Fire and Smoke Update:

Wildfire activity has increased across BC as well as the Cascade and coastal ranges in the Northwest in recent days. On Monday, smoke is heaviest across Southern BC and across Western Washington, Oregon, and California thanks to recent offshore winds (blowing from east to west).

On Tuesday, stronger onshore winds (blowing from west to east) will help to clear out the smoke near the West Coast, with heavier smoke shifting eastward into the Inland Northwest.

More smoke maps:

• Smoke (Surface) Map → Regular | U.S. Lower 48 States & Canada
• Smoke (Surface) Map → High Resolution | U.S. Lower 48 States

Smoke should eventually reach the Northern Rockies by mid-week, then a gradual northern shift is expected late this week as winds shift to south/southwest.

Rain and cooler temperatures are also expected for Oregon and NW California late this week, which should help with the fire situations, and over time lead to a reduction in smoke coverage and intensity.

Weather Pattern – Big Picture:

A trough of low pressure will move across the Northwest early this week with widespread showers and thunderstorms developing on Monday night and Tuesday, thanks to an abundance of Pacific moisture along with an unstable atmosphere.

Monsoonal moisture will also be retreating southward out of the Central/Northern Rockies as a ridge of high pressure briefly builds across the Great Basin. Moisture will favor the Colorado Front Range and New Mexico on Monday, with a drying trend expected for these areas starting on Tuesday.

Temperatures on Monday will remain above-average across most of the Pacific Northwest and Western Canada, with near to below-average temperatures near the West Coast and also east of the Divide in Wyoming and Colorado.

By Wednesday, a much cooler airmass will have taken hold across the Northwest U.S. while the Southwest will be warmer than average.

Forecast for Monday:

Thunderstorm activity from lingering monsoonal moisture will favor the Colorado Front Range, Sangre de Christo Range, and New Mexico with frequent lightning and locally heavy rain possible. Storm coverage will be more isolated near and east of the Divide in Wyoming and Montana.

Isolated thunderstorms are possible across the Washington Cascades and BC Coast Range on Monday afternoon, and then more widespread showers and thunderstorms will develop across these regions overnight. The Olympics are expected to receive less rainfall compared to the Cascades.

Forecast for Tuesday:

Showers and thunderstorms will become more widespread across the Washington Cascades, Inland Northwest, Southern BC Coast Range, and Northern U.S. Rockies with significant rainfall expected, while Oregon and the BC Interior will see lighter and more spotty rainfall.

Strong to severe thunderstorms are also possible across the Inland Northwest and Northern Rockies with strong wind gusts, moderate to large hail, and frequent cloud-to-ground lightning possible.

Further south, lingering monsoonal moisture will result in scattered thunderstorms in New Mexico favoring the Sangre de Christo Range. Colorado will see a decrease in thunderstorm activity with only some isolated activity expected near the Continental Divide.

Forecast for Wednesday:

Showers will continue across Northern Idaho and NW Montana with cooler temperatures expected, while moisture will also spread northward into the BC Interior with scattered showers and thunderstorms developing.

Meanwhile, another trough of low pressure will reach the PNW Coast on Wednesday night with showers developing across the BC Coast Range as well as the Olympics and Cascades in Washington.

Further south, only isolated thunderstorms are expected near the Continental Divide in Colorado and across the higher terrain in Central Arizona and New Mexico with light and spotty rainfall.

Forecast for Thursday to Friday:

Late this week, an active pattern will take hold across a larger portion of the West. A trough of low pressure will deepen along the West Coast with widespread showers and significant rainfall expected across western and central portions of BC, Washington, and Oregon as well as Northern California. Larger wildfires burning in Oregon and NW California should benefit from this moisture, too.

Meanwhile, increasing south/southwest winds downstream of this trough will bring deeper monsoonal moisture back into the Interior West. As a result, an uptick in thunderstorms can be expected across Arizona, Utah, and Colorado along with the potential for locally heavy rain and slot canyon flash flooding. 

Outlook for Saturday (Sep 2) to Wednesday (Sep 6):

Labor Day weekend is looking unsettled with daily chances of showers across the Pacific Northwest and southward into the Central Sierra Nevada Range including Tahoe as a trough lingers across the West Coast.

The Southwest may start out active, but then deeper monsoonal moisture is expected to be transported northward into the Wasatch and the Northern Rockies where wetter conditions are expected, while the Southwest will start to trend drier.

Temperatures are expected to be near to below average near the West Coast during this period, while temperatures will be warmer than average across the Rockies.

Thanks so much for reading! Next update on Wednesday (August 30).

Alan Smith