# Flood warning

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about whether warnings of floods should be issued to the general public or whether previous warnings should be rescinded or retracted.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* The need to avoid situations where a warning condition is rescinded only for the warning to be re-issued within a short time, again because of the wasted efforts of the general public and because such occurrences would bring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.

## By country

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

### Philippines

In the Philippines, the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) branch of the Department of Science and Technology (DOST) issues general flood advisory (for non-telemetered river basins whenever there is a significant amount of rainfall recorded).
Flood Warnings issued by the PAGASA :
| Warning name | Description | Forecast | Action/Response |
| Flood Monitoring | Telemetered: Slow rise in water level but still below alarm level Non-Telemetered: Monitor for possible flooding area | Telemeterd: Flood is possible Non-Telemeterd: Light to Moderate Rainfall | Non-Telemetrd: Advised to take Precautionary Measures |
| Flood Alert | Telemetered: Water level is continuously rising but still below critical level Non-Telemetered: Alert for possible flash floods and landslides | Telemeterd: Flood is threatening Non-Telemeterd: Moderate to Heavy Rainfall | Non-Telemeterd: Advised to be alert for possible flood, flash flood and landslides |
| Flood Warning | Telemetered: Water level is above critical level Non-Telemetered: Flood is occurring immediate action is recommended | Telemeterd: Flood is occurring Non-Telemeterd: Heavy to Intense Rainfall | Non-Telemeterd: Advised to take appropriate action |
| Severe Flood Warning | Telemetered: Water level is continuously rising above critical level Non-Telemetered: Flood is persisting force evacuation is recommended | Telemeterd: Flood is persisting Non-Telemeterd: Intense to Torrential Rainfall | Non-Telemeterd: Advised to force evacuation |
| Final | Telemetered: Slow recession of water level Non-Telemetered: Light rains | Flood is no longer possible | |

### United Kingdom

Arrangements for flood warnings vary across the United Kingdom with several agencies leading on warnings for emergency responders and the public. The Environment Agency, Natural Resources Wales and Scottish Environment Protection Agency all undertake location specific flood warning activities for communities at risk depending upon the scale of flood risk, technical challenges and investment needed to deliver a reliable service.
Prior to issuing a flood warning consideration is given to:
* the needs of communities to activate emergency response plans
* the nature of the catchment or coastline and the lead time that may be provided
* meteorological observations and forecast information on rainfall and coastal water levels
* hydrological observations and flood forecasts
* reference to thresholds of historic or forecast flood levels
Dissemination of flood warnings has moved towards a service whereby those at risk can pre-register to receive warnings by phone, email or text message from an automatic system, Floodline. Both warnings and updates about current conditions are also carried by local radio stations. In addition, live updates are carried by the Environment Agency's website, showing which locations have flood warnings in place and the severity of these warnings.
There is currently no flood warning system in Northern Ireland, but the Met Office does issue weather warnings. Flood risk management is the responsibility of Rivers Agency in Northern Ireland. Consideration will be given to the introduction of a warning system as part of the implementation of the EU Floods directive.

### United States

In the United States, the National Weather Service issues flood watches and warnings for large-scale, gradual river flooding. Watches are issued when flooding is possible or expected within 12–48 hours, and warnings are issued when flooding over a large area or river flooding is imminent or occurring. Both can be issued on a county-by-county basis or for specific rivers or points along a river. When rapid flooding from heavy rain or a dam failure is expected, flash flood watches and warnings are issued.
In the U.S. and Canada, dissemination of flood warnings is covered by Specific Area Message Encoding (SAME) code FLW, which is used by the U.S. Emergency Alert System and NOAA Weather Radio network and in Canada's Weatheradio Canada network.
"Flood statements" are issued by the National Weather Service to inform the public of flooding along major streams in which there is not a serious threat to life or property. They may also follow a flood warning to give later information.

#### Example of a flood warning

The following is an example of a "Flood Warning." The Sabine River is used as an example:
Source:

#### Example of a flood statement

Source:

### Iowa Flood Center

The Iowa Flood Center at the University of Iowa operates the largest real-time flood monitoring system of its kind in the world. It includes more than 200 real-time stream stage sensors that feed data into the Iowa Flood Information System where data can be viewed, online, by disaster management staff and the general public. The stream stage sensors, mounted on bridges and culverts, use ultrasonic sensors to monitor stream and river levels.

### Australia

In Australia, flood warnings are handled by the Bureau Of Meteorology as well as local councils. The method of distributing the warning can vary but is usually transmitted by radio, social media and dedicated apps.