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FLOOD FAQs

First of all, never, ever, set up your tent or camper by a stream bed - even a dry one - when you camp in the mountains! Be aware that flash floods can occur from distant thunderstorms well upstream from where you are. The most important safety tip is to climb to higher ground and safety as quickly as you can when you sense the danger, getting as high above the stream bed or roadway as possible or out of the canyon and low-lying areas.

Detecting Floods

Flash floods tend to be associated with many types of convection, 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 key to detecting flash flood producing rainfall is determining the location, areal extent, and time duration of the most intense rainfall cores associated with MCS's. The main tools used to detect heavy rainfall signatures associated with flash floods are satellite, lightning observing systems, radar, and rain gauges.

Satellite evidence

Satellite imagery can indicate the presence of larger and smaller-scale systems associated with heavy rainfall. It clearly indicates the organization of larger features associated with Mesoscale Convective Systems, and also aids in estimating rainfall using IR-based cloud-top temperatures.

Lightning detection evidence

Lightning detection systems can indicate the presence of deep moist convection, which can help detect SOME potential flash flooding situations. The heaviest rainfall seems to occur where the lighting strikes are most intense.

Radar evidence

WSR-88D radars graphically display precipitation on a map. Radar can show the location of the intense rainfall cores associated with deep moist convection, and estimate the duration of rainfall. Radar can also track the evolution of convective systems over time. Forecasters are able to watch existing cells intensify, and see when new cells begin to develop aloft. Animation of radar provides specific information on the movement of convective 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.

Ground evidence

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.

Some places that are subject to frequent floods are protected by warning systems that are activated when stream depths or rainfall amounts exceed certain levels. In a typical system automated rain and/or stream level gauges transmit information to a central computer which then activates an alarm, giving critical decision makers enough time to enact emergency measures.

What does a flash flood look and sound like?

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 oftern caught off guard because rain may not be heavy or falling at all where they are.

HOW DOES NSSL CONTRIBUTE?

NSSL's hydrometeorology group is working with software that utilizes many different sensors (radar, satellite, rain gauges, lightning) for accurately depicting storms and storm evolution, and improving flash flood monitoring and prediction. This technology will support operations for domestic and international weather services, water resources and flood control, aviation, and power generation and distribution industries.

National Radar Mosaic – NSSL's hydrometeorology research scientists seamlessly pieced together all 130 NWS and Department of Defense radars across the U.S. The mosaic provides high-resolution depiction of storms and quantitative precipitation estimation products from coast-to-coast in real time.

AMBER – Areal Mean Basin Estimated Rainfall algorithm – A flash flood alert tool implemented in NSSL's Warning Decision Support System that monitors the amount of precipitation that falls into a water-shed or basin and alerts the forecaster to a potential flash flood situation.

Flash Flood Monitoring and Prediction System – developed by NSSL scientists computes average basin rainfall accumulations and rates based on WSR-88D precipitation estimates. These calculations allow NWS forecasters to monitor precipitation in flash-flood-scale basins, potentially improving their ability to make accurate and timely flash flood warning decisions. NSSL's group was awarded a Bronze Medal by NOAA for this work.

National Basin Delineation Project – The foundation of the FFMP is the National Basin Delineation Project. NSSL and CIMMS accomplished the task of defining flash-flood scale basins for the conterminous U.S. , Puerto Rico, and Guam .

Taiwan's Hydrometeorological Decision Support System – NSSL collaborated with NOAA's Forecast Systems Laboratory and the Central Weather Bureau and Water Resources Agency of Taiwan to help improve the country's capabilities to issue flash flood and flood warnings and to improve their river and reservoir management.

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