Climatologists John Abatzoglou, David Rupp, and Larry O’Neill break down the forces that enabled Oregon’s historic fires in September 2020
John Abatzoglou, David Rupp, and Larry O’Neill
Over half an inch of rain fell over Salem, Oregon, and much of western Oregon, on September 18. In most years this ordinary rainfall event would not have attracted attention. But there is nothing ordinary about 2020. Had this rain arrived two weeks earlier, residents of western Oregon likely would not have had a front-row seat to the wildfires that tore downslope along the western flank of the Cascade Range. Instead, a set of climate and weather extremes enabled and drove one of the largest sets of fires in western Oregon in the past century. This article traces the origins of the extensive fires in western Oregon that began over Labor Day 2020, highlighting the specific roles played by multiple weather and climate extremes in enabling these fires.
During Labor Day weekend, an uncommonly strong meteorological pattern called an Omega block led to unusually warm air across the West Coast, while unusually cold air descended into Colorado and the Four Corners region. This early season event brought snow to parts of Colorado but high heat to western Washington, Oregon, and California. The pattern also contributed to strong, dry, north to easterly winds that stretched from the Canadian border into northern California. Offshore, downslope winds from the east, known locally, and rather unimaginatively, as east winds, produced warming along the western slope of the Cascade Range. The unfortunate trifecta of warm, dry, and windy conditions created extreme fire danger.
The easterly winds that began on September 7 persisted for about 48 hours, spreading into the southern part of Oregon and northern California by September 8 to become the second-strongest September wind event in at least 70 years. That said, the event was forecasted. Red Flag Warnings were issued, and some power companies, such as Portland General Electric, proactively de-energized transmission lines in high-risk areas. Nevertheless, numerous wildfires ignited. In addition, several existing fires, including the Lionshead and Beachie Creek fires, which had ignited during a widespread lightning storm in mid-August, roared downhill to the west on the afternoon of September 7.
Images from these wildfires will stay with many of us for a long time. So will the numbers: over a million acres burned, 40,000 Oregonians evacuated and a half-million under some form of evacuation notice during the event, over 4000 structures destroyed, and nine human lives lost. Millions of others were exposed to wildfire smoke. The Air Quality Index across Oregon reached levels higher than those in any other major city worldwide. The Air Quality Index in Portland was considered hazardous for three consecutive days, and unhealthy for seven consecutive days.
Climate and weather play distinct roles in individual wildfires and wildfire seasons. So too do humans. Human activity ignites fires. Throughout much of Oregon, the number of human-ignited fires during 2020 was unusually high, possibly reflecting an increase in outdoor recreation during the pandemic. Initial reports implicated downed electrical transmissions lines in igniting some of the wildfires that began during September 2020. Humans also suppress fires, particularly under more benign weather conditions. However, by September, fire-suppression capacity was stretched by wildfires in California, which included the largest fire on record in that state and the greatest annual area burned. Fire-suppression resources also were needed in the Southwest and Colorado, where the failure of the monsoon contributed to a prolonged fire season. And to the north, the same Omega block whipped up a few large wildfires in central and eastern Washington, including one that destroyed a majority of the structures in the small town of Malden.
From mid-July to September 10, the Cascade Range in Oregon was unusually warm and dry. Climatologically, summer is the warmest and driest season, but this eight-week period in 2020 was the third warmest and third driest in the past 42 years.
As a result, standing and downed vegetation became quite dry by early September. The Energy Release Component (ERC), a measure of the potential amount of energy released in a wildfire, increased along the west slope of the Cascade Range throughout summer, peaking during the second week of September. Had the previous 60 days been substantially cooler or wetter, fuels would not have carried fire as readily when east winds became strong.
Another measure, the Burning Index, incorporates fuel dryness and potential rates of wildfire spread, providing a proxy for the difficulty of fire containment. During September 2020, values of the Burning Index were the highest in at least four decades for much of western Oregon.
Large wildfires such as those that occurred during 2020 are rare, but not unprecedented, in western Oregon. Infrequent, extensive wildfires that are documented in the historical record typically coincided with similar conditions: a summer that was warmer and drier than normal and a September east wind event. The events that unfolded in Oregon this year resemble the wind-driven wildfires that often impact suburban areas in northern and southern California.
The long historical return intervals of wildfires from the crest of the Cascade Range to the coast may lull society into thinking these areas are not particularly prone to direct fire impacts. But 2020 exposed the region’s vulnerabilities. The best science available indicates that the conditions that enable large wildfires and wildfire seasons will become more common as a result of climate change and past and current land management and land use. In short, wildfire is becoming more prevalent across much of the Pacific Northwest. To address the growing risk of wildfire and other climate-related hazards, CIRC continues to work with communities that wish to plan for and adapt to wildfire and other climate impacts over the next several decades.
This post is part of Northwest Climate Currents, an ongoing series that uses the Climate Toolbox and the data it contains to help Northwest residents understand and prepare for regional climate events. The Climate Toolbox is a suite of free online applications designed by CIRC researchers and intended to inform responses to and preparation for climate variability and change and related impacts.
John Abatzoglou is a CIRC team member, climatologist, and professor at the University of California, Merced.
Larry O’Neill is Director of Oregon Climate Service and an associate professor at Oregon State University.
David Rupp is a CIRC team member, climatologist, and assistant professor at Oregon State University.
Featured Image: Image of western Oregon from the Sentinel satellite, captured on September 8, 2020. (Pierre Markuse, all rights reserved).