Flash Flood Forecasting Over Complex Terrain With an Assessment of the Sulphur Mountain NEXRAD in Southern California (2005) / Chapter Skim
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3 The National Weather Service and Flash Floods
3 The National Weather Service and Flash Floods
Pages 26-34
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From page 26... ...
is especially relevant for supporting the NWS prior to and during heavy precipitation events. This chapter gives additional information about the NWS and its role pertaining to flash floods, including a brief description of the modernization effort that led to the development of the NEXRAD network and of the extensive process that local NWS meteorologists go through to forecast and warn of flash flood events.
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From page 27... ...
THE MODERNIZATION AND ASSOCIATED RESTRUCTURING OF THE NATIONAL WEATHER SERVICE Vast improvements in instrumentation, computer, communications, and data processing technologies since the 1950s led to much improved weather observing technologies in the 1970s and 1980s. These advances, together with a better scientific understanding of atmospheric processes, allowed for major improvements in NWS capabilities to accurately observe, forecast, and warn of severe weather.
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From page 28... ...
issuing forecasts, watches, and warnings for severe local weather hazards, flash floods, floods, heat waves, winter storms, fire weather conditions, agricultural weather conditions, and all other local weather information; 2. issuing local aviation weather forecasts and warnings as well as forecasts for en route pilots in the United States; 3.
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From page 29... ...
THE PROCESS OF ISSUING FLASH FLOOD FORECASTS, WATCHES, AND WARNINGS The NWS serves as the sole official voice for issuing warnings during life-threatening weather situations. The flash flood forecasts, watches, and warnings are issued only by the WFOs, but the RFCs and the national centers -- such as the NCEP, the HPC, the Storm Prediction Center, and the National Hurricane Center -- provide essential supporting information and guidance to the WFOs.
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From page 30... ...
Although the general idea remains the same, the collection of more and newer remotely sensed and in situ data, combined with continually improving probabilistic and deterministic tools, has greatly improved NWS procedures and lead times for issuing flash flood forecasts, watches, and warnings during the past 15 years. Successful forecasts of a heavy rainfall event depend on interpretation of the meteorological and hydrological aspects of a particular situation.
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From page 31... ...
A multitude of types of data, collected around the world, provide input to the NWP models. Measurements are made of air pressure, temperature, humidity, wind direction and speed, cloud coverage, and cloud base heights from observations at surface weather stations, buoys, and ships.
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From page 32... ...
In any case, human forecasters typically adjust and improve on the numerical guidance by using their experience, including dealing with the influence of regional terrain on the upcoming weather system. When it is determined that rainfall is imminent, forecasters must assess flash flood potential in order to decide whether and when to issue flood and flash flood watches and warnings.
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From page 33... ...
For these reasons, forecasters rely heavily on radar estimates of current and approaching rainfall intensities and accumulated amounts -- ground-truthchecked against available rain gauge data when and where possible -- in the warning process, particularly to identify small-scale areas of maximum precipitation. However, radar measurements of rainfall rate and accumulation must be considered only estimates.
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From page 34... ...
The weighing and tipping-bucket types of rain gauges can be equipped to report automatically to the NWS each hour or more fre quently, whereas many gauges staffed by human observers may report only once per day unless special provisions are made for additional reporting in heavy rain situations. Although it may seem that rain gauges should provide exact rainfall amounts, in practice their values can be in error or unrepresentative.
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