Big Picture:

A trough of low pressure (Trough #1) that contributed to showers and thunderstorms across the Northwest early this week will work its way across the Northern Rockies on Wednesday with additional showers expected.

Another trough (Trough #2) will then reach the PNW Coast on Wednesday night and Thursday and will begin to deepen to the south with wetter conditions arriving in Oregon and Northern California. Trough #2 will remain over the West through Labor Day weekend with widespread impacts expected in terms of rainfall and thunderstorm activity.

Monsoonal moisture will also increase across Arizona and Utah on Thursday, resulting in an uptick in thunderstorm activity. Moisture will then expand northward throughout the Intermountain West heading into the weekend.

In terms of temperatures, a cooler airmass will take hold across the West Coast and eventually the Intermountain West over the next 5 days with warmer temperatures expected east of the Continental Divide.

Fire and Smoke Update:

The good news is that most areas in the Western U.S. are seeing a decrease in smoke. Heavier smoke exists in Eastern and Northern British Columbia and Western Alberta. Light smoke will drift across the Great Basin and Northern Rockies on Wednesday but is expected to be short-lived.

Further improvement in smoke conditions and wildfire activity is expected late this week and into the weekend as a cooler and wetter pattern takes hold.

Forecast for Wednesday:

Widespread moderate to heavy showers will continue across Northern Idaho and Northwest Montana with lighter showers to the west over the Washington Cascades. Activity will expand northward into Southeast BC as well. Occasional lightning is possiblebut to a much lesser extent compared to the past couple of days.

Further south, only isolated thunderstorms with light/spotty rain can be expected near the Continental Divide in Colorado, and across portions of Arizona and New Mexico.

Forecast for Thursday:

Showers will return to the PNW with the heaviest and most widespread activity expected across Oregon and Southwest Washington. Eastern BC and Western Alberta will also see heavier rainfall.

Southerly winds will transport deeper monsoonal moisture back into the Southwest with scattered thunderstorms developing across Arizona and Southern Utah. Locally heavy rain and slot canyon flash flooding will be possible. 

Forecast for Friday:

Widespread moderate to heavy showers and possibly thunderstorms can be expected across Central Oregon with activity extending into Northern California as well. There is some uncertainty as to how much rain makes it into the Tahoe region, with possibilities ranging from light/spotty rain to heavy rain.

Monsoonal moisture will continue to increase across the Southwest with more widespread showers and thunderstorms expected across Arizona, Utah, Southeast Nevada, Western Colorado, and parts of Wyoming. Frequent lightning and heavy rain will be possible across these areas.

Forecast for Saturday to Sunday:

A wet and stormy Labor Day weekend can be expected across much of the West as the trough of low pressure near the West Coast slowly works its way inland over time. Scattered to widespread showers and thunderstorms can be expected across most mountainous regions of the West along with locally heavy rain.

Outlook for Monday (Sep 4) to Friday (Sep 8):

On Labor Day Monday, the Southwest will begin to dry out while significant moisture will remain in place across the Northern Rockies with showers and thunderstorms expected. Unsettled/showery conditions will remain possible on Monday across the PNW as well.

As the week progresses, additional disturbances could bring more showers to the Northwest and Northern Rockies at times. Meanwhile, the monsoon is expected to weaken significantly with a drier pattern taking hold across the Southwest.

Temperatures are generally expected to remain below average during this period west of the Continental Divide and above average near and east of the Divide, as well as in the Four Corners region.

Thanks so much for reading! Next update on Friday (Sep 1).

Alan Smith