Comparison of the Labor Day 2020 cold front and fire event to similar past events

Mark Ingalls, AgWeatherNet Intern

The Labor Day cold front that moved in from the northeast (rather than the typical westerly/northwesterly direction) and helped to initiate numerous fires in Washington and Oregon was unusually potent  to the point where it may have been a generational event. It was not, however, entirely unprecedented. Strong weather systems from the Canadian interior have initiated or enhanced fire activity in the Pacific Northwest in the past. This includes playing a contributary role in some of the largest fires to occur in the region in the early 20th Century.

Meteorological background
Before diving into years past, it is important to know what occurred on Labor Day to have something to compare to. Conditions leading up to the event were dry, with drought conditions in place in Central Washington and much of Oregon.

On September 7, a low pressure system centered near the southern end of the Hudson Bay was draping a cold front across much of North America. This is the same cold front that dropped Denver from 93° to 31° within a 24-hour period, produced snow in the Northern Plains and Rockies, and heavy rain in Oklahoma.

Washington and Oregon were not graced with the moisture that was seen on the east side of the Rocky Mountains. With the cold front advancing into the Pacific Northwest from Alberta, the moisture was drained from the air as it crossed the mountains that stand between Spokane and Calgary. Relative humidity values dipped to below 10% in spots, but extremely dry conditions were not confined just to Eastern Washington. The AgWeatherNet site in Vancouver recorded 11%, and the airport in Tillamook on the Oregon Coast recorded 9% relative humidity. Dew points were so low in Tillamook on September 8 that relative humidity values there only peaked at 20%.

That was not the only effect the mountains had on the system, however. Simply put, large mountain ranges like the Rockies form a dam for advancing air, forcing it into narrow valleys and passes causing what would otherwise be breezy conditions to accelerate. This is the same process as what causes windy conditions in the Kittitas Valley and through the Columbia River Gorge as westerly systems move through the region.

While many of the fires are likely to have been started through some form of human activity (deliberate and otherwise), it was these strong and dry easterlies that spread the fires quickly. Following the frontal passage on Labor Day, high pressure in the Northern Plains and Canadian Prairies dominated wind flow patterns in the Pacific Northwest. This kept winds easterly and northeasterly, further spreading fires. While some smoke was observed in Washington during this period, it wasn't until winds shifted southerly and southwesterly a few days later that it became hazardous for most locations in the state.


Smoke

This article will not look deeply into comparing regional smoke conditions to past events. Historical air quality data does not extend very far into the past and a survey of contemporary sources for the events compared to below does not offer any information about smoke conditions. It is assumed that the lack of information infers dense smoke was not observed at a region-wide level, but this is just an assumption. There probably is information hiding in newspaper articles somewhere, but those were not readily available.

That said, weather data and pictures of smoke from throughout the Northwest following Labor Day bring to mind memories of the 1980 eruption of Mount St. Helens, whether that is direct experience for those who lived through that event, or stories told to those who were born or moved to the region later. In 2020, such as Salem experienced significantly darkened skies hours before sunset on some days. In 1980, the Tri-City Herald reported "The sun blacked out Spokane about 3 p.m. Sunday and an hour later a fine gritty ash began sifting to the ground."

Volcanic ash is significantly different than wildfire smoke and ash so even if air quality records did exist from Eastern Washington in 1980, it would be hard to compare something like air quality index values between 2020 and that event. Volcanic ash consists primarily of pulverized rock rather than the burned remains of vegetation and structures. Returning to the May 19, 1980 edition of the Tri-City Herald: "A Richland lung specialist said most of the particles falling here are too large to be inhaled. But some finer particles could cause lung irritation." By contrast, wildfire smoke contains much smaller particles that are easier to inhale and would read differently on air quality sensors.

September 1902

A low pressure system located where Alberta, Saskatchewan, and Montana meet on the morning of September 7 had produced a strong cold front the crossed the Pacific Northwest. Winds were noted as being easterly at Spokane, Seattle, and Victoria, with winds at Portland becoming easterly between 5 am on the 7th and the same time on the 8th. The presence of the cold front is noted in highs through the Columbia Gorge on the 7th. Highs on the day before and after the passage were in the 80s, but Portland, Cascade Locks, Hood River, and The Dalles all reported highs in the 70s on that day. Presumably the front came through that morning, bringing daytime heating to a halt.

The Yacolt Burn spread swiftly during this period, but a long period of easterly winds at the surface was not observed thanks to high pressure setting up over the Pacific Northwest in the following days. Small fires in the Columbia Gorge and Mount St. Helens areas grew together to form the Yacolt Burn and would end up growing to 500,000 acres in four days.

September 1929

A low pressure system located midway between Lake Winnipeg and the Hudson Bay had draped a cold front across the continent on September 15. The Daily Weather Map produced by the Weather Bureau the morning of the 15th notes that since the weather map from the previous morning, temperatures had dropped by 20° or more from about Calgary to Great Falls.

From the 14th to the 15th, the low pressure center had moved from Western Saskatchewan to Northern Ontario. For comparison, the track of the 2020 low took it from Southern Manitoba on September 6 to Northern Ontario on the 7th.

High pressure was in place across Montana the morning of the 15th, with northerly winds at Spokane, easterly at Seattle, and southeasterly at Portland. These conditions were at least partially responsible for the rapid spread of the Dole Valley Fire in Clark and Skamania Counties, which eventually burned 227,500 acres.

September 1951

A low pressure system located near Lake Winnipeg on September 20 draped a cold front across much of the continent, producing snow in parts of Montana and dropping temperatures significantly across much of the Western United States north of a line from Reno to Salt Lake to Denver. As the front passed, blowing dust was reported at Pendleton, demonstrating another similarity between that event and the one in 2020.

Behind the front, high pressure set up, centered over Western Montana but extending into the Canadian Rockies. This kept winds easterly and northeasterly across the Pacific Northwest for the following days, funneling dry air into the region. High winds associated with this pushed a wildfire 18 miles in just eight hours, burning 38,000 acres overnight in the Olympic Peninsula. This fire threatened Forks and was later known as the Great Forks Fire. Rainfall during the summer leading up to this event was less than 15% of normal and other large fires were also started in Oregon and British Columbia.

Conclusion

The Labor Day 2020 frontal passage and associated fire event were rare, though not unprecedented for the Pacific Northwest. Arid regions of Washington and Oregon are particularly fire prone throughout the dry season, but large fires in and west of the Cascades often require an unusually dry event combined with high winds due to the ample moisture in these areas. Though infrequent, strong cold fronts originating from the Canadian interior create the ideal conditions for such events.

Today, there is more human activity in the Cascades, leading to more individual fire starts as up to 90% of wildfires in the United States are human caused. This figure includes arson, but more frequently human-caused fire starts result from unintentional triggers such as improperly parked or maintained vehicles, electrical transmission infrastructure, campfires left unattended, etc. On top of this, the way forests are managed is far different than in decades past, leading to different fuel loads when a fire does start. Human induced climate change also can contribute to more favorable fire conditions, though recent research from the University of Washington suggests that easterly/northeasterly cold front events may become more infrequent as the climate gradually changes.

Sources:

AgWeatherNet, Washington State University

Olympic Peninsula Community Museum, University of Washington

Tri-City Herald, May 19, 1980 (in the author's personal library)

US Daily Weather Maps, National Oceanic and Atmospheric Administration

Washington State Hazard Mitigation Plan, Washington Department of Emergency Management

"Yacolt Burn, largest forest fire in recorded Washington history to that point, rages for three days beginning on September 11, 1902", HistoryLink


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