Improving Short Term Flash Flood Forecasting Across Southern Utah's Canyon Country: An Assessment of Environmental Conditions Associated with Flash Flood Days

Forecasting flash flooding across the Desert Southwest, and specifically southern Utah, poses a significant and unique challenge. The landscape across this area consists largely of impervious sandstone resulting in rapid runoff through narrow channels and normally dry washes during heavy rainfall events. These areas are often populated with hikers and other outdoor enthusiasts of all skill, knowledge, preparedness levels, and nationalities. This flash flooding often impacts locations many miles downstream from where rainfall originally occurred, with recreationists in these locations often unaware of the flood threat due to a perceived lack of precipitation at their location. Deep, narrow canyons also hinder most telecommunication methods making it difficult for those within these locations to receive timely alerts regarding flash flooding. Consequently, providing notification of enhanced flood potential days in advance, before these groups embark on trips into flash flood prone areas, is particularly important.

This study uses a combination of Stormdata reports and daily River Forecast Center (RFC) Quantitative Precipitation Estimates (QPE) to identify days in which flash flooding and/or heavy rain occurred during the warm season (June through October) across southern Utah. These days, termed flash flood days, were included in the subsequent statistical analysis, along with carefully selected null days. This analysis examines both plan-view composites of various synoptic parameters as well as kinematic and thermodynamic data from Rapid Refresh (RAP) BUFR soundings for representative locations across southern Utah. The primary goal of this analysis is to identify synoptic patterns and environmental parameters conducive to flash flooding in southern Utah and potentially the broader desert Southwest. Beyond looking at southern Utah as a whole, the study area is also divided into sub-regions (southwest, south central, and east central Utah) to identify subtle differences in the environmental data resulting in flooding specific to each sub-region. It is anticipated that by better pinpointing the meteorological factors most correlated to flash flooding within a specific sub-region, forecasters will be able to more successfully assess the potential for a flash flood day to occur within that sub-region in the critical 24 to 72 hour period. This would provide valuable and timely information so users can make better informed decisions and avoid putting themselves in potentially dangerous situations.