Significant Flood Outlook Product

The NWS will provide a graphical product identifying areas where there is the potential for significant river flooding over a 5-day period. It is intended to provide a "heads up" to users like emergency managers who may need to be aware of where flood problems might occur. Because outlooks 5 days in the future have a considerable degree of uncertainty, it should not be used for detailed planning.

Because hydrologic conditions vary greatly from one part of the country to another, the NWS relies on 13 different River Forecast Centers (RFC) to provide hydrologic predictions for the country. This allows them to focus on locally unique conditions that can impact forecasts and outlooks. Each day an appraisal will be made at each RFC of how forecasts of future precipitation could affect river levels. During winter months, the effects of river ice, and in particular, ice jams, as well as snow melt will also be considered. Assessment of these conditions will be translated into an evaluation of how these factors will affect future river levels.

When the forecaster feels rises will cause significant river flooding, s/he will identify those areas at risk on a map. Based on the forecaster’s confidence, significant flooding areas identified on the outlook map will be characterized as either possible or likely. When flooding is described as likely, it indicates that the forecaster expects there is at least a 50% chance of significant river flooding someplace in the area identified. It does not necessarily mean that all rivers in the risk area will see significant flooding. If the forecaster is less confident that significant flooding will occur, the area will be characterized as including rivers where significant river flooding is possible. Specifically, in the judgment of the forecaster, there will be a 30-50% chance of significant river flooding someplace in the area indicated.

After coordination among the RFCs, input for all of the contiguous 48 states will be combined into a single map. There will be an additional map showing flood potential for the rivers of Alaska.

The outlook was developed with the emergency management community in mind. It is intended to identify when and where river flooding is expected to be severe and extensive enough to require action by emergency managers. Significant flooding is defined as flooding that adversely affects roads, residential, commercial, industrial and/or agricultural areas and may require evacuation of people and/or livestock. Significant flooding includes moderate and major flooding categories commonly used by the NWS. Minor flooding is not included in the outlook as it implies only minimal or no property damage, with possibly some public inconvenience.

The outlook also does not address flash flooding. It is not included because it is usually due to localized, intense rainfall that is very difficult to predict more than a day or so in the future. Outlook information is already available in the form of flash flood guidance issued by RFCs and both QPF (quantitative precipitation forecasts) and excessive precipitation guidance issued by HPC.

What do you mean by moderate or major flooding?

Moderate flooding indicates inundation of secondary roads; transfer to higher elevation necessary to save property, some evacuations may be required.

Major flooding causes extensive inundation and property damage, usually characterized by evacuations of people and livestock and the closure of both primary and secondary roads.

The outlooks are only intended to identify broad areas where forecast conditions indicate the possibility of flooding. In most cases, not all rivers in the area identified will experience significant flooding, and the outlook does not attempt to identify specific locations that will flood. The forecaster will provide an assessment of how susceptible an area is to flooding by characterizing flooding as either being possible or likely.

Because many factors must to come together to create significant flooding, the flood outlook will sometimes produce "false alarms." This is a consequence of its intended purpose: to alert users to areas that should be monitored. River forecasts with greater reliability are issued by NWS forecast offices for almost 4,000 specific river locations.

Because the flood outlook provides a broad-brush assessment of flood potential, it should not be used for detailed planning. Although sharp lines are used to identify areas of flood potential, in reality these are only approximations of the area at risk. Uncertainty will generally be greater around the edges of an area identified as having potential to flood.

Also, normally not all rivers in the areas with significant flood potential will flood. Within a risk area, there will be a range of rivers, from small streams to larger rivers. Larger rivers require more precipitation and runoff to cause them to flood. Therefore, localized rain may cause smaller streams to flood, but have minimal impact on very large rivers. On the other hand, widespread rain may cause only moderate flooding on many smaller rivers and tributaries, but, if there are enough tributaries contributing to flow into a major river, the cumulative effect may result in significant flooding of the larger river.

The outlook will be distributed using traditional NWS dissemination options (i.e., NOAA Port, Family of Services, EMWIN) and on Web pages on the Internet. The Web presentation will feature a "drill down" capability. Users who click on a specific point on the national map will be connected to the map produced by the appropriate RFC. Finally, the RFC Web page will provide access to local information produced by NWS Weather Forecast Offices.

More questions -- contact:

Frank Richards
(301) 713-1630x128
Francis.Richards@noaa.gov