NOAA’S FLOOD FORECASTING HELPS SAVE LIVES AND PROPERTY

March 31, 2005 — Floods have on average killed more than 100 people and are responsible for $4.6 billion in damages in the United States each year — making it among the deadliest of all weather-related events. In fact, three-fourths of all presidential disaster declarations are associated with flooding. Fortunately, thanks to the NOAA National Weather Service, flood fatalities are one of the most preventable of all weather related fatalities.

Part of the NOAA National Weather Service, the NOAA Hydrologic Services Program, monitors conditions that lead to flooding 24 hours a day, seven days a week, throughout the United States. Its hydrologic forecasts, watches and warnings (in combination with flood outreach and educational efforts) have been extremely effective at saving lives and reducing costly flood damages throughout the nation. Some studies suggest that as little as one hour of lead-time can result in a ten-percent reduction in flood damages.

“No state or territory is immune from the dangers of flooding, but getting accurate, timely weather information and knowing how to react when floods threaten can mean the difference between life and death,” said Brig. Gen. David L. Johnson, U.S. Air Force (Ret.), director of the NOAA National Weather Service. “The National Weather Service strives to improve flood forecasts and warnings as part of NOAA’s continuing mission to serve society’s increasing need for weather and water information.”

Flooding Can Occur Anywhere at Anytime
Flooding occurs whenever water accumulates faster than the ground can absorb it or rivers (and other drainage routes) can carry it away. Rain intensity, duration and topography, as well as soil condition and ground cover, all contribute to varying degrees of flooding.

One of the more frightening things about flooding is that it is one of the most common of all natural disasters and can occur nearly anywhere, at any time. Flooding can occur from ice jams or when melting snow combines with rain in the winter and early spring; when severe thunderstorms bring heavy rain in the spring and summer; or as tropical cyclones bring intense rainfall to both coastal and inland states in the summer and fall. Coastal flooding can also be produced by tsunamis, large waves produced by earthquakes or volcanic activity.

The impacts of flooding also increase as population and urban development grows. As land is converted from fields or woodlands to roads and parking lots, it loses its ability to absorb rainfall, thereby increasing the speed and amount of water runoff. If these areas do not have the proper drainage and/or storm drains become clogged, streets and low-lying areas will become flooded. Therefore, towns and cities built in low lying areas or near rivers are especially prone to flooding.

Flash Flooding vs. Broad Scale Flooding
There are two basic types of flooding — flash flooding and broad scale riverine floods. Each has its own different causes, characteristics, time scales — and forecasting methods.

• Flash Flooding: A flash flood is a rapid rise of water along a waterway or low-lying area that cannot be dispersed by soil absorption, runoff or drainage. Flash floods typically occur when slow moving or multiple intense thunderstorms travel over the same geographic area — depositing huge amounts of water in a very short period of time. Deserts and urban areas are especially vulnerable to flash floods, because their hard, dry surfaces do not absorb water quickly. Flash flooding is also associated with dam breaks, levee failure and ice jams. The onset of flash floods can be sudden, as water can travel at powerfully high rates of speed and can reach peak levels in just a few minutes — making them the most life threatening type of flood. If you live in areas prone to flash flooding, plan now to protect your family and property and monitor information provided by the NOAA National Weather Service. Fortunately, most flash floods are short-lived and their effects are concentrated in a relatively small areas. While much work has been done by weather services around the world to monitor rainfall and warn of flooding, flash floods remain difficult to predict.
• Broad Scale Flooding: While broad scale riverine flooding can result from widespread, prolonged flash flooding, it is often associated with frontal systems, such as cold low-pressure cells that produce prolonged periods of rain over a very broad area. Broad scale flooding develops more slowly than flash flooding, often taking days or even weeks to reach its peak. It is the cumulative effect of both local and remote water sources that contributes to broad scale flooding. Rainfall from routine storms does not usually cause broad scale flooding, but when successive storms pass over a particular river basin, the river rises until eventually it overflows its banks. If it is late winter or spring, melting snow can produce added runoff that can compound flood problems. Ice jams and torrential rains from decaying hurricanes or tropical systems can also contribute to broad scale river flooding. Because broad scale flooding is a cumulative effect that occurs over a wider geographic area and longer period of time, the NOAA Weather Service is able to provide reliable forecasts and issue appropriate warnings well before the flood peaks.

Because of the very different nature of these two types of floods, the NOAA National Weather Service uses different techniques to forecast them. Flash flood forecasts are made primarily using current local hydrological conditions and Doppler radar, while broad scale flood forecasts are issued using current (and predicted), local and regional hydrological information and sophisticated computer models.

NOAA National Weather Service Flood Forecasting
There are two levels to the hydrologic services offered by the NOAA National Weather Service. First the NOAA River Forecast Centers are staffed by professional hydrologists and hydrometeorlogists who issue river, reservoir and flood forecasts for almost 4,000 locations across the United States using a wide variety of data and sophisticated computer models. The RFC staff then provides forecast guidance to personnel at the NOAA National Weather Service Forecast Offices, who in turn issue forecasts, outlooks and related hydrologic information to the media, public and water resource managers. Used by a wide range of customers (e.g., barge operators, power companies, recreational users, farmers, households, businesses and environmentalists), these forecasts are a basis for operation and management of flood-control structures. Emergency managers also use the forecasts to fight floods, evacuate residents and to take other measures to mitigate the impact of flooding.

NOAA River Forecast Centers
The NOAA National Weather Service has 13 regional River Forecast Centers that prepare hydrologic forecasts. When developing the forecasts, both current and historical data are used from the NOAA National Weather Service, other NOAA offices (i.e., NOAA Satellite and Information Service, and the NOAA Oceans and Coasts Service) and non-NOAA partners (i.e., U.S. Army Corps of Engineers, U.S. Geological Survey, and local and regional authorities). Data are collected from automated gauges, geostationary satellites, Doppler radars, weather observation stations, and the computer and communications system, called the Advanced Weather Interactive Processing System. RFC hydrologists must also have extensive knowledge of the river basin to ensure that data used in their models are accurate and reliable. Computer models are used to forecast floods for selected locations along major rivers or small streams near urban areas that have a history of flooding. Other operational functions are performed on a seasonal or as needed basis. These functions include production of water supply outlooks, seasonal flood outlooks and low-flow forecasts.

NOAA National Weather Service Flood Products
The NOAA National Weather Service produces a number of flood products. As with severe thunderstorms and tornadoes, the NOAA National Weather Service uses a two-tiered system to alert the public of the dangers of flooding by issuing flood/flash flood warnings and watches. A warning is issued to inform the public that flooding is imminent or in progress. A watch is issued to inform the public and cooperating agencies that current and developing hydrometeorological conditions are such that there is a threat of flooding, but the occurrence is neither certain nor imminent.

NOAA's Advanced Hydrologic Prediction Service
During the past 10 years, new technologies have enabled the NOAA National Weather Service to make significant strides in improving its hydrologic program with earlier, more detailed flood warnings and forecasts. One of the newest innovations is the Advanced Hydrologic Prediction Service, which incorporates a host of new forecasting tools to provide more timely, site-specific information ranging from short-term flash flood events to long-range river forecasting.

AHPS uses a combination of remote sensing, data automation and advanced modeling to analyze data, create forecasts and generate graphical displays. This information is posted on the AHPS Web site in a timely and user-friendly manner. AHPS has significantly improved river forecasts and water resource management in the United States, providing lead times of as much as several months in advance of a flood. Furthermore, AHPS information is not limited to floods alone — AHPS also provides critical information to water resource managers (particularly important in times of drought). The National Hydrologic Warning Council estimates that benefits from AHPS in the areas of hydropower, irrigation, navigation and water supply could total as much as $766 million per year.

Flood Safety and Awareness
Providing up to date, accurate flood information, however is only half the battle since such information is only useful if the public takes the appropriate actions when faced with threatening flood conditions. Therefore, NOAA encourages the public to consider the following:

• Everyone in flood-prone areas should develop a personal/family flood safety plan and purchase flood insurance.
• Your community should also have a flood safety plan. The NOAA National Weather Service has designed StormReady, to arm America's communities with the communication and safety skills necessary to save lives and property.
• Listen to NOAA All-Hazards Weather Radio, commercial radio, television, or go to the Internet for the latest flash flood and flood watches, warnings, and weather advisories.
• Participate in national flood safety and awareness campaigns, such as Turn Around, Don't Drown™ and Flood Safety Awareness Week. The NOAA National Weather Service, the Federal Alliance for Safe Homes and other partners developed TADD to enhance public awareness of the dangers of driving or walking into flooded areas. The NOAA National Weather Service also hosted its first annual Flood Safety Awareness Week from March 21-25, 2005, to raise public attention to the dangers of flooding and ways to protect life and property.

Flooding may be one of nature's leading killers, but you don't have to be one of the victims. Knowing your flood risk and taking the proper precautions is the best way to avoid becoming a weather statistic.

Relevant Web Sites

NOAA Hydrologic Information Center

NOAA's Current Flood Conditions

NOAA's Current Hydrologic Conditions

NOAA's Hydrologic Outlooks

NOAA's Historic Flood and Hydrologic Information

NESDIS Flash Flood Home page

NESDIS Flood Events

ADVANCED HYDROLOGIC PREDICTION SERVICE: WATER INFORMATION FOR A
STRONGER AND SAFER AMERICA

NOAA’s STORMREADY AND TSUNAMIREADY PROGRAMS

NOAA WEATHER RADIO: THE VOICE OF THE NATIONAL WEATHER SERVICE

Media Contact:
Greg Romano, NOAA Weather Service, (301) 713-0622 ext. 164 or Chris Vaccaro, NOAA Weather Service, (301)713-0622 x134