How NSSL research provides real-time precipitation estimations and flash flood prediction for high-impact events

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.

Flash flooding — the rapid rise of water in a normally dry area — is mostly related to excessive rainfall resulting in significant groundwater runoff and quick rises in waterways. NOAA National Weather Service (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.

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.

A gif loop of radar reflectivity over middle Tennessee showing the increase flash flood levels.
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.)

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 Mesoscale Meteorological Studies (CIMMS) at the University of Oklahoma developed two systems to help with forecaster analysis and warning decision making — the Multi-Radar Multi-Sensor (MRMS) system and the Flooded Locations and Simulated Hydrographs (FLASH) system.

The Systems

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

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.

NSSL and CIMMS researchers regularly analyze MRMS QPE 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.

Using 24-hour analysis centered around 1200 UTC (7:00 AM local time) to collect both daily CoCoRaHS rain gauges 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.

A screenshot of MRMS dual-polarization QPE data.
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.)

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 — new model runs are generated every ten minutes — in real-time for the entire country.

The FLASH system also provides products that compare QPE values to flash flood guidance — a measure of how much rainfall is needed to flood small waterways — in addition to the average recurrence intervals — 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.

Three separate screenshots of the FLASH model products showing QPE and flooding.
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.)

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 — approximately 0.5% chance of occurring per year).

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. The maximum unit streamflow values — the amount of water runoff normalized by its basin area — have been shown to capture the spatial coverage of flash flooding and provide context to its potential severity.

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

  • How quickly the flash flood threat escalated.
  • How the extreme values pointed to a potentially catastrophic event.
  • How the model routed the water to show the impacts on local rivers even after the rainfall ended.
A graphic of the CREST maximum unit streamflow from the FLASH system. The graphic shows flood waters maxing out over time.
CREST maximum unit streamflow from the FLASH system from 0600–2100 UTC 21 August 2021. (Graphic provided.)

Researchers at NSSL and CIMMS continuously work to enhance the performance of the MRMS and FLASH systems to improve precipitation estimations and flash flood predictions. Efforts with machine learning and artificial intelligence are paving the way for increased performance in areas where radars struggle to accurately capture precipitation. Probabilistic hydrologic modeling with the use of forecast precipitation with the FLASH system looks toward the future of warning for flash floods within the FACETs (Forecasting a Continuum of Environmental Threats) paradigm.

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Forecasters now have richer data to predict fast-moving floods

 

Flash floods — raging torrents of water caused by heavy rainfall and overtopped rivers and streams — can become deadly in a matter of minutes.

Now, a new forecasting tool is helping NOAA National Weather Service forecasters predict when and where these devastating events will occur.

Developed by researchers at NOAA’s National Severe Storms Laboratory and its partners, FLASH — short for the Flooded Locations and Simulated Hydrographs Project — combines real-time rainfall estimates with real-time surface models to supply forecasters better information and more confidence with which to issue flood warnings.

How it works
FLASH gathers rainfall data from multiple radars, combines it with information about the type of surface where rain is falling and creates a highly detailed forecast for when specific rivers and streams will flood and by how much. FLASH also provides historical context, helping forecasters communicate the significance of a predicted flash flood.

FLASH was tested by researchers and NWS forecasters during the HMT-Hydro testbed experiments from 2014-2016, allowing the product to be fine-tuned.

What’s Next

FLASH is being reframed into a fully probabilistic framework called Pro-FLASH. This consideration of uncertainty in the modeling process accommodates forcings from ensemble precipitation forecasts.

Such can increase the lead time with the distributed hydrologic forecasts, but must consider the associated uncertainties with the forcings. We are also developing FLASH products for the outer-continental United States domains including Alaska, Hawaii, Guam, and the Caribbean.

 

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NSSL researchers lead project to evaluate experimental flash flood products

DSC_0052During July, NOAA National Weather Service forecasters from forecast offices and river forecast centers will assess emerging hydrometeorological concepts and products in the Multi-Radar / Multi-Sensor (MRMS) Hydro Experiment 2015. Their goal is to improve the accuracy, timing, and specificity of flash flood watches and warnings.

MRMS-Hydro is led by NSSL and is part of the 2015 United States Weather Research Program (USWRP) Hydrometeorological Testbed (HMT). Operational activities will take place Monday through Friday for three weeks (July 6 to 24).

During the experiment, participating forecasters will evaluate short-term predictive tools derived from MRMS quantitative precipitation estimates (QPE) and Flooded Locations and Simulated Hydrographs (FLASH) hydrologic modeling framework. Forecasters will also explore the utility of experimental watch and warning products conveying uncertainty and magnitude issued through the Hazard Services software from the Earth Systems Research Lab/Global Systems Division (GSD). Research scientists will investigate human factors to determine operationally relevant best practices for the warning decision making process and the system usability of the Hazard Services platform.

HMT-Hydro will coordinate operations with the third annual Flash Flood and Intense Rainfall experiment (FFaIR) at the NOAA/NWS Weather Prediction Center (WPC) to simulate the collaboration that occurs between the National Weather Service’s national centers, river forecast centers, and local forecast offices during flash flood events.

HMT-Hydro and FFaIR will simulate the real-time workflow from WPC 6-24 hour forecast and guidance products to experimental flash flood watches and warnings issued in the 0-6 hour period. The HMT-hydro team will shift its area of responsibility on a daily basis to where heavy precipitation events and associated flash flooding is anticipated.

Researchers will collect feedback from NWS operational forecasters through comments during their shifts, electronic surveys, de-briefings, and a webinar at the end of each week. NWS feedback is critical for future development and eventual implementation of new applications, displays, and product concepts into AWIPS2 and other operational systems.
HMT-Hydro 2015 provides a real-time environment to rapidly test the latest observational and modeling capabilities so they may be improved and optimized for transition to operational decision-making in the National Weather Service to support a Weather-Ready Nation.

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Experimental flash flood forecasting system tested by Texas rain event

Screen Shot 2015-06-04 at 10.13.41 AMRecent flooding in Texas and Oklahoma put NSSL’s experimental Multiple Radar Multiple Sensor (MRMS) Flooded Locations and Simulated Hydrograph (MRMS-FLASH) system to a rigorous test, and researchers were pleased.

The real-time MRMS-FLASH hydrologic modeling suite produces forecasts of flash flooding that are compared to historical simulations that use more than a decade of NEXRAD-based inputs at each 1km grid point. On May 25, the City of Houston, Texas, experienced deadly flash flooding. For this event, FLASH predicted extreme water flows out to six hours in advance.

The Coupled Routing and Excess Storage (CREST) distributed hydrologic model, also a part of the MRMS-FLASH modeling suite, generates maps of streamflow and unit streamflow (cubic meters per second per square kilometer) every 15 minutes. Comparisons between the observations of flash flooding in Houston and the maps of unit streamflow show a good correspondence between areas of high unit streamflows and flash flooding.

The FLASH model represents surfaces that do not absorb water, such as in urban zones, and is able to model dynamic soil moisture conditions, and how water will be routed downstream. MRMS-FLASH has run in real-time demonstration mode for several years.

NSSL’s MRMS-FLASH system provides information and services to make communities more resilient, focusing on enhanced water forecasting and delivery services, and also helps support the NWS to evolve its operations. https://inside.nssl.noaa.gov/flash/2015/05/flash-performance-with-recent-flood-events-in-oklahoma-texas/

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The 2013 Flash Flood and Intense Rainfall experiment (FFaIR)

flash-raindrop-strippedA team from NSSL will partner with the NOAA Hydrometeorological Testbed at the Weather Prediction Center to host the 1st annual Flash Flood and Intense Rainfall Experiment (FFaIR).  FFaIR will explore using high-resolution atmospheric and hydrologic models to improve short-term forecasts of both precipitation amounts and flash flooding. The project runs from July 8-26, 2013.

NSSL’s Flooded Locations And Simulated Hydrographs (FLASH) system will be one of several modeling systems evaluated during FFaIR. The FLASH system uses radar-estimated rainfall from NSSL’s National Mosaic and QPE System (NMQ/Q2) as input into the CREST (Coupled Routing and Excess STorage) hydrologic model.  FLASH then creates real-time 6-hour forecasts on a 1km grid that is updated every 15 minutes.

The 2013 FFaIR experiment will provide, for the first time, a pseudo-real time environment where participants from across the weather enterprise can explore the interface of meteorology and hydrology.  Working together through the forecast process will foster collaboration between National Centers for Environmental Prediction, National Weather Service Forecast Offices, NOAA labs, and the academic community.

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