Projected Changes in the Timing of Autumn Rain, Wind, and Their Co-Occurrence Over western North America

In the western United States, the largest and most severe wildfires are typically associated with strong wind events that occur when fuels are dry. Across the coastally adjacent parts of the region, these conditions are most common from late summer through fall, prior to the onset of rain. While the likelihood of strong wind events increases from late summer into fall as atmospheric baroclinicity strengthens, so does the likelihood of precipitation, making the timing of wind in relation to rain of critical importance for wildfire risk. In this study we assess future projections of changes in the timing of rain and wind in a suite of downscaled climate models. Models are found to represent historical patterns of rain and wind timing reasonably well, with some biases. Models project that by the end of the century under an aggressive but plausible warming scenario, the average date of the first strong wind day will occur later in the year. A majority of models also show a shift to a later onset of fall rains over the Pacific Northwest, and drier conditions in Southern California in November. The cooccurrence of strong wind and dry conditions is found to increase slightly in southern California and decrease in most of the Pacific Northwest, but models disagree on statistical significance and sign of change. However, with drier conditions lasting longer into the fall in some regions, a longer window of opportunity for wind to co-occur with dry conditions relative to the current climate is possible.