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Forecast Summary:

• Strongest monsoon surge of the season to bring widespread showers and thunderstorms to much of the West through August 30.

• Flash flooding risks are highest in the Sierra Nevada Range, Colorado, New Mexico, Arizona, Utah, Idaho, Wyoming, and Southern Montana. 

• Pacific Northwest heatwave continues with well-above-normal temps, high fire danger, and a risk of lightning-triggered wildfires from scattered thunderstorms.

• Temperature pattern: Below-normal temps dominate the Interior West under cloud cover, while the Northwest and California stay hot and dry.

• Daily evolution:

  • Tue–Thu: Most widespread storm activity, with moisture plume shifting north.

  • Fri–Sat: Heaviest focus along/east of the Divide (Montana → New Mexico), more scattered storms west of the Divide.

• Extended outlook (Aug 31–Sep 9): Monsoon activity gradually shifts to a more seasonal pattern with emphasis on NM/AZ; Rockies keep afternoon storm chances; Northwest may see occasional troughs, but confidence is low; temps stay cool in the Rockies, hot in the PNW and California.

Big Picture Weather Pattern:

The most significant monsoon moisture surge of the season (by far) will continue to impact the West with moisture expanding northward and eastward as the week progresses.

Check out the projected rainfall totals through Saturday, August 30th. While many areas of the West will see some beneficial rainfall (flooding concerns and lightning-triggered wildfires aside), the heaviest totals are projected across Idaho, Wyoming, Northern Utah, and the northern and eastern ranges of Colorado. 

5-Day Temperature Forecast:

The increase in cloud cover associated with the monsoonal push, along with cooler air aloft, will lead to below-normal temperatures across much of the Interior West.

However, the Pacific Northwest will be a notable exception where a heatwave will continue with well-above-normal temperatures. Fire danger will remain high in this region as a result. 

Tuesday (Aug 26):

Deep monsoonal moisture will be in place across the West with widespread showers and thunderstorms expected. This will not be a typical afternoon thunderstorm setup, as rain will be possible at any time of day or not.

Also, showers will be more common than thunderstorms with heavy cloud cover and cooler temperatures limiting surface heating in many areas, though thunderstorms will still be possible, especially in the afternoon.

The Southwest, the Central Rockies, and the Sierra Nevada Range will all be favored for numerous showers and thunderstorms with significant rain amounts possible, including locally heavy rainfall rates. 

Areas that will have the highest risk of flash flooding include Southern and Western Colorado and Northern New Mexico (San Juans and Sangres), Southeast Arizona, Southern Utah Canyon Country, Northern Utah (including the Wasatch), and Southern Idaho.

Also, isolated thunderstorms are possible in the Cascade Range from Central Oregon to Washington. Moisture levels will be comparatively lower in this region, and a heat wave will also be ongoing, which will pose a risk for lightning-triggered wildfires. 

Wednesday (Aug 27):

Another active day is expected with the deepest moisture plume shifting north as a shortwave trough moves across Idaho and Northwest Wyoming.

Numerous showers and thunderstorms can be expected throughout the West from the Sierra to the Rockies. The Northern Rockies, including Southern and Central Idaho and Western Wyoming, will be favored for the heaviest rainfall amounts and will also see the highest potential for flash flooding.

Further south, shower and thunderstorm coverage looks comparatively lower, but there will still be plenty of moisture in place for showers and thunderstorms capable of heavy rain across Utah, Colorado, Arizona, and New Mexico.

To the north, the Cascades in Oregon and Washington will see isolated to scattered afternoon thunderstorms with a risk of lightning-triggered wildfires, though locally heavy rain is also possible with stronger storms. The eastern slopes of the Cascades in Oregon are favored for higher thunderstorm coverage and a greater potential for heavy downpours.

Thursday (Aug 28):

The deepest moisture will continue to lift northward and eastward, with the Northern Rockies favored for the most widespread showers and thunderstorms. The Northern Rockies and the Front Range of Colorado will see the highest risk of heavy rainfall and flash flood potential.

A backdoor cold front sliding into Eastern Wyoming and Eastern Colorado will be the culprit of the uptick in moisture and flash flood risk from the Bighorns in Wyoming to the Front Range in Colorado.

The Southwest and the Sierra Nevada Range will see fewer and more scattered thunderstorms compared to prior days, mainly confined to the afternoon hours.

However, there will be enough lingering moisture to result in heavy rain potential with stronger thunderstorms, and vulnerable areas such as slot canyons and burn scars will have a flash flooding risk.

Isolated to scattered thunderstorms can also be expected across the Pacific Northwest once again. This will be a mixed wet and dry thunderstorm setup, with some storms producing locally heavy downpours, while the risk of lightning strikes and fire ignitions will also exist away from rain cores.

Friday (Aug 29):

Moisture will favor Southern Montana and Northern Wyoming, where numerous showers and thunderstorms can be expected, both in the morning and in the afternoon and evening. Localized flash flooding will also remain possible across this region. 

The Southwest and the Central Rockies will see more sunshine with an afternoon thunderstorm setup expected. Stronger storms will be capable of producing heavy rain, and flash flooding will also be possible in slot canyons and burn scars. 

Elsewhere, more isolated thunderstorm activity can be expected over the Sierra Nevada Range and SoCal Mountains, as well as the Interior Northwest (Eastern Washington and Northern Idaho) and in British Columbia. 

Saturday (Aug 30):

The deepest moisture and highest rain and thunderstorm chances will be along and east of the Continental Divide from Southern Montana to New Mexico, and also west of the Divide across Southwest New Mexico and Eastern Arizona. These areas will also see the greatest potential for heavy rain.

West of the Divide in the Rockies, thunderstorm coverage looks more isolated to scattered in nature. Most storms will produce light to moderate rain west of the Divide but stronger storms could produce locally heavy downpours.

Rain showers could also develop over Western Washington as a trough approaches, but confidence is low as some models are keeping this area dry on Saturday, while others have rain moving in during the day.

Outlook for August 31-September 4:

The monsoon will revert back to a more "seasonal" pattern with medium-range models hinting at a focus over New Mexico for heavy rain potential. Seasonal monsoonal moisture levels should continue to result in routine afternoon thunderstorm chances elsewhere across the Southwest and the Rockies, and maybe the Sierra as well, though confidence is lower.

The Pacific Northwest (mainly Washington) could also see some showers on Sunday, but confidence is low as not all models are projecting this.

Temperatures will remain near to below normal across the Southern and Central Rockies, while above-normal temperatures are expected to persist over the Pacific Northwest and in California.

Outlook for September 5-9:

Confidence in the forecast decreases somewhat for this period, but it looks like a fairly active monsoon by September standards should continue. Arizona, New Mexico, Southern Utah, and Southern Colorado look the most favored in terms of rainfall, while the Central Rockies will likely hang onto thunderstorm chances as well.

There are some hints of Pacific disturbances eventually reaching the Northwest, but confidence is low as models have been projecting this in the 7+ day timeframe recently, only to back off as we get closer.

Also, we'll need to keep an eye on areas east of the Divide in the Rockies, as we are seeing hints of cold fronts sliding in from the north, which could lead to moisture increases and thunderstorm areas for areas such as the Beartooths, Bighorns, and Colorado Front Range.  

Temperatures are projected to remain cooler than normal by the European AI Ensemble Model across the Central Rockies. Other models have a warmer projection, but the European AI Ensemble has performed better in the 10-15 day range recently.

Also, it looks like above-normal temperatures will likely persist across the Pacific Northwest, though we are getting past the typical climatology for PNW heatwaves by this point in the year, so perhaps there will be some cooling compared to this current week. 

Thanks so much for reading! Next update on Wednesday (August 27).

Alan Smith