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G lossary Basin: The entire land area with a common outlet for sur- face runoff. Basin boundary: The perimeter of a basin (or on a map, the line demarcating the perimeter) beyond which surface runoff drains away into another basin. Calibration: The process of optimizing parameters of a hy- drologic model to provide the best simulation of histori- cal stream flow by use of precipitation, temperature, and other hydrometeorological data from the historical record. Climate Data Continuity Project (CDCP): A NOAA- funded project to document differences in several hy- drometeorological observations (i.e., temperature, pre- cipitation, and wind) measured by pre-Automated Sur- face Observing System instrumentation and ASOS in- strumentation at airport locations. Climatic areas: Regions that can be grouped according to distinctive types of average weather conditions, espe- cially patterns of seasonal precipitation and temperature. Differences in climate between two areas can have a strong effect on the corresponding hydrologic charac- teristics of each area. Cooperative observer: An individual (or institution) who takes precipitation and temperature observations and in some cases other observations such as river stage, soil temperature, and evaporation at or near his or her home or place of business. Many observers transmit their reports by touch-tone telephone to an NWS com- puter, and nearly all observers mail monthly reports to the National Climatic Data Center to be archived and published. Data collection platforms (DCPs): Devices that acquire, record, and transmit data from various types of environ- mental sensors. pops often operate on a timed basis. The term pop is often modified to describe a data col- lection network to which the pop belongs (e.g., Geo- stationary Operational Environmental Satellite pop for 44 pops that report environmental data by way of the Geo- stationary Operational Environmental Satellite). Doppler radar: A radar capable of measuring the change in frequency of a radar wave caused by the relative mo- tion of an object in the atmosphere within the area of radar coverage. Doppler capabilities are not used in the process of converting radar echoes to precipitation estimates. Drought: A period of abnormally dry weather sufficiently prolonged to cause a serious hydrological imbalance. Ensemble forecasting: In a hydrologic modeling context, a process whereby a hydrologic model is successively ex- ecuted several times for the same forecast period by the use of varied data input scenarios or a perturbation of a key variable state for each model run. A common method employed to obtain a varied data input scenario is to use the historical meteorological record, with the assumption that several years of observed data covering the time period beginning on the current date and ex- tending through the forecast period comprise a reason- able estimate of the possible range of future conditions. Flash flood: A flood caused by water that rises rapidly from a normal level to inundation. Within the context of NWS hydrologic services, "rapidly" means within six hours. Flood: A higher than normal flow on a stream, triggered by a causative event such as heavy rainfall, snowmelt, or reservoir release, in which the stream level equals or exceeds a predetermined flood stage. Flood forecast: Any numerical forecast product made for a stream in which at least one forecast value exceeds flood stage or flow (same as "river flood forecast". Flood stage: An established stream level at a given gauge location at which high water begins to have an adverse impact either upstream or downstream from the gauge location. Flood warning: An NWS product issued to provide ad- vance notice that a specific river or rivers will equal or
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GLOSSARY exceed flood stage. Actions to be taken to mitigate losses are determined by the level of severity indicated in the flood warning product. Forecast skill: A statistical measure of how well forecasts predict what actually occurs. Forecast verification: The process by which forecasts are later compared to observed values for the same time period to evaluate the accuracy and timeliness of fore- casted values. Hydrograph: A graphical plot of stream stage or discharge versus time that shows how stream flow varies over time at a given stream location. Hydrologic cycle: The continual process by which water evaporates from the surface of oceans, lakes, other wa- ter bodies, and land; exists in the form of atmospheric moisture; forms into clouds and falls to the ground as precipitation; and flows through a stream network or groundwater system back to the oceans to repeat the process. Hydrologic model: A conceptual or physically based proce- dure to simulate numerically a process or processes that occur in a stream basin. Hydrologic services: A general term referring to the opera- tions, products, verbal communication, and related forms of support provided by the NWS regarding the streams, rivers, and other areas affected by surface water. Hydrology: The science dealing with the processes that con- trol the movement of water across and under the surface of the land. Hydrometeorology: An interdisciplinary science involving the study and analysis of the interrelationships between the atmospheric and land phases of water as it moves through the hydrologic cycle. Kalman filter: A mathematical formulation that weighs two comparable pieces of information (e.g., an observation and a simulated value) to provide an optimized output based on the error characteristics of both pieces of in- formation. Local-area forecast: In the pre-modernization and associ- ated restructuring of NWS, a locally tailored weather forecast that was prepared and issued by a WSO based on more general forecast products received from a "par- ent" Weather Service Forecast Office. This process did not apply to hydrologic forecasts, which were prepared initially by River Forecast Centers and issued to the public by NWS offices with hydrologic service area re- sponsibility. Local Climatological Data (LCD): A National Climatic Data Center publication that contains monthly and an- nual summaries of hydrometeorological parameters for surface-observing sites primarily at airport locations. The LCD is an important source of climate records. 45 Local flood warning system: A network of stream and rain gauges usually linked with one or more computerized terminal systems to monitor hydrologic events as they occur. These data, in conjunction with historical data and seasonal trends, provide a basis for flood prediction and forecasting. Manual weather observation: An observation of a meteo- rological parameter such as temperature or precipitation that is made with the human eye and recorded by hand before it is forwarded to the NWS. Meteorology: The science concerned with weather and at- mospheric phenomena. NEXRAD base reflectivities: Raw radar data obtained by the receiver (dish) and converted to output by the radar data acquisition computer. Base reflectivity is propor- tional to the sixth power of the diameter of rain drops encountered by the radar beam. Operational test and evaluation: Demonstration of a cen- trally developed system at selected field offices, per- formed prior to nationwide implementation to ensure that field office operational requirements have been met. Precipitation gauge: A device that measures the water- equivalent depth of precipitation that has fallen in the frozen (e.g., snow) or liquid (i.e., rain) states. Probabilistic QPF (pQPF): A form of QPF (see definition for QPF below) that includes an assigned probability of occurrence for each numerical value in the forecast product. Quantitative precipitation forecast (QPF): A numerical forecast of precipitation amount provided in a format such that the forecasted depth of precipitation is given for specified locations, depicted graphically for an area, or computed for each grid in an area that can be used as input to hydrologic forecast models or other applications. Restructuring: In reference to NWS field offices, the pro- cess of converting from the two-tiered Weather Service Forecast Office-Weather Service Office configuration to the single-tiered Weather Forecast Office configuration. Risk reduction: In the context of the NWS modernization, the process of implementing newly developed software and hardware technology and operations at selected field offices to demonstrate their operational viability and to obtain forecaster input on needed refinements prior to widespread implementation at field offices. River crest: The highest stage or level of a flood wave as it passes a given stream location. River-flow modeling: The continuous process by which flows and corresponding river stages are numerically simulated (see also "hydrologic model"~.
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46 ASSESSMENT OF HYDROLOGIC AND HYDROMETEOROLOGICAL OPERATIONS AND SERVICES River forecasts: One or more numerical values prepared to predict the level (stage), flow rate (discharge), or veloc- ity at a given location along a river for a future time or time period. Severe weather: Weather activity of an exceptional nature that exceeds predetermined criteria at which life and/or property are known to be threatened. Site-specific: Term used in conjunction with the words "forecast" or "warning" to highlight the fact that a hy- drologic (stream) forecast is produced for an individual stream gauge location as opposed to a general area (e.g., a city or county), as is commonly done in many types of weather forecasts. Spatially lumped parametric models: In the context of hydrologic models, one that assumes that each hydro- meteorological input (e.g., rainfall) or basin variable state (e.g., soil moisture) can be represented by a param- eter that characterizes average or aggregate conditions across the entire basin. Stage I (precipitation processing): The first level of pre- cipitation processing, occurring within the NEXRAD computer and performed for each volume scan of the radar. Base reflectivity data are converted to a precipita- tion estimate for each grid in the radar umbrella, using a complex algorithm that includes quality control proce- dures, a ZIR relationship, and a bias adjustment using data from a ground-based precipitation gauge network. Several graphical and digital products are produced for Weather Forecast Office operations and subsequent processing. Stage II (precipitation processing): The second level of precipitation processing, occurring within the Weather Forecast Office Advanced Weather Interactive Process- ing System and performed on an hourly basis. Stage I precipitation estimates are further refined using data from additional precipitation gauges and other sources of information such as rain/no rain determinations from satellite imagery. Stage II may also be executed at RFCs for backup purposes. Stage III (precipitation processing): The third level of pre- cipitation processing, performed interactively at RFCs. Stage II precipitation estimates from multiple NEXRADs are mosaicked into an RFC-wide product for use in river basin hydrologic modeling operations. RFC forecasters can review the mosaicked product, in- teractively edit areas of bad data, and substitute gauge-only data fields into portions of the "mosaicked" radar-based product. State variable (also variable state): The current numerical value simulated for a given unknown (e.g., soil mois- ture, snow-water content) in the hydrologic modeling process. The state of hydrologic model variables at any given time control how the model will simulate response to new input such as precipitation or snowmelt. Stream gauge: Any site along a stream where the stage (water level) is read either manually, by eye, or mea- sured with recording equipment. Technology transfer: The process of providing hydrologic models, systems, and procedures developed for NWS operations to the private sector, to other federal, state, re- gional, and local agencies, and to international interests. Telemetered data: Hydrometeorological data, measured by a conventional sensor such as a rain gauge or thermom- eter, but which are read, digitized, stored, and transmit- ted electronically. Telemetry networks: A collection of hydrometeorological sensors that have the capability to transmit data electroni- cally. Transmission of data may be by land phone lines, satellites, or land-based radio-microwave equipment. Trace: A hydrograph or similar plot for an extended-range time horizon showing one of many scenarios generated through an ensemble forecast process. ZIR relationship: The empirical conversion relationship between radar reflectivity and precipitation rate.