A flood warning is closely linked to the task of flood forecasting. The distinction between the two is that the outcome of flood forecasting is a set of forecast time-profiles of channel flows or river levels at various locations, while "flood warning" is the task of making use of these forecasts to make decisions about whether warnings of floods should be issued to the general public or whether previous warnings should be rescinded or retracted.
The task of providing warning for floods is divided into two parts:
decisions to escalate or change the state of alertness internal to the flood warning service provider, where this may sometimes include partner organisations involved in emergency response;
decisions to issue flood warnings to the general public.
The decisions made by someone responsible for initiating flood warnings must be influenced by a number of factors, which include:
The reliability of the available forecasts and how this changes with lead-time.
The amount of time that the public would need to respond effectively to a warning.
The delay between a warning being initiated and it being received by the public.
The need to avoid issuing warnings unnecessarily, because of the wasted efforts of those who respond and because a record of false alarms means that fewer would respond to future warnings.
The need to avoid situations where a warning condition is rescinded only for the warning to be re-issued within a short time, again because of the wasted efforts of the general public and because such occurrences would bring the flood warning service into disrepute.
A computer system for flood warning will usually contain sub-systems for:
flood forecasting;
automatic alerting of internal staff;
tracking of alert messages and acknowledgements received;
diversion of messages to alternates where no acknowledgement received.
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