has fallen as a function of time. The tipping bucket is so-called because a small bucket on an arm-like device tips and empties each time 0.01 inch of rain accumulates. The weighing and tipping-bucket types of rain gauges can be equipped to report automatically to the NWS each hour or more frequently, 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. During high rainfall rates, the tipping bucket may not be able to keep up with the intensity of the rain and result in an underestimate. In high wind conditions, some rain may blow over the gauge, even if wind shields are positioned around it. Spatial variations in rainfall due to topographic or other factors may also make the gauge value unrepresentative of the basin-average rainfall that contributes to flood conditions. Thus, the blending of radar and rain gauge data tends to give the most complete picture of the basin-average rainfall.
During a rain event, operational meteorologists and hydrologists also monitor measurements of streamflow and stage that are available at strategic locations on streams and rivers. Streamflow indicates the volume of water flowing down the channel in a fixed amount of time, such as cubic meters per second. Stage indicates the water level in the stream or river and is compared against flood stage values, beyond which the water overflows the banks and flooding begins. In the case of slower-evolving stream and river floods, as opposed to flash floods, NWS forecasters have used floodplain mapping and experience to determine what areas will be affected for various stage values, and they are able to convey this information to the public in flood warning messages. Reports also are received from trained spotters and observers, emergency managers, law enforcement officials, and the public to indicate where flooding has begun or is imminent.
In summary, NWS forecasters go through an extensive, multiday process of assessing the flash flood potential of approaching weather systems. They monitor numerous data from a myriad of sources and assess models and real-time observations in order to determine whether a flash flood situation is developing. The forecast process is highly regionalized in that forecasters’ assessments are based on their local knowledge and experience, which often serve as the most valuable and effective forecasting tools.