SDI (Rodriguez y Silva et al. 2020) factors in topography, fuels, expected fire behavior under prevailing conditions, fireline production rates in various fuel types with and without heavy equipment, and access via roads, trails, or cross-country travel. SDI is currently classified into six categories representing low through extreme difficulty. Extreme SDI zones represented in red are “watch out” situations where engagement is likely to be very challenging given the combination of potential high intensity fire behavior and difficult suppression environment (high resistance fuel types, steep terrain, and low accessibility). Low difficulty zones represented in blue indicate areas where some combination of reduced potential for dangerous fire behavior and ideal suppression environment (low resistance fuel types, mellow terrain, and high accessibility) make suppression activities easier. SDI does not account for standing snags or other overhead hazards to firefighters, so it is not a firefighter hazard map. It is only showing in relative terms where it is harder or easier to perform suppression work. SDI incorporates flame length and heat per unit area from basic FlamMap runs (Finney et al. 2019). SDI is based on fire behavior modeled using regionally appropriate percentile fuel moisture conditions and uphill winds. This product uses the wind blowing uphill option to represent a consistent worst-case scenario. Input fuels data are updated to the most recent fire year using a crosswalk for surface and canopy fuel modifications for fires and fuel treatments that occurred after the most recent LANDFIRE version. For example, LANDFIRE 2016 model inputs are modified to incorporate fires (Monitoring Trends in Burn Severity (MTBS), Geospatial Multi- Agency Coordination (GeoMac), and Wildland Fire Interagency Geospatial Services (WFIGS) and fuel treatments (USFS Forest Activity Tracking System (FACTS) and DOI National Fire Plan Operations and Reporting System (NFPORS) hazardous fuels reduction treatments) from 2017-present. Road and trail inputs are developed from a combination of HERE 2020 Roads, USFS, and DOI road and trails databases. Hand crew and dozer fireline production rates are from FPA 2012 (Dillon et al. 2015). Classification of topography and accessibility thresholds are detailed in Rodriguez et al. (2020). Dillon, G.K.; Menakis, J.; Fay, F. (2015) Wildland Fire Potential: a tool for assessing wildfire risk and fuels management needs. In: Keane, R.E.; Jolly, M.; Parsons, R.; Riley, K., eds. Proceedings of the large wildland fires conference; May 19-23, 2014; Missoula, MT. Proc. RMRS-P-73. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 345 p.Finney, M.A.; Brittain, S.; Seli, R.C.; McHugh, C.W.; Gangi, L. (2019) FlamMap:Fire Mapping and Analysis System (Version 6.0) [Software]. Available from https://www.firelab.org/document/flammap-softwareRodriguez y Silva, F.; O'Connor, C.D.; Thompson, M.P.; Molina, J.R.; Calkin, D.E. (2020). Modeling Suppression Difficulty: Current and Future Applications. International Journal of Wildland Fire.