Wildfire smoke plume spreads across a large portion of North America

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The recent explosion of wildfires across the western US, particularly in California, is generating an immense amount of smoke. The smoke can readily be seen on visible satellite. The fuzzy gray in the image below is all smoke–covering much of the California, the Rockies, northern Plains, and Upper Midwest.

NASA MODIS visible satellite image on August 24, 2020, image via Earthview.

AirNow.gov has a snazzy new fire+air quality interactive map that I highly recommend checking out. It depicts the ongoing ugly situation in California. Air quality is degraded across much of the intermountain west. Farther east, the smoke is mostly aloft.

Screenshot from fire.airnow.gov taken on August 24, 2020. Color-filled circles represent air quality.

The human impact has been severe, with evacuations complicated by the need to social distance to prevent the spread of COVID-19.

Smoke forecast

How will the smoke situation evolve? The HRRR-smoke model has become one of the best sources for monitoring the evolution of wildfire smoke in the 0-48 hour timeframe. The vertically-integrated smoke forecast over the next 48 hours shows little change from the current view.

HRRR-smoke vertically-integrated forecast for Wednesday morning August 26, 2020.

For health impacts it is better to look at the near-surface smoke. Large portions of northern California, eastern Oregon, Idaho, and the northern Rockies will continue to experience severely degraded air quality. There are additional large fires in northern Colorado that are also producing smoke. It appears that some smoke will mix down to the surface farther east, but the impacts will be less severe.

HRRR-smoke near-surface smoke forecast for Wednesday morning August 26, 2020.

Looking longer term, the NASA Global Modeling and Assimilation Office produces a number of smoke/aerosol model products using the GFS model. The details of longer range forecasts are likely to change, but the general story of the 210 hour forecast, captured below, is that smoke will persist in California into September. The northern Rockies may see some relief if this model forecast holds.

The good news for the coastal PNW is that severe smoke is not expected in the upcoming 7-10 days, although that is always subject to change if a new fire breaks out.

Climate change influence

Unfortunately, conditions were primed for wildfires thanks to extremely dry conditions across southern and central California. While it is true that these areas are always “dry” in late summer, there is still a range of variability to the dryness. By one of the most important metrics, vapor pressure deficit, the early August conditions were some of the driest in decades:

The increase in vapor pressure deficit is part of a climate change-induced trend that is expected to accelerate in the coming decades. A 2019 paper by A. Park Williams and coauthors showed a clear upward trend in both warm season maximum temperature and vapor pressure deficit (VPD) with further increases in coming decades predicted by the CMIP5 climate model ensemble.

Observed (black) vs. climate modeled (red) maximum temperature (left side) and vapor pressure deficit (right side) for California. Image reproduced from Williams et al. 2019.

This result lead the authors to make some sobering conclusions:

The observed rates of warming and increasing vapor‐pressure deficit are consistent with those simulated by climate models when forced by anthropogenic emissions, indicating that these trends are extremely likely to continue for decades to come. The large increase in California’s annual forest‐fire area over the past several decades is very likely linked to anthropogenic warming.

Williams et al. 2019

While the climate influence is clearly important, it is often a single weather event that leads to a severe wildfire season. In recent years, Diablo Wind events have triggered several major fires by causing high-voltage power lines to malfunction. In 2020, a highly-unusual lightning storm, fueled by tropical moisture, was to blame:

Such weather events will occur some years and not others, but the general trend is clear.

What about Washington State?

Here in Washington State, we are fortunate to have avoided any major wildfires or smoke so far this year. While smoke from California fires reached Seattle in 2018, a large-scale southerly flow pattern needed to bring the smoke up to Seattle is not expected to occur in the next 10 days.

Fires from British Columbia are more commonly responsible for severe smoke in the Pacific Northwest. Fortunately there has been enough rain in BC to keep the wildfire danger in the low-to-moderate category across most of the province.

BC wildfire service daily wildfire danger.

The eastern slopes of the Cascades (both in Washington and Oregon) remain under drought conditions, so wildfire risk will remain elevated in those regions until more significant fall rains arrive.

Unfortunately, climate change is also likely to increase wildfire risk in portions of the Pacific Northwest. A recent study by UW research scientist Jessica Halofsky and coauthors found that by the second half of the century, large increases in wildfire frequency, extent, and severity are likely in dry coniferous forest and woodlands at low to mid elevations (i.e. the east side of Cascades, southern Idaho, drier areas of Rocky Mountains).

If there is one bit of good news from the Halofsky et al. study it is that fire-climate interactions in moist coniferous forests, including the Olympics and the west side of the Cascades, are only expected to result in a small increase in frequency and extent of wildfires.

On a further optimistic note, it is possible to mitigate the risk of future wildfires, both through forest management practices and through the reduction of CO2 emissions that are the primary cause of climate change. Hopefully as a society we will take more steps in that direction.

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