National Weather Service – Flood Severity Category Evaluation

The National Weather Service (NWS) includes in flood warnings (optionally in flash flood warnings) a category to communicate the magnitude of expected flooding on rivers or streams (NWSI 10-922, 2002). The categories and definitions are as follows:

• Minor flooding – minimal or no property damage, but possibly some public threat or inconvenience.
• Moderate flooding – some inundation of structures and roads near stream. Some evacuations of people and/or transfer of property to higher elevations are necessary.
• Major flooding – extensive inundation of structures and roads. Significant evacuations of people and/or transfer of property to higher elevations.
• Record flooding – flooding that equals or exceeds the highest stage or discharge at a given site during the period of record keeping (NWSM 10-950, 2002).

The NWS needed to understand if the current severity categories communicated necessary information for public servants and the public to take appropriate actions. In addition, the NWS needed to understand how the use of river gage station flood history along with severity categories was recognized and used by public servants and the public.

Riverside Technology, inc. (RTi) assembled a team with the expertise and experience necessary to complete this evaluation. Subcontractors David Ford Consulting Engineers, Inc. and the University of Colorado at Colorado Springs conducted the evaluations of flood severity information users and made recommendations based on those findings. They were supported by RTi hydrologists familiar with NWS flood warnings. The responses of the flood severity information users were analyzed to identify patterns, trends, and common opinions of the users. A final report titled “Evaluation of National Weather Service Flood Severity Categories and Use of Gage Station Flood History Information” was produced and provided to the NWS.

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