Weather Factors Associated with Extremely Large Fires and Fire Growth Days.

"Megafires" are of scientific interest and concern for fire management, public safety planning, and smoke-related public health management. There is a need to predict them on time scales from days to decades. Understanding is limited, however, of the role of daily weather in determining their extreme size. This study examines differences in the daily weather during these and other smaller fires, and in the two sets of fires' responses to daily weather and antecedent atmospheric dryness. Twenty fires of unusual size (over 36 400 ha), were each paired with a nearby large fire (10 100–30 300 ha). Antecedent dryness and daily near-surface weather were compared for each set of fires. Growth response to daily weather was also examined for differences between the two sets of fires. Antecedent dryness measured as the evaporative demand drought index was greater for most of the fires of unusual size than it was for smaller fires. There were small differences in daily weather, with those differences indicating weather less conducive to fire growth for the unusually large fires than the smaller fires. Growth response was similar for the two sets of fires when weather properties were between 40th and 60th percentiles for each fire pair, but the unusually large fires' growth was observably greater than the smaller fires' growth for weather properties between the 80th to 100th percentiles. Response differences were greatest for wind speed, and for the Fosberg fire weather index and variants of the hot-dry-windy index, which combine wind speed with atmospheric moisture. Significance Statement: Some wildfires grow substantially larger, and more rapidly, than the vast majority of others. This growth makes them more difficult to manage and increases their potential to cause injury, death, or property damage. We looked at how the growth of these fires relates to daily weather, either with more fire-conducive weather or with a different growth response to the weather. We found evidence suggesting that it is the growth response, not the weather itself, that differs. Results consistently pointed to high wind events as being related to disproportionate fire growth. Our results could help improve planning for these types of events both in the short term and in terms of climate impacts on fire. [ABSTRACT FROM AUTHOR]