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Prescribed burning is a primary tool for habitat restoration and management in fire-adapted grasslands. Concerns about detrimental effects of burning on butterfly populations, however, can inhibit implementation of treatments. Burning in cool and humid conditions is likely to result in lowered soil temperatures and to produce patches of low burn severity, both of which would enhance survival of butterfly larvae at or near the soil surface. In this study, we burned 20 experimental plots in South Puget Sound, Washington, USA, prairies across a range of weather and fuel conditions to address the potential for producing these outcomes. Risk to diapausing butterfly larvae, assessed by measuring subsurface soil temperatures and heat dosages, was lower when air temperature was less than 26 degrees C and dead fuel moisture was greater than 9 %. The likelihood that unburned or low-severity patches would be left was affected by dead fuel moisture, but also required pre-existing fuel discontinuities. Burns conducted in the morning hours during the summer drought season (the main prescribed-fire season in this system) were cooler and had lower severities. This research increases our understanding of how fine fuel moisture and fuel continuity during grassland burning can affect fire intensity, severity, and spread. It also provides support for burning earlier in the day as a way to increase burn heterogeneity and has allowed us to create recommendations for burning in sensitive butterfly habitat.