Flood and Flash Flood Warnings
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.
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
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.
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
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
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/
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.