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Flooding typically occurs when prolonged rain falls over several days, when intense rain falls over a short period of time, or when an ice or debris jam causes a river or stream to overflow onto the surrounding area. Flooding can also result from the failure of a water control structure, such as a levee or dam. The most common cause of flooding is water due to rain or snow melt that accumulates faster than soils can absorb it or rivers can carry it away.
Three fourths of all presidential disaster declarations are associated with flooding. Year in and year out, flooding has a significant economic impact on the Nation. In most years it causes more damage in the United States than any other severe weather related event – an average of $5.3 billion a year for the 30-year period, 1975-2004. During this same period, an annual average of 93 people lost their lives due to flooding. While final totals are not available for 2005, flooding from Hurricanes Katrina and Rita took hundreds of lives and caused tens of billions of dollars in damages, dwarfing these long-term averages.
Because flooding can occur any place and at any time, NOAA’s National Weather Service (NWS) monitors conditions that lead to flooding 24 hours a day, 7 days a week, and issues forecasts, watches and warnings.
Through the constant infusion of technology, the NWS continues to improve flood forecasts with the goal of saving lives and reducing damage. The NWS’ Hydrologic Services Program is in the midst of deploying its Advanced Hydrologic Prediction Service (AHPS). In addition to taking advantage of scientific improvements, AHPS provides a rich, Web-based resource for current river conditions.
The NWS monitors conditions that will cause flooding using a wide range of observation systems, including radar, satellites, aircraft, and automated and manual land-based observation systems.
 A NEXRAD Doppler radar with a tornado in the background. |
In addition to providing information on winds associated with severe weather and tornadoes, the NWS’ NEXRAD Doppler radars provide estimates of rainfall that is critical to monitoring conditions that cause flooding. This information,nexrad doppler radar wsr 88d combined with rain gage observations, serves as the key input to flood forecast models. The primary forecast tool used by the 13 NWS’ River Forecast Centers (RFCs) is known as NWSRFS, or the NWS River Forecast System. NWSRFS also depends on observations of current river conditions. This information is provided by a number of partners, including the U.S. Geological Survey and the Army Corps of Engineers. NWSRFS combines both observations and forecasts of precipitation with other parameters, such as terrain slope, soil type, and vegetation, to produce forecasts of river conditions.
NWSRFS also provides estimates of soil moisture which is a critical
factor in predicting flash flooding. Soil type and moisture content determine
how much water the ground can absorb and are used to develop what is
known as flash flood guidance. When rainfall is expected to exceed
flash flood guidance, flooding can be expected. NEXRAD precipitation
estimates, combined with detailed flash flood thresholds are used in
an application known as flash flood monitoring and prediction which
has increased the precision and lead time for flash flood forecasting.
Flash flooding typically occurs when intense rains fall faster
than the ground can absorb it. Flash floods do not always affect
stream, but can impact low-lying areas and urban areas where drainage
systems can be overwhelmed by torrential rains.
 Aircraft used to perform snow surveys |
In northern areas, snow accumulates over the winter. This reservoir of water is released when it melts. If accompanied by warm rains, the snowmelt and rain can combine to cause flooding. In flat areas without sloping terrain to remove it, snowmelt alone can cause flooding. Knowing the amount of water stored in the snow pack is key to providing accurate snowmelt flood forecasts. The NWS’ National Operational Hydrologic Remote Sensing Center (NOHRSC) uses satellite and ground based snow observations in a snow modeling system to produce detailed snow water equivalent estimates. In areas of particular concern, the NOHRSC deploys aircraft with gamma ray sensors that provide high quality estimates of snow water equivalent. This information is used by a component of NWSRFS to provide snow melt forecasts.
Forecast information prepared by the RFCs is provided to 122 NWS Weather Forecast Offices that translate the forecast guidance into watches and warnings tailored to the local areas they serve.
In the case of flash flooding, rapid dissemination of these forecasts, watches and warnings is especially critical. NOAA Weather Radio provides up to the minute flood warnings. Receivers can be set to provide audible alarm even when they are turned off. This technology is critical to saving lives, particularly during nighttime disasters.
In addition, NWS works closely with national, state and local emergency managers to disseminate forecasts and warnings as well as to support their flood response activities. Another key partner in communicating weather threats is the media, including television, radio and the Internet.
Unfortunately, most flood fatalities are not due to limitations in the forecast system. All too often, people in vehicles literally drive into harm’s way. As little as two feet of water can float an average car. Currently popular sports utility vehicles (SUVs) require only slightly more water to be swept away. While it may appear that water is not deep enough to cause problems, there is almost no way of knowing if the roadbed itself has been eroded or undermined. The NWS urges people to AVOID WATER, no matter how benign it may look: Turn Around, Don’t Drown.
