ForWarn is a satellite-based monitoring and assessment tool that provides an overview of potential forest disturbances for the conterminous U.S. ForWarn produces maps every eight days, using satellite imagery to recognize and track changes in vegetation health and status.


Forest change maps produced at eight day intervals, with forest change measured as the departure from seasonal 'greenness' at a location relative to three different historical baselines (% difference Normalized Difference Vegetation Index or NDVI). Products also include an archive of past change maps, an archive of seasonal vegetation phenology maps, and many other disturbance maps from other sources.

Developed by

The U.S. Forest Service Eastern Forest and Western Wildland Environmental Threat Assessment Centers, in partnership with other federal and academic organizations (see below).


Online map viewer that requires no user ID or password, and WMS and WCS servers for GIS applications.


Conterminous U.S.

Scale (range)

MODIS pixel (231 meter nominal resolution)

Training Requirement

2 (scale of 1-3). Moderate time investment < 1 week


Released for use

Potential Applications

Identifying forest disturbances and threats across the nation’s landscape.

Caveats, Restrictions

ForWarn is sensitive to detecting all disturbances, therefore false positives are possible; the tool becomes more sensitive when longer baselines are used. Clouds may look like disturbances, however a true color cloud assessment product is available to detect where this may be occurring. The  24-day temporal maximum compositing may cause a delay in seeing known disturbances.

Overview and Applicability

ForWarn is a satellite-based monitoring and assessment tool that provides an overview of potential forest disturbances.  ForWarn produces national maps (lower 48 states) every eight days, using NASA MODIS (Moderate Resolution Imaging Spectroradiometer) satellite imagery to recognize and track changes in vegetation health and condition.  Current satellite "greenness" is compared with expected or maximum greenness seen over a historical baseline period. Maps in the form of percent of expected greenness (% difference in Normalized Difference Vegetation Index or NDVI) are delivered through a suite of three products, using the web-based Forest Change Assessment Viewer.  A users guide is provided to help interpret these maps and values for changes in seasonal greenness. Maps from outside sources detailing specific disturbances (e.g. wildfire locations) are also provided.

ForWarn is intended to complement existing forest monitoring programs, helping to direct attention and resources to locations where forest behavior seems unusual or abnormal. Many forest disturbances are expected to increase with climate change, including insects, diseases, wildfires, and extreme weather.

Maps generated by Forwarn can provide an overview of potential forest disturbances.Maps generated by Forwarn can provide an overview of potential forest disturbances.



ForWarn was developed by the U.S. Forest Service Eastern Forest and Western Wildland Environmental Threat Assessment Centers in partnership with NASA Stennis Space Center, US Geological Survey, the US Department of Energy, and the University of North Carolina Asheville’s National Environmental Modeling and Analysis Center. The prototype version of ForWarn has been operating successfully since January 2010. The tool was officially released online and to the public in March of 2012.

Inputs and outputs

The principal historical input is NASA MODIS (Moderate Resolution Imaging Spectroradiometer) MOD 13 satellite imagery, which is compared with current views obtained from eMODIS, from the USGS EROS Data Center. The web-based viewer also provides access to datasets from other sources, including USGS drought assessment products and U.S. Forest Service aerial disturbance surveys.

ForWarn standard products include near real-time forest disturbance maps (produced at eight day intervals), showing potential forest disturbances of different ages.  These disturbance maps show the prevailing conditions compared to a similar 24-day period of time during one of three possible historical baselines. The baseline ‘˜normals’ are the maximum ‘˜greenness,’ as measured by the Normalized Difference Vegetation Index (NDVI) during the prior year, the last three years and the full period of record (nine years). An archive of forest change datasets dating back to early 2010 are accessible under the theme name "Archive of Forest Disturbance Detection Maps."

ForWarn products can be accessed through the project’s Forest Change Assessment Viewer using a standard internet browser. For more advanced GIS users, data can also be accessed through a Web Map Service (WMS). This allows ForWarn data to be viewed apart from the Assessment Viewer, such as on a desktop GIS.  Users can also download a KML file for a subset of data products, and can save and share what they are looking at with others in email, documents, and web pages by using the "Share this map" URL feature.

For a detailed discussion of ForWarn products, or to access these map products, see the ForWarn data page.

Restrictions and limitations

Before using the Assessment Viewer to access and interpret products, please read the Forest Change Assessment Viewer Users Guide.

This tool is meant to provide a national warning system on forest disturbances. Therefore, ForWarn will suggest that there is a potential disturbance whenever the current greenness is less than the maximum greenness seen during the baseline period. The longer the baseline, the more sensitive ForWarn becomes. Users should be aware of this sensitivity and the possibility of detecting a disturbance where there is none.

Clouds may also act to produce false forest disturbances in particular locations and periods.  A True color cloud product is produced and available within the Assessment Viewer for locating cloud contamination within potentially disturbed areas.  Persistence of disturbance in a particular location through multiple 8-day ForWarn products usually means the disturbance is not caused by clouds.  There may be a delay of up to 3 8-day intervals in detecting a particular known forest disturbance, due to the 24-day maximum compositing period that is used by ForWarn to obtain cloud-free satellite views of current forest greenness.

A help video on how to use and interpret ForWarn results shown in the Assessment Viewer, and how to attribute the most-likely causes of particular disturbances is available below.

Accessing the tool and additional information

Please visit the ForWarn webpage to learn more:

A series of nine help videos are available at demonstrating various features of the Assessment Viewer. A special contact email,, has been created for users to communicate with both Threat Centers in a timely way regarding interpreting ForWarn results.

Be sure to read the Forest Change Assessment Viewer Users Guide before using online data products.