Big Picture:

There is a lot going on across the West over the next week or so.

First, monsoonal moisture will continue to result in daily rounds of thunderstorms from the Sierra to the Colorado Rockies. Frequent lightning will be possible with stronger storms, while locally heavy rain and isolated flash flooding are also possible, especially in slot canyons and dry washes. 

A heat wave will continue across the Northwest on Wednesday and Thursday, before cooling off from Friday on as high pressure breaks down and westerly winds bring cooler marine air into the region.

Unfortunately, winds will be on the increase leading up to Friday's cold front, which combined with hot and dry conditions will result in elevated fire weather concerns and increasing smoke.

Taking a look at temperatures, the Northwest will be well above average on Wednesday:

By this weekend, big changes are expected with below-average temperatures for much of the West on Saturday as a trough of low pressure deepens along the West Coast.

Monsoonal moisture will also begin to spread northward by late this week and into the weekend. Meanwhile, a tropical disturbance in the Eastern Pacific is projected to move from south to north along the Baja Peninsula and spread significant moisture into parts of the West. But when and where this moisture reaches is uncertain.

Fire and Smoke Outlook:

Fire activity has increased significantly in recent days as a heat wave has taken hold across the Northwest. This trend is expected to continue through Friday with increasing smoke across the Inland Northwest and Northern Rockies. 

Fire behavior may decrease somewhat over the weekend as cooler air arrives, though winds could still be a factor. Smoke will likely linger across portions of the Interior Northwest and Northern Rockies this weekend as well.

Forecast for Wednesday:

Scattered thunderstorms can be expected throughout the Four Corners region near and west of the Continental Divide, as well as the Great Basin and Sierra Nevada Range. Locally heavy rain and frequent lightning are possible under stronger storms.

Forecast for Thursday:

We will see a subtle westward and northward shift in moisture across some parts of the West. Scattered thunderstorms can be expected across the Four Corners region once again. However, Central Colorado and the Front Range/Sangre de Christo Range will see more isolated activity with a higher concentration of storms over the San Juans as well as Northern New Mexico.

Further west, thunderstorm coverage may also decrease slightly across the Sierra compared to prior days. However, moisture will increase further north into NorCal and Oregon with a mix of wet and dry thunderstorms expected. Isolated thunderstorms are also possible across portions of Idaho and Western Wyoming.

Forecast for Friday:

Cooler air will arrive from the Pacific on Friday as two low pressure troughs set up across the West – one near the West Coast in California and another over Western Canada near the BC/Alberta border.

An active day of thunderstorms can be expected across Arizona, Utah, and Colorado (mainly west of the Divide) while an uptick in thunderstorms is also expected across the Northern Rockies. Scattered thunderstorms can also be expected further west in Oregon and across NorCal down to around Tahoe.

More widespread showers and thunderstorms are expected across Eastern BC and Western Alberta in association with the northern trough. For now, the heaviest/most widespread rain is expected northward around Jasper with lighter rain further south across the Banff area and BC Powder Highway region. 

Forecast for Saturday to Sunday:

All eyes are on the tropical disturbance brewing in the Pacific and its impacts on the weather across the Western U.S. this weekend.

A trough will remain in place across the West, and how the tropical disturbance interacts with this feature along with the ongoing monsoon across the Southwest will determine who gets rain and storms and who doesn't.

Weather models are not in good agreement at the moment with the eastward vs westward extent of moisture so the map displayed below is highly subject to change.

For now, the best chance of rain looks to be across parts of Arizona, California, Nevada, Utah, and perhaps Idaho, Eastern Oregon, and Western Wyoming. A northern trough of low pressure could keep conditions unsettled and showery across Montana as well.

Colorado and New Mexico may end up on the drier side of this feature with a decrease in thunderstorm chances compared to prior days. But we are still far enough away that this could change.

Outlook for Monday (Aug 21) to Friday (Aug 25):

Moisture levels are expected to be elevated throughout the West next week, due to a combination of moisture from tropical activity in the Pacific as well as moisture from the ongoing monsoon. 

Some of this moisture is expected to reach the PNW as well, but confidence is low in terms of rain amounts and coverage and whether or not it will help with the fire situation.

Temperatures are expected to be cooler than average across the Southern Great Basin (cool being a relative term in August) while temperatures should rebound to above-average levels elsewhere. However, no major heatwaves are anticipated west of the Continental Divide with abundant moisture expected.

Thanks so much for reading! Next update on Friday (August 18).

Alan Smith