The Future is Here: Incorporating Mesoscale Forecasts into Predictions for a Flood Disaster in the Rio Grande Valley of Texas

During the evening of 24 June 2019, a mesoscale convective system (MCS) produced 8 to 15 in (20.3 to 38.6 cm) of rainfall in a 6 h period, leaving at least 1500 homes and businesses with major to destructive flooding and hundreds of roads under 2 to 4 ft (0.6 to 1.2 m) of water. Initial damage estimates totaled at least $40 million. Despite nighttime rainfall rates of up to 3 in/hr (7.6 cm/hr) for several hours, there were no direct deaths or injuries from the flood. Unlike a similar event in the same region on 20 June 2018, there were few precursor synoptic-scale clues to accurately forecast the location, duration, and amount of rainfall that occurred. The suite of traditional model and model output statistics guidance 24 h prior to the onset of the event indicated quantitative precipitation forecasts (QPF) near zero.

Convection-allowing models (CAMs) told a much different story. Though precipitation output varied among these models and their ensembles, the consensus of the CAMs suggested that a vigorous outflow boundary in North Texas during the evening of 23 June 2019 would evolve into an MCS 24 h later in the Rio Grande Valley. This allowed forecasters at NOAA’s Weather Prediction Center (WPC) to provide deterministic and probabilistic QPF sufficient for local forecasters at the Weather Forecast Office (WFO) in Brownsville to take note. This led to a collaborative discussion for heavy rain and potential flooding early on 24 June 2019. The deterministic forecast for the period of heaviest rainfall, issued around 24 h prior to the initial convection, indicated a maximum between 2 and 2.5 in (5.08 to 6.35 cm) of rainfall in the area which received the highest total.

While the initial WPC QPF was well below observed values, it was far above those shown by traditional synoptic models that WFO forecasters normally use for next-day predictions. We detail how WPC confidently used CAMs to create a locally heavy rainfall forecast, including a “slight risk” of excessive rainfall over the Rio Grande Valley during antecedent dry conditions. We also describe a new forecast paradigm that incorporates WFO/WPC collaborated, convectively-driven QPF into local operations - a paradigm that improves forecast accuracy to deliver timely impact-based decision support to core public safety partners. Finally, we’ll explain how critical mesoscale precipitation forecast updates issued by WPC as the situation was unfolding on 24 June 2019 were used by the WFO to inform and direct public safety partners to critical flood areas, and how the combination of these updates and information from first responders were used to provide Flash Flood Emergencies to the most impacted locations.