The mission of the National Weather Service (NWS) is to provide weather, hydrologic, and climate forecasts for the protection of lives and property and the enhancement of the national economy. To this end, the NWS issues watches and warnings for severe weather, flash floods, and river floods.

Flash floods can occur at any time and any place with little or no warning. Most often resulting from runoff from heavy rains, flash floods can fill streams, creeks, low-water crossings and low-lying areas, turning them into raging torrents in minutes. A Flash Flood Watch means that conditions are right for rain to cause flash flooding in the watch area. A Flash Flood Warning means that flooding is imminent or occurring. Flash Flood Watches and Warnings are issued by NWS forecast offices. They are disseminated on radio, television, internet sites, and National Oceanic and Atmospheric Administration (NOAA) Weather Radio. Flash floods account for the greatest number of weather-related deaths by far in Texas.

River flooding generally occurs on a longer time scale than flash flooding on smaller streams. River flooding most often results from prolonged periods of heavy rain across large areas sufficient to bring a mainstream river or its tributaries above flood stage, the threshold elevation at which damage occurs. A Flood Warning means river flood conditions are forecast, imminent, or in progress. Flood Warnings include the estimated height and time of the flood crest for forecast points.

Rain Gages

Gathering rainfall information is a critical part of the NWS's river forecasting and flood warning program. Rainfall measurements and estimates are available from a number of sources, some on demand and in real time and others after the fact. Measurements are made using a variety of equipment including standard 8-inch-diameter rain gages, weighing/recording rain gages, and tipping buckets.

The standard 8-inch rain gage consists of a large cylinder with a funnel and a smaller measuring tube inside. The dimensions of this instrument are very specific. Water that collects in the measuring tube has exactly one-tenth the cross-sectional area of the top of the funnel. Precise rainfall measurements can be made by reading the exaggerated height of the water in the tube. For example, 0.1 inch of rainfall would actually fill 1 inch of the measuring tube. A special measuring stick inserted into the measuring tube and (or) markings on the side of the tube, take into account the exaggeration of the vertical scale.

The tipping bucket catches precipitation in a collector, then funnels the water to the tipping bucket. The bucket works much like a seesaw with a container ("bucket") on each side. The bucket on the raised end of the tipper is positioned directly beneath the collector spout. The seesaw tips when the bucket collects the equivalent of 0.01 inch of rainfall. Each tip empties one side of the seesaw and positions the other bucket under the funnel. After each tip, the measured water is funneled out the bottom of the gage. Each time a bucket tips, an electronic signal is sent to a recorder. The tipping bucket rain gage is especially good at measuring drizzle and very light rainfall events. It is less exact in heavy rain when rainfall may be is missed during the time the buckets are moving into place.

The weighing rain gage uses a container sitting on a scale to measure the weight of the collected rain water. The weight of water collected is translated to inches and recorded in an ink trace on a chart for a permanent rain record.

Observing Programs
Official weather observing sites are operated by the NWS and Federal Aviation Agency (FAA). Automated equipment at these locations is carefully sited and calibrated to obtain the best possible readings. Precipitation, measured by tipping buckets, is available in 5-minute increments and is delivered automatically to NWS and FAA information systems.

The NWS also partners with agencies such as the U.S. Geological Survey (USGS) and U.S. Army Corps of Engineers (COE) in collecting rainfall amounts at USGS streamflow-gaging stations and COE installations. This information is collected every 15 minutes and delivered by satellite. In addition, many local river authorities support river and rainfall reporting systems and share these data with the NWS.

The cooperative program is an organized effort to collect high-quality observations in addition to official sites. At least one cooperative observing site is located in each county. Cooperative observers are provided with equipment to collect daily temperature and rainfall data. Observations are transmitted daily by phone and monthly by mail. These observers use standard 8-inch rain gages as well as recording rain gages. More information on the cooperative program and equipment is available at http://www.nws.noaa.gov/om/coop/index.htm

Unofficial observers collect 24-hour rainfall data and report it to the local NWS office by phone each morning after rain occurs. Many also report interim rainfall amounts on request. Unofficial observations may come from Federal, State, and local government entities, corporations, parks, and weather enthusiasts. Other collections of rainfall observations that may be available to the NWS include mesonets operated by universities, State departments of transportation, school weather networks, and ham radio networks.

Radar precipitation estimates are derived from radar reflectivity returns. Complex algorithms relate the returned radar echos to estimates of precipitation intensity and amounts. These data are available every 6 minutes. Many observing systems support the same 6-minute period to provide a means for evaluating the reliability of radar estimates. Radar algorithms may overestimate or underestimate rainfall depending on atmospheric conditions, meteorological conditions, and distance from the radar site. More information on the NEXRAD radar program is available from the Radar Operations Center at http://www.osf.noaa.gov/

Satellite precipitation estimates are obtained from infrared satellite images that correlate cloud-top temperatures with rainfall production. These estimates are available several times an hour and are particularly valuable in data-sparse regions. Information on satellite precipitation estimates is available from the National Environmental Satellite, Data, and Information Service at http://orbit net.nesdis.noaa.gov/arad/ht/ff/auto.html

River Forecasting
Rainfall amounts are input into hydrologic models that are used to produce river-level forecasts. The West Gulf River Forecast Center in Fort Worth provides hydrologic forecasts for rivers in Texas. Their Web pages include information on how hydrologic forecasts are made at http://www.srh.noaa.gov/wgrfc/

Bucket surveys
The nature of rainfall in Texas is often described as feast or famine, long periods of dry weather interspersed with heavy rains. Rain, when it does come, can be capricious even in flood events. Rainfall amounts may vary widely across distances of only a few miles. Isolated rainfall amounts are often much larger than the average amounts over one or several counties. After significant or record flood events, the NWS may conduct a bucket survey. This process involves phone calls and visits to the area of concern in an effort to collect as much rainfall information as possible in addition to that acquired in the programs mentioned above; in essence, to survey all the buckets the rain may have fallen into. This kind of high-density survey can provides very high resolution of rainfall for historical records, hydrologic model calibration, verification of radar estimates, and statistical climate research.