Flash Flood Emergenices

Flash Flood Emergencies are one of two high impact products the National Weather Service issues. Due to the increased threat and urgency portrayed in the product, flash flood emergencies have played a key role in the decision support services provided by the NWS since they were first issued in 2003. This study first examines the history of the product and how the product has added to NWS operations and services. NWS policy instructions state flash flood emergencies should only be issued in exceedingly rare situations, and when a severe threat to human life and catastrophic damage is imminent or ongoing. Example situations include the need for immediate evacuation from flooded areas, multiple swift water rescues, flooding above previous record values, or failure of a high risk dam. Unlike a tornado emergency, which can be issued at forecaster discretion, flash flood emergencies are typically coordinated with local emergency management partners before issuance. Collation of basic information such as the date/time of issuance, issuing Weather Forecast Office (WFO), and the geographic warned area, create a foundation for the frequency and geographic coverage of these events. Identification and compositing of synoptic parameters and patterns and proximity soundings, are compared to previous synoptic typing studies and conceptual models for heavy rain and flash flood events. This study shows tropical systems, slow moving synoptic scale waves, high precipitable water environments, and geographic areas with shallow soil depth and runoff-concentrating topography result in the most flash flood emergencies.