Multifactor Change in Western U.S. Nighttime Fire Weather

Abstract Reports from western U.S. firefighters that nighttime fire activity has been increasing during the spans of many of their careers have recently been confirmed by satellite measurements over the 2003–20 period. The hypothesis that increasing nighttime fire activity has been caused by increased nighttime vapor pressure deficit (VPD) is consistent with recent documentation of positive, 40-yr trends in nighttime VPD over the western United States. However, other meteorological conditions such as near-surface wind speed and planetary boundary layer depth also impact fire behavior and exhibit strong diurnal changes that should be expected to help quell nighttime fire activity. This study investigates the extent to which each of these factors has been changing over recent decades and, thereby, may have contributed to the perceived changes in nighttime fire activity. Results quantify the extent to which the summer nighttime distributions of equilibrium dead woody fuel moisture content, planetary boundary layer height, and near-surface wind speed have changed over the western United States based on hourly ERA5 data, considering changes between the most recent decade and the 1980s and 1990s, when many present firefighters began their careers. Changes in the likelihood of experiencing nighttime meteorological conditions in the recent period that would have registered as unusually conducive to fire previously are evaluated considering each variable on its own and in conjunction (simultaneously) with one another. The main objective of this work is to inform further study of the reasons for the observed increases in nighttime fire activity. Significance Statement Western U.S. firefighters have reported a problematic rise in nocturnal wildfire activity. Verifying their hypothesis that meteorological variability is responsible is a first step toward better understanding the predictability of the underlying processes. This study expands upon our previous investigation of multidecadal change in seasonally averaged nocturnal vapor pressure deficit by looking at changes in the frequency of dry-fuel nights over the western United States and their coincidence with other fire-conducive nocturnal meteorological conditions. Dry-fuel nights have become >10× more frequent in the 2010s compared to the 1980s and 1990s in some regions. Over 81% of the study area, increasing dry-fuel night frequency has been compounded by the double, or triple, threats of simultaneously windier and deeper planetary boundary layers.