Flood warning

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about whether warnings of floods should be issued to the general public or whether previous warnings should be rescinded or retracted.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 flood warning can often be 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, both 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* The need to avoid situations where a warning condition is rescinded only for the warning to be re-issued within a short time, again because of the wasted efforts of the general public and because such occurrences would bring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 subsystems for:
* flood forecasting;
* automatic alerting of internal staff;
* tracking of alert messages and acknowledgements received;
* diversion of messages to alternates where no acknowledgement received.

National Flood Warning Services

The type of flood warning service available varies greatly from country to country, and a location may receive warnings from more than one service.

United Kingdom

For example, in the UK, the UK Met Office may issue warnings of flooding likely to occur because their meteorological forecasts of rainfall indicate that heavy rainfall will occur - such forecasts might be available 6 to 24 hours or more before the predicted rainfall, but are not fully reliable as to whether or precisely where such rainfall will occur. Thus such warnings would need to appropriately cautious and would have to apply to wider regions than will eventually be affected by the heavy rainfall. In conjunction with this service, more location-specific flood warnings are the role of the Environment Agency (covering England and Wales) who undertake flood forecasting tasks over shorter lead-times, on the basis of:
* observed rainfall over the recent past;
* forecasts of rainfall over lead-times of up to about 6 hours;
* current and past river conditions.

In the UK, the dissemination of flood warnings is moving or has moved towards a service whereby those potentially at risk of river flooding can pre-register to receive warnings by phone from an automatic system. Both warnings and updates about current conditions are also carried by local radio stations. In addition, internet sites have pages showing what locations have flood warnings in place and the severity of these warnings.

[http://www.environment-agency.gov.uk/subjects/flood/floodwarning/ Environment Agency (EA) Flood Watch (live warnings) - England/Wales]

[http://www.environment-agency.gov.uk/subjects/flood/ EA scheme description]

[http://www.sepa.org.uk/flooding/warnings/ Scottish Environment Protection Agency (SEPA) Live Flood Warning Information]

[http://www.sepa.org.uk/flooding/schemes/ SEPA scheme description]

Éire

[http://www.flooding.ie/prepared4.htm Flood warning for Ireland (Eire)]

Others