{"id":5595,"date":"2021-09-10T17:29:51","date_gmt":"2021-09-10T22:29:51","guid":{"rendered":"https:\/\/inside.nssl.noaa.gov\/nsslnews\/?p=5595"},"modified":"2022-03-31T11:13:29","modified_gmt":"2022-03-31T16:13:29","slug":"how-nssl-research-provides-real-time-precipitation-estimations-and-flash-flood-prediction-for-high-impact-events","status":"publish","type":"post","link":"https:\/\/inside.nssl.noaa.gov\/nsslnews\/2021\/09\/how-nssl-research-provides-real-time-precipitation-estimations-and-flash-flood-prediction-for-high-impact-events\/","title":{"rendered":"How NSSL research provides real-time precipitation estimations and flash flood prediction for high-impact events"},"content":{"rendered":"

Some of the costliest and deadliest weather events in the United States are flash floods. On average, more fatalities are attributed to flash flooding than other short-fused severe weather hazards, like tornadoes, hurricanes, and lightning.<\/p>\n

Flash flooding<\/a> \u2014 the rapid rise of water in a normally dry area \u2014 is mostly related to excessive rainfall resulting in significant groundwater runoff and quick rises in waterways. NOAA National Weather Service<\/a> (NWS) forecasters rely on accurate quantitative precipitation estimations (QPEs). QPE are input into diagnostic tools and models to help NWS forecasters predict and warn on the potential for flash flooding, like flash floods that occurred in Tennessee on Aug. 21, 2021.<\/p>\n

Areas west of Nashville, particularly in Humphreys County, received over 1 foot of rain in a matter of hours. This included a period where 3-4 inches of rain fell per hour over multiple consecutive hours. Approximately 17.02 inches of rain was recorded in McEwen located in Humphreys County. This preliminary total eclipses the state record for rainfall in a 24-hour period, which was 13.60 inches in 1982. Twenty people perished in this Tennessee flood event.<\/p>\n

\"A<\/a>
A Multi-Radar Multi-Sensor reflectivity loop covering the duration of the western Middle Tennessee flash flood event ton Aug. 21. (Gif provided by Randy Bowers.)<\/figcaption><\/figure>\n

NWS forecasters can use a series of products to diagnose an ongoing weather event to determine what might be happening. Researchers at the NOAA National Severe Storms Laboratory (NSSL) and the Cooperative Institute for Severe and High-Impact Weather Research and Operations (CIWRO<\/a>, formerly CIMMS) hosted\u00a0at the University of Oklahoma developed two systems to help with forecaster analysis and warning decision making \u2014 the Multi-Radar Multi-Sensor (MRMS) system<\/a> and the Flooded Locations and Simulated Hydrographs (FLASH) system<\/a>.<\/p>\n

The Systems<\/h3>\n

The MRMS system is a platform that combines various weather observations and model data to create a suite of products, including various QPE fields.<\/p>\n

A key to the MRMS system is the quality control of radar data. Quality control algorithms remove radar artifacts from blockages, wind farms, biological scatter (like birds and bugs), and other data contaminations. The MRMS system then applies the latest scientific advancements in precipitation estimation using dual-polarization radar technology to provide accurate precipitation data in real-time every two minutes.<\/p>\n

NSSL and CIMMS researchers regularly analyze MRMS QPE<\/a> performance, including historic events like the Tennessee flash flooding. Product evaluations are conducted through internal web pages that allow for statistical comparisons of MRMS QPEs to independent gauge observations.<\/p>\n

Using 24-hour analysis centered around 1200 UTC (7:00 AM local time) to collect both daily CoCoRaHS rain gauges<\/a> along with hourly automated gauge observations, a few notable trends appear in the data. The overall analysis showed well correlated and clustered comparisons between the MRMS radar-based QPE and the gauge observations with rather small errors. The MRMS dual-polarization radar QPE had some overestimations with totals less than two inches, while some slight underestimation was observed with totals exceeding four inches. Yet, the nearly equivalent values between the gauges and MRMS in the area of greatest rainfall shows how well the system handled the event.<\/p>\n

\"A<\/a>
Analysis of MRMS dual-polarization QPE ending 1200 UTC on Aug. 21 (left column) and Aug. 22 (right column) with bubble plots (top row) and scatterplots with statistics (bottom row) using hourly and daily gauge observations. (Screenshot provided.)<\/figcaption><\/figure>\n

The second application developed by NSSL and CIMMS researchers to help with flash flood prediction is the Flooded Locations and Simulated Hydrographs (FLASH) system. The FLASH system is the first system to generate hydrologic modeling products specific to flash flooding at the flash flood time scale \u2014 new model runs are generated every ten minutes \u2014 in real-time for the entire country.<\/p>\n

The FLASH system also provides products that compare QPE values to flash flood guidance \u2014 a measure of how much rainfall is needed to flood small waterways \u2014 in addition to the average recurrence intervals \u2014 a measure to determine the rarity of the precipitation totals based on how frequently they occur. All products within the FLASH system use the MRMS dual-polarization radar QPE as their input.<\/p>\n

\"Three<\/a>
Analysis of the following FLASH products at 1300 UTC 21 August 2021: maximum QPE-to-FFG ratio (left), maximum QPE average recurrence interval (center), and CREST maximum unit streamflow (right). (Screenshot provided.)<\/figcaption><\/figure>\n

At the peak of the rainfall over Humphreys County, Tennessee, the QPE comparison products were at the upper end of the plotted scales. The accumulated rainfall was at least four times that of the NWS flash flood guidance for the area, and the average recurrence interval of the rain was beyond the plotted scale in the system (at least 200 years \u2014 approximately 0.5% chance of occurring per year).<\/p>\n

The product that best conveys the flash flood potential and its possible severity is the maximum unit streamflow product from the Coupled Routing and Excess Storage (CREST) hydrologic model<\/a>. The maximum unit streamflow values \u2014 the amount of water runoff normalized by its basin area \u2014 have been shown to capture the spatial coverage of flash flooding and provide context to its potential severity.<\/p>\n

The projected unit streamflow values based on MRMS precipitation rates during the Tennessee flash flood event on Aug. 21, 2021, showed three key features:<\/p>\n