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Forecast Summary:

• Moisture Focus: Monsoon activity will favor the Northern and Eastern Rockies through Saturday before shifting into New Mexico and Arizona Sunday–Monday.

• Heavy Rain/Flash Flood Risk: Colorado, Northern New Mexico, Utah, and parts of Montana and Wyoming will see repeated rounds of storms with localized flash flooding potential, especially Thursday–Saturday.

• Pacific Northwest: Multiple troughs will support scattered showers and thunderstorms, but rainfall will be spotty with a mix of wet/dry storms — wildfire risk remains elevated due to lightning.

• Temperature Pattern: The PNW and California will stay hotter than average through Labor Day, while the Rockies will run slightly cooler from cloud cover; Southwest trends cooler with more clouds and storms.

• Holiday Weekend Outlook: Strongest storm focus shifts east of the Divide (CO/NM) Saturday, expands in AZ/NM Sunday–Monday; Colorado trends drier after Saturday.

• Longer Term (Sep 2–11): Ridge builds over the Northwest (late-season heatwave), while a late-season monsoon continues across the Southwest and Southern Rockies, keeping thunderstorm chances going.

Big Picture Weather Pattern:

Monsoonal moisture and shower/thunderstorm chances will favor the Northern and Eastern Rockies from Thursday to Saturday, then the focus of moisture will shift into the Southwest (New Mexico and Arizona) on Saturday and Sunday.

A low pressure system off the coast of the Pacific Northwest will also bring some moisture and instability into Western Washington, resulting in scattered showers and thunderstorms this weekend, though widespread soaking rains are not expected. 

5-Day Temperature Forecast:

Temperatures will remain above-average across the Northwest and California over the holiday weekend, while the Rockies will be slightly cooler than average thanks in part to lingering cloud cover.

Thursday (Aug 28):

A shortwave trough will slowly work its way across the Northern Rockies with deep monsoonal moisture in place. This will result in additional showers and thunderstorms at all hours of the day, with heavy rainfall rates expected at times, along with the potential for localized flash flooding.

A backdoor cold front will also move into Eastern Wyoming and Eastern Colorado with easterly upslope winds behind the front, contributing to an uptick in heavy rain potential with showers and thunderstorms. This includes the Colorado Front Range.

Elsewhere, a shortwave trough will also move into Washington and will pull some monsoonal moisture northward into this area. As a result, scattered showers and thunderstorms can be expected across much of Washington.

The setup over Washington will feature a mix of wet and dry thunderstorms, so while some storms will produce heavy rain, lightning strikes that occur away from rain cores could potentially trigger new wildfires.

Showers and thunderstorms will decrease in coverage compared to prior days across much of the Interior West, including Western Idaho, Southern Utah, and Western Colorado. However, thunderstorms will still be capable of producing heavy rain.

Friday (Aug 29):

There will be a few features in play here. A shortwave trough will continue to linger over Montana, while a potent shortwave will also track across the Four Corners region, leading to an uptick in thunderstorm coverage across Utah, Colorado, and New Mexico.

Easterly upslope winds east of the Continental Divide will continue to favor the easternmost ranges of the Rockies for scattered showers and thunderstorms with heavy rain potential.

By and large, most areas of the Rockies will see scattered showers and thunderstorms, with the potential for locally heavy rain and also some risk of flash flooding.

A low pressure area will remain over the Pacific Northwest and will slowly lift northward into Canada. Scattered showers and thunderstorms will largely be confined to the Washington/BC border region and also Southern BC.

Once again, a mix of wet and dry thunderstorms can be expected, with some storms producing heavy rainfalls, while lightning-triggered wildfires will remain a risk away from rain cores.

Saturday (Aug 30):

The focus of shower and thunderstorm activity will be near and east of the Continental Divide, especially in Colorado and Northern New Mexico, and to a lesser extent in Eastern Wyoming. Stronger thunderstorms in these areas will be capable of producing heavy rain with some risk of flash flooding.

In the Northwest, a trough will move just offshore from Washington and will move from south to north, with enough moisture and instability for isolated showers and thunderstorms. Most storms will only produce light to moderate rain, with elevated fire concerns due to lightning.

Sunday (Aug 31):

Monsoon moisture will increase across New Mexico and Arizona, resulting in an uptick in aftenroon thunderstorms with heavy rain and localized flash flooding possible.

Moisture will decrease over Colorado, with only some lingering moisture in Southern Colorado (mainly along/south of Highway 24) supporting an isolated afternoon thunderstorm setup, with storms producing light to moderate rain.

In the Pacific Northwest, an inverted trough will inch a bit further onshore with moisture and instability supporting a threat of scattered showers and thunderstorms. This will a bet a mixed wet and dry thunderstorm setup, with some storms capable of heavy downpours (mainly over the North Cascades and Southern BC).

Monday (Sep 1):

On Labor Day, monsoon moisture will expand across Arizona and New Mexico with scattered afternoon thunderstorms. Some storms will produce heavy rain, along with a risk of flash flooding in slot canyons, dry washes, and burn scars.

Colorado will remain just on the edge of the monsoon moisture. Thunderstorm coverage may expand northward a bit compared to Saturday, but overall coverage looks to remain isolated and confined to the higher terrain and most storms will only produce brief light to moderate rain and gusty winds.

September 2-6:

Next week, a ridge-dominant pattern is expected over the West, while a trough-dominant pattern is expected over the East.

This will result in warmer and drier conditions across the Northwest, but a seasonal monsoon pattern for early September will remain in place across the Southwest, favoring Arizona and New Mexico. Also, Colorado and Utah should see some thunderstorm activity (favoring the southern parts of these states), and some moisture and thunderstorm chances should also sneak into the Sierra Nevada Range.

This pattern will favor a late-season heatwave over the Pacific Northwest with well-above-normal temperatures expected. Temperatures are expected to be below normal over Arizona and New Mexico, where more abundant moisture and cloud cover are projected. 

September 7-11:

A late-season monsoon is expected to chug along across the Southwest and Southern Rockies with routine thunderstorm chances expected. The high pressure ridge over the Northwest should weaken a bit, potentially opening the door to Pacific disturbances and shower chances. 

Temperatures are expected to remain above normal over the Pacific Northwest and Northern Rockies, but with less extreme anomalies compared to before. Temperatures are expected to be near normal over the Four Corners states.

Thanks so much for reading! Next update on Friday (August 29).

Alan Smith