The spring flood risks portrayed in this map are explained in the National Hydrologic Assessment issued by the National Oceanic and Atmospheric Administration (NOAA), National Weather Service (NWS) on March 20, 2025 (https://www.weather.gov/owp/2025NHA). This map shows the basins with a 50% or more likelihood of exceeding significant flooding (defined as moderate and major flood levels) during April through June, 2025. The National Hydrologic Assessment offers an analysis of flood risk and water supply for Spring 2025 based on late summer, fall, and winter precipitation, frost depth, soil saturation levels, snowpack, current streamflow, and projected spring weather. NOAA's network of 122 Weather Forecast Offices, 13 River Forecast Centers, National Water Center, and other national centers nationwide assess this risk and summarize at the national scale. 

The 2025 NOAA National Spring Flood Outlook Dataset is generated using the "Long-Range River Flood Risk" for approximately 2,600 river and stream forecast locations across the nation for this spring provided by the National Weather Service.  The "Long-Range River Flood Risk" from the river and stream forecast locations are then extrapolated to a basin scale using the Watershed Boundary Dataset (WBD)'s hydrologic unit codes (HUC) provided by the USDA-NRCS, USGS, and EPA.  The current "Long Range River Flood Risk" is available at: https://water.noaa.gov. Using the "Long Range River Flood Risk", stakeholders can access a single, nationally consistent map depicting the 3-month risk of minor, moderate, and major river flooding.  This risk information is based on NOAA's Ensemble Streamflow Prediction (ESP) forecasts which are generated for approximately 2,600 river and stream forecast locations across the nation. With this capability, stakeholders can quickly view flood risk predicted to affect their specific area of concern.

The Long-Range River Flood Risk improves the value of the National Hydrologic Assessment by clearly and objectively communicating flood risk at the local level. The National Weather Service (NWS), in coordination with local officials, defines flood levels for each of its river forecast locations, based on the impact over a given area. The flood categories are defined as follows:

Minor Flooding: Minimal or no property damage, but possibly some public threat (e.g., inundation of roads).

Moderate Flooding: Some inundation of structures and roads near the stream. Some evacuations of people and/or transfer of property to higher elevations.

Major Flooding: Extensive inundation of structures and roads. Significant evacuations of people and/or transfer of property to higher elevations.