Abstract : Mass fires are likely to spread rapidly and burn intensely when strong winds are combined with low humidities and high temperatures, particularly after a rainless period. To identify synoptic weather types that create such periods of critical fire weather, the 48 contiguous states were divided into 14 regions and fire danger indexes were computed from weather data at 89 stations for the years 1951-60. Surface weather types and upper-air patterns associated with high fire danger are described for each region.