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Forecast Summary:

• Heavy wildfire smoke from British Columbia and Washington will linger across the Northwest and Northern/Central Rockies through Saturday before gradually lifting northward early next week as winds shift to southwest.

• A cooler, wetter pattern develops next week with showers and thunderstorms expected across the Northwest and parts of the Interior West.

• Saturday–Sunday: Scattered thunderstorms expected across the Southwest, Rockies, and Northwest, with locally heavy rain and flash flooding possible in slot canyons and burn scars.

• Monday–Tuesday: More widespread rain in Northern California, Oregon, and Washington, especially the Cascades; Rockies trend drier except for very isolated storms over the higher peaks.

• Wednesday: Showers continue across the Northwest; monsoonal moisture may also return to the Southwest.

• Sept 11–15: Slow-moving trough supports showers and storms across the Rockies, while a weaker system may bring additional rain to the Northwest.

Wildfire Smoke:

Unseasonably hot temperatures (for early September) and significant lightning activity have both contributed to an outbreak of wildfires across the Pacific Northwest, Canada, and California this week.

The biggest smoke-producing fires are in British Columbia and Washington and northwest winds aloft are transporting heavy smoke from these fires into the Interior Northwest and Northern/Central Rockies.

This widespread smoky pattern is expected to persist through Saturday, then the smoke plume will gradually start to lift northward on Sunday as winds aloft shift from northwest to southwest (blowing from the southwest).

High-Res Smoke Model – Friday PM to Sunday Midday:

Heading into early next week, the widespread heavy smoke should continue to lift northward as southwest flow takes over, though locally heavy smoke will remain possible in the vicinity of ongoing active fires over California and the Interior Western U.S. 

Low-Res Air Quality Forecast Model – September 6-10:

Big Picture Weather Pattern:

The good news is that, in addition to a wind shift, a cooler and wetter pattern will gradually take hold over the Northwest and portions of the Interior West next week as a stronger fall-like trough of low pressure arrives. This trough will be slow-moving and will keep shower chances going across the Northwest for much of the week.

5-Day Temperature Forecast:

Below-average temperatures are expected across California, Oregon, and Nevada this week, while initial above-average temperatures over the Rockies and Northwest will give way to a cooling trend as the week progresses. 

Saturday (Sep 6):

As a shortwave trough moves ashore, monsoonal moisture will spread eastward across the Southwest and the Rockies, leading to an uptick in thunderstorm activity.

Showers and thunderstorms will be scattered in nature, with most occurring during the afternoon, but stronger storms will be capable of producing locally heavy rain. There is also a risk of flash flooding, especially in slot canyons and burn scars. 

As the nose of the shortwave trough pushes northward, moisture and instability will also increase over the Pacific Northwest, where scattered showers and thunderstorms can be expected on Saturday afternoon. Showers will also develop over the Olympics, Puget Sound, and Western Washington Cascades on Saturday night.

Sunday (Sep 7):

A broad area of scattered showers and thunderstorms will cover the Northwest to the Northern and Central Rockies. For most areas, this activity will largely be confined to the afternoon hours, except over Western Washington, where rain showers are also expected in the morning.

Most areas in the green shaded region will receive light to moderate rainfall from showers, though locally heavy downpours are also possible with thunderstorms.

Monday (Sep 8):

A stronger and more fall-like trough will reach the West Coast on Monday with more numerous rain showers developing across Northern California, Oregon, and Washington during the afternoon and overnight hours. Isolated thunderstorms are also possible. 

The heaviest and most widespread rain is expected over the Oregon Cascades, where many locations will pick up a half-inch to one inch of rain. 

A drying trend is expected across the Rockies, though a few stray afternoon thunderstorms couldn't be ruled out over the higher terrain. 

Tuesday (Sep 9):

The trough will slowly move inland with an area of low pressure in the upper atmosphere setting up over Southwest Washington. Scattered to numerous rain showers can be expected throughout the Northwest, with the Washington and Oregon Cascades favored for the most widespread activity and heaviest rainfall.

The Inland Northwest, including Idaho and Western Montana, will see more scattered activity, but a better chance of thunderstorms can also be expected here, with stronger storms producing locally heavy rainfall.

Tahoe and the Tetons/Yellowstone will be right on the edge of the pattern on Tuesday, with showers possible, but models are in poor agreement in terms of rainfall amounts and shower coverage.

To the south, Colorado will be mainly dry, though a stray afternoon thunderstorm couldn't be ruled out over the Front Range Mountains near the Divide.

Wednesday (Sep 10):

The trough will dig a bit further south and will push inland slightly with showers favoring Northern California, Oregon, and the Interior Northwest, including Idaho and Western Montana. Tahoe will also have a better chance of showers, though models are in poor agreement on just how much rain will fall there.

Southerly flow ahead of the trough may also pull some monsoonal moisture back into the Southwest, with isolated afternoon thunderstorms possible across Arizona, Southern Utah, and Western Colorado.

September 11-15:

A slow-moving trough will continue to work its way across the Intermountain West late next week, with showers and thunderstorms possible across the Rockies from roughly September 11-13.

Another weaker trough centered further north is projected to impact the Northwest around September 13-15 with additional showers possible. 

Temperatures are projected to be cooler than average across California and Nevada (good news for firefighting efforts) and near-average across the Central Rockies and Southwest. Further north, above-average temperatures are expected near the Canadian border.

September 16-20:

Confidence in the pattern decreases once we get beyond 10 days, but for now, it looks like the pattern could relax somewhat with no major "troughing" signals projected at this time.

There are some hints of weaker Pacific disturbances tracking across the Northwest and Northern Rockies, while a low-grade monsoon could also re-emerge over the Southwest. 

Temperatures are also expected to be warmer-than-average across most of the West, though keep in mind that "average" temperatures are quickly decreasing by this point in the year, so it won't be the same as above-average warmth in July or August. 

Thanks so much for reading and have a great weekend!

My final post of the season will be on Wednesday, September 10th.

Alan Smith