Quantifying Flash Flood Warning Thresholds in Pima County, AZ

Tucson meteorologists currently lack quantitative rainfall thresholds to inform flash flood warning issuance. In response to a request from the National Weather Service (NWS) in Tucson, AZ, students in HWRS 350 Principles of Hydrology analyzed precipitation rates that triggered flash floods in Pima County and determined peak precipitation rates during flash flood events from 2018-2024. Nine students continued the research in spring 2026, expanding the dataset to include 1996-2017 and 2025. This research directly supports NWS objectives to safeguard communities and minimize economic losses by improving warning accuracy and potentially reducing emergency response times. 

Flash flood event dates from 1996-2025 using the National Oceanic and Atmospheric Administration's (NOAA) National Centers for Environmental Information (NCEI) Storm Events Database. Corresponding rainfall data at 15-, 30-, and 60-minute intervals were retrieved from the Pima County Regional Flood Control District ALERT webpage using a web-scraping Python script. To ensure accuracy, all of the maximum precipitation values from Python are manually checked using the Pima ALERT website. Frequency distributions of precipitation rates during flash flood events will be generated through histogram analysis.

Thresholds in maximum precipitation analyses for different land uses (urban basin, rural basin, mountain range areas) will also be reported. Understanding different maximum precipitation thresholds for urban, rural, and mountainous areas is essential because the same amount of rainfall produces vastly different hydrological, economic, and safety impacts depending on the land's topography and usage.