The Rapid Response Flood Team (RAFT) Paradigm

For decades, National Weather Service River forecast Centers (RFCs) have been at the forefront of river forecasting for the United States. By providing flood guidance to the Weather Forecast Offices (WFOs) that issue the various river flood products out to the public, RFCs have provided life-saving information that is consumed by various entities to aide in public safety, as well as billion dollar decisions, on a routine basis. Forecasting the quick responding basins of the Texas Hill Country poses a unique challenge for the West Gulf River Forecast Center (WGRFC). Many headwaters in the Texas Hill Country peak in less than three hours and several basins peak on a sub-hourly time scale. The existing WGRFC forecast workflow has been constructed to effectively model basins with time peak values of six hours or greater. The primary hydrologic model used by WGRFC is a spatially lumped Sacramento Soil Moisture Accounting (SAC-SMA) model that is ideal for larger basins and has been calibrated for basins with six-hour or greater time to peak, but not all basins behave this way. Because this model framework operates as a “one size fits all,” it forecasts the smaller, quicker-responding rivers with the same lumped SAC-SMA modeling used to forecast the slower, more steady rivers. This becomes very problematic when a flood is occurring on one of these faster-responding river basins because the model and subsequent forecasts are typically too late and do not capture the flood peak before impacts have already occurred. Additional constraints on the workflow include the ingest frequency of gage adjusted precipitation forcings and transmit frequency of the stream gage observations. Forecaster experience has helped to mitigate some of these shortcomings; however, WGRFC is piloting an approach in the Texas Hill Country to refine the existing forecast workflow using new technologies and a recalibration of the SAC-SMA model in order to increase forecast lead-time for quick responding basins. This pilot study is anticipated to demonstrate improvements in the assessment and dissemination of the essential flood forecast information to WFOs that will allow the public to be warned sooner regarding impending river floods on quick responding basins and can potentially be applicable across other RFCs.