydrometeorological variables. Radar-derived quantitative precipitation estimation (QPE) and rain gauge data were also used to understand the spatial and temporal characteristics of the flood-producing NCFRs. New hydrological insights for the region: NCFRs make up nearly 40 percent of the urban flood events in the selected Southern California watersheds between 1995 and 2020. We tested the impact of maximum reflectivity, storm speed, and direction (azimuth) on the streamflow and runoff ratio of the four urban watersheds. Maximum reflectivity played an insignificant role, whereas storm speed and azimuth played a significant role in some of the watersheds. Overall, NCFRs contributed around 8-58% of the average annual precipitation in the Los Angeles and San Diego metropolitan areas—producing average rain rates of 0.4-1.8 mm h-1 during the entirety of an event and average storm totals of 12-36 mm. These findings suggest that NCFRs play a significant role in urban flooding and in the climate regime of Southern California.
