A small percentage of wildfires account for a large majority of total area burned in California, and an even smaller percentage account for a majority of detrimental impacts to human communities and natural ecosystems. Often these impacts are associated with extreme fire growth on one to two days, leading to so-called “megafires”. What conditions allow for such rapid fire growth? This question is critical because occurrence of megafires appears to be increasing in spite of increasing money, labor, and technology used to suppress and contain fires (expenditures reached an all-time high of $10.4B in 2020). Our research will investigate whether there is a discrete set of weather, fuel, topographic, and human factors that lead to the extremely rapid growth of these fires. Because this extremely rapid fire growth is often limited to one or a few days per large fire, it will be crucial to evaluate fire spread on a daily basis, or even a finer, subdaily or hourly basis. The core activity of this project is to use such a finer-time-scale analysis to identify the fuels, weather, and/or firefighting resources conditions associated with rapid fire growth. Once identified, these conditions could be used to target wildfire mitigation efforts and to alert fire managers and the public of extreme fire risk in particular locations or during high-risk time periods.