Precipitation
A mid latitude location ensures that most of the Western U.S. is exposed to precipitating weather systems from both the polar (northern) and subtropical (southern) jet streams through much of the year, although this assurance is more likely for the Pacific Northwest and less so for the southern Rockies. Although almost the whole region experiences regular precipitation, there are great geographical differences. To the east of any mountain ranges (e.g. the Rockies, Sierras, Cascades, Olympics), there is less precipitation because weather systems crossing from west to east deposit most of their precipitation on the windward slopes, and therefore, in general, areas east of mountain ranges experience more severe fires. Average annual precipitation totals vary from 38 inches in Seattle to 25 inches in San Francisco to 14 inches in Los Angeles to 9 inches in Phoenix to 15 inches in Denver.
The three-to-seven-year cycling of El Niño Southern Oscillation (ENSO) events and other periodic climate patterns expose the Western U.S. to periods with increased or decreased likelihoods of rainfall. This periodicity is favorable for growing vegetation and then drying it – creating fuel for fires. Furthermore, throughout the region, droughts can develop and exist for many months in part because they can be self-reinforcing when a dry air mass builds a dome of high-pressure on a semi-continental scale that steers precipitating systems away. Long droughts in regions with dense vegetation create conditions for severe wildfires.
Although it may seem somewhat counter-intuitive, it has been known for a long time (e.g., Crimmins and Comrie, 2004) that, in some areas, the increased growth of vegetation for fine fuels such as brush, shrubs, and tall grass due to an increase in precipitation is a direct factor in increased wildfire in subsequent seasons. This is generally found to be the case in the Southwest U.S., where there are more grassy areas for fine fuels, but less so in more densely forested areas with a higher abundance of coarse fuels. A number of studies, described below, have addressed this:
- Littell et al. (2009) noted that in the Southwest and other arid regions, moist conditions during seasons prior to a fire season have more impact than the conditions during the year that fires occurred.
- Barbero et al. (2014), as part of their comprehensive analysis to develop climate–fire relationships for very large fires, analyzed many temperature- and moisture-related parameters across many regions. They found that for two arid regions in Western U.S., the Palmer Drought Severity Index of the preceding year, which is related to the amount of precipitation in the preceding year, is an important variable for predicting fire.
- Pilliod et al. (2017) found that years with more fires and area burned tend to occur after one or more years of above-average precipitation in areas like the Great Basin (in Nevada).
- Hernandez Ayala et al. (2021) examined 20 years (from 2001-2020) of precipitation, fire, and vegetation data for California and found that in more than half the years there was above average precipitation and vegetation growth that preceded enhanced wildfire season activity.
- A study by Balch et al. (2013) found that regions with invasive cheatgrass (ubiquitous in most of the Western U.S.) show a stronger interannual increased fire response to wet years than regions with native grass species.1
Understanding how climate change will impact wildfire risk in arid fuel-limited regions is challenging, owing in part to the fact that understanding how precipitation may change in the future – particularly in the Western U.S., has some uncertainty. Unlike the expectation that temperatures will increase, which alone will cause an increase in VPD, GCMs vary considerably on basic information such as whether seasonal and even annual precipitation will increase or decrease in a given region, and how more extreme events will change, although there is more confidence that extreme rainfall events will likely increase in magnitude as well as frequency.
Almazroui et al (2021), in a study that examined how temperature and precipitation would change over the U.S. using the latest CMIP6 model simulations, found that over the Western U.S., precipitation increases, on average, by only a few percent by mid-century with not much change across the climate scenarios considered by the Intergovernmental Panel on Climate Change (IPCC). Uncertainty ranged from about -2% to +10%, with most of the precipitation increase occurring in the winter months, and drier summers. This result was essentially independent of time horizon but did become more pronounced at end-of-century.
Even though the increase in average annual precipitation seems small, because the increase is expected to be concentrated in the winters it might set the stage for fine fuels to develop. The warmer drier conditions during summer would then promote drying the vegetation into combustible fuel. This information alone suggests the wildfire risk in regions with fine fuel availability in the Western U.S., where burned area correlates with prior year or prior years' precipitation, is expected to increase.