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 created a powerful research and development tool for the creation of new techniques, strategies, and applications to better estimate and forecast precipitation amounts, locations, and types. The tool is called NMQ, the National Mosaic and Multi-sensor Quantitative Precipitation Estimation. NMQ uses a combination of observing systems ranging from radars to satellites on a national scale to produce precipitation forecasts. As new concepts are developed, they can be tested by easily plugging them in and out of NMQ. Current projects include integration of polarimetric radar data since it has strong potential in improving the accuracy of precipitation estimates. Special techniques are also being employed to reduce effects of radar beam blockage, especially troublesome in the West but fairly common elsewhere.
NMQ's prototype QPE products are also known as “Q2”—next-generation products combining the most effective multi-sensor techniques to estimate precipitation. Q2 products have been used in various meteorological, aviation, and hydrological applications. NSSL also works with NWS forecasters to improve science applications and to transfer these new ideas to operations.
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 uses real-time high-resolution, accurate rainfall observations from the NMQ/Q2 project to produce flash flood forecasts at 1-km/5-min resolution. FLASH project development continues to be an active collaboration between members of NSSL's Stormscale Hydrometeorology and Hydromodeling Groups, and the HyDROS Lab at the University of Oklahoma.
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.