Severe Weather 101
Flash floods tend to be associated with many types of storms, all capable of producing excessive rainfall amounts over a particular area, so detection remains a challenge. Sometimes a flash flood threat is overshadowed by other severe weather events happening at the same time. The main tools used to detect heavy rainfall associated with flash floods are satellite, lightning observing systems, radar, and rain gauges.
What we do: NSSL's research team includes hydrologists, hydrometeorologists and civil engineers to approach flash flood detection and forecasting from all angles.
WSR-88D radars graphically display detected precipitation on a map. Radar can show the location of the intense rainfall cores, and estimate the duration of rainfall. Radar can also track the evolution of storm systems over time. Forecasters are able to watch existing storm cells intensify, and see when new cells begin to develop. Animation of radar provides specific information on the movement of storm systems and helps in the assessment of the flash flood threat.
Currently, the NWS uses products developed for WSR-88D radars to aid in issuing flash flood statements, watches, or warnings. One product estimates one-hour precipitation accumulation to assess rainfall intensities for flash flood warnings, urban flood statements and special weather statements. Another product estimates accumulated rainfall, continuously updated, since the last one-hour break in precipitation. This product is used to locate flood potential over urban or rural areas, estimate total basin runoff and provide rainfall accumulations for the duration of the event.
What we do: NSSL developed and implemented the real-time Multi-Radar Multi-Sensor system in 2004, integrating data from multiple radar networks, surface and upper air observations, lightning detection systems, satellite and numerical weather prediction models. The data is used to estimate and forecast precipitation locations, amounts, and types.
MRMS was transitioned into operations at the National Center for Environmental Prediction in 2014 and provided severe weather and precipitation products for improved decision-making capability within NOAA. The operational MRMS QPE products have high resolution and rapid updating capabilities. The products are also used for verification of satellite rain products and for verification of quantitative rain forecasts from numerical weather prediction models. MRMS serves as a powerful tool for the creation and evaluation of new techniques, strategies and applications to better QPE. As new concepts are developed, they can be tested by easily plugged in and out of MRMS. This process facilitates a rapid science-to-operations transition of new MRMS applications and products for flood and flash flood predictions and water resources management.
Rain gauges provide the most accurate method of measuring rainfall at a single geographic point. To have operational value, the rain gauge report must be available in real time, and automated reporting networks are increasing. Real-time rain gauge networks are most useful for flash flood detection when WSR-88D rainfall estimates can be compared with the actual rain gauge values to determine the accuracy of the radar estimate.
The Flooded Locations And Simulated Hydrographs project (FLASH) was launched in early 2012 to improve the accuracy and timing of flash flood warnings. FLASH introduces a new paradigm in flash flood prediction, using MRMS and producing flash flood forecasts with products generated as frequently as every 2 minutes. The primary goal of FLASH is to improve accuracy, timing, specificity, and severity levels of flash flood warnings in the U.S., thus saving lives and protecting infrastructure. The FLASH team is comprised of researchers and students who use an interdisciplinary and collaborative approach to achieve the goal. The FLASH system was transitioned to the National Weather Service in November 2016.
Estimates of rainfall from satellite data are less direct and less accurate than either gauges or radar, but have the advantage of high resolution and complete coverage over oceans, mountainous regions, and sparsely populated areas where other sources of rainfall data are not available. Since flash flood events often originate with heavy rainfall in sparsely instrumented areas that goes undetected, satellite-derived rainfall can be a critical tool for identifying hazards from smaller-scale rainfall and flood events.
You may notice a stream starting to rise quickly and become muddy. Sometimes flood debris temporarily blocks the water flow upstream. When it breaks free the debris may release a “wall of water” downstream. You may hear a roaring sound upstream as a flood wave moves rapidly toward you. People are often caught off guard because rain may not be heavy or falling at all where they are.