Developing a Flash Flood Climatology for the Varied Terrain of the Hanford, California, County Warning Area

Flash flood forecasting is one of the biggest challenges across the Hanford CWA, due to elevation which varies from nearly 300 feet on the valley floor, to 14,505 feet at Mt. Whitney. This abrupt change in topography creates many opportunities for high flows from not only heavy rain, but also snowmelt during the late winter and spring. Impactful runoff can also occur along the western boundary of the County Warning Area (CWA) as some of the coastal ranges of California reach 4,000 feet. Other concerns are slow moving or stationary thunderstorms, which can create water problems through the urbanized locations within the CWA, like Fresno and Bakersfield, where rainfall infiltration is unlikely to occur. Previous events and ongoing collaboration with Fresno emergency managers will help to identify areas of the city which are most prone to flood, and will aid in warning decision and lead times. Another concern is the coverage of hydrophobic soils across the forecast area through the deserts and burn scars. For example, a recent event from April of 2017, near the Fresno International Airport, could be classified as an event described by Maddox et all (1980): Meteorological Characteristics of Flash Flooding Over the Western United States. A Flash Flood climatology will also assist with defining more localized warning thresholds to be used with emerging technologies such as FLASH (Flood Locations and Simulated Hydrographs) and the National Water Model, which attempt to simulate impactful streamflows along unregulated waterways. Categorizing these events, based upon the causes referenced above, will raise the pattern recognition and situational awareness of the warning forecaster, required to better communicate confidence in these events to our core partners.