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Wildfire risk likely to increase into late September as hot, dry weather builds across West

By Alex Sosnowski, AccuWeather senior meteorologist
September 02, 2018, 6:18:31 PM EDT

The western United States remains a tinderbox, and wildfire conditions may worsen as September weather patterns progress.

A persistent northward bulge in the jet stream allowed temperatures to soar to well above average and at times record levels in parts of the West this summer.

That same weather pattern may return later in September.

Multiple, big fires have set the tone so far this year

Most of the large wildfires in California that have burned 875,000 acres as of Aug. 28 are now mostly contained. However, multiple large fires in the Northwest continue to burn with with substantially less containment.

Wildfires 9.2 AM

The Klondike Fire in Oregon is only 40 percent contained and has burned over 100,000 acres.

The 875,000 acres burned in California thus far this year, including the Mendocino Complex and Carr fires, are well above the five-year average of 157,000 acres to date, according to Cal Fire.

On a national basis, 6.5 million acres have burned so far this year, compared to a 10-year to date average of 5.2 million acres, according to the National Interagency Fire Center.

Temperatures to be kept at bay, but fire risk to continue in early September

In recent days, lower temperatures have allowed the wildfire situation to stabilize in parts of the western United States.

The jet stream dipped farther south during late August and is forecast to linger over the Northwest U.S. during the first part of September.

Static Wildfire Risk Early Sep

While this configuration will allow lower temperatures and some moist Pacific air to flow across the Northwest, no big rainfall is anticipated, according to AccuWeather Long-Range Meteorologist Max Vido.

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"Instead, there may be some episodes of wind in the Northwest that could add to firefighting difficulties and spotty thunderstorms may bring little rainfall and lightning strikes that could ignite new wildfires," Vido said.

A positive aspect of breezes moving the air will be to allow smoke to disperse, rather than linger for days.

Warmth, fire risk to build over western US during mid- to late September

As bad as the fire season has been so far, conditions are likely to get worse and could get much worse in the weeks and months ahead.

Similar to that of the middle of the summer, the jet stream is forecast to bulge northward over the Western states during mid- to late September.

Static Wildfire Risk Later Sep

High pressure areas tend to get stronger as the autumn progresses.

This setup will lead to above-average temperatures and continue the lack of rainfall. The same pattern may result in light winds over the interior West, mainly east of the Cascades and Sierra Nevada. Smoke from existing fires may gather once again.

The brush will remain very dry into the autumn. Months of intense sunshine and heat have baked the landscape all summer long.

"Around the middle of September this year, we expect conditions to be conducive for strong wind events in Southern California and perhaps Northern California as well," Vido said.

Southern California Santa Ana winds and their Northern California cousin, Diablo winds, blow from the coast ranges to the beaches as areas of strong high pressure build across the interior West. As the air blows downhill, the wind speed and air temperature increase dramatically.

The hot, dry winds can greatly enhance the chance of wildfire ignition and may cause existing fires to spread wildly.

"Santa Ana wind events typically increase in frequency during September and continue through the autumn," Vido said.

Even during the peak of the Pacific Ocean storm season during the late autumn and winter, episodes of strong winds in lieu of much rain and high country snow can lead to an explosive wildfire situation.

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