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Forecast Summary:

• Monsoon Moisture Surge: A major push of monsoonal moisture, enhanced by Tropical Storm Lorena, will spread from Arizona/California midweek into the Rockies and Pacific Northwest by the weekend.

• Temperature Trends: Heat wave persists in the Pacific Northwest; cooler-than-normal conditions develop in the Southwest under monsoonal influence.

• Midweek Storms (Sep 3–4): Higher thunderstorm coverage in Arizona, Southern Utah, Nevada, and California with heavy rain/flash flood potential; dry lightning risk in Northern California and Southern Oregon.

• Late Week (Sep 5–7): Highest thunderstorm coverage and flash flooding threat across the Four Corners and Rockies, with showers & thunderstorms also possible in the Northwest.

• Next Week (Sep 8–12): A fall-like trough brings cooler temps, rain chances for the Northwest & Northern Rockies, and possible light snow on high peaks.

Big Picture Weather Pattern:

A surge of monsoonal moisture, aided by Tropical Storm Lorena in the Eastern Pacific, will influence much of the West this week, with Arizona and California favored initially, before moisture expands northward and eastward across the Rockies as the week progresses. Some of this moisture will also reach the Pacific Northwest.

5-Day Temperature Outlook:

Temperatures will remain well above normal across the Pacific Northwest as a heatwave persists. The Southwest will be cooler than normal thanks to the uptick in monsoonal moisture. 

Wednesday (Sep 3):

Deep monsoonal moisture will be in place across Arizona, Nevada, and California, with scattered showers and thunderstorms expected. Stronger storms or training of thunderstorms will be capable of producing heavy rain along with the potential for flash flooding.

Thunderstorm coverage looks highest in Arizona and more spotty across the mountainous terrain of Nevada and California.

Further north, isolated to scattered thunderstorms will also develop across Northern California and Southern Oregon in response to a shortwave moving over the area. However, this region will see a mix of wet and dry thunderstorms, with an elevated risk of lightning-triggered wildfires with any dry thunderstorms. 

Thursday (September 4):

Monsoonal moisture will expand eastward a bit into Utah, Southwest Colorado, and New Mexico, while moisture levels will remain high over Arizona. Moisture will also linger over California and Nevada.

This monsoon moisture surge will be enhanced by Tropical Storm Lorena in the Eastern Pacific as it makes landfall on the Baja Peninsula.

Green-shaded areas will see scattered afternoon thunderstorms with stronger storms or training of storms capable of producing heavy rain and flash flooding. The highest coverage of storms will be in Arizona and Southern Utah as a shortwave trough approaches, with more isolated to scattered coverage elsewhere.

Friday (September 5):

Monsoonal moisture will continue to increase across the Southern and Central Rockies as a subtle shortwave trough moves through, while another trough along the West Coast will help to pull some of this moisture northward into the Northwest and Northern Rockies.

The Four Corners states will see the highest thunderstorm coverage, along with the greatest potential for locally heavy rainfall and flash flooding with thunderstorms. 

Further north, thunderstorm coverage will be more isolated and scattered in nature from the Cascades to the Sawtooths to the Tetons and Wind Rivers, but moisture levels will be high enough that locally heavy rain could occur with any stronger storms. 

Saturday (Sep 6):

Monsoonal moisture will continue to expand across the Rockies, with an active afternoon for thunderstorms expected. Stronger storms or training of storms will be capable of producing locally heavy rain and flash flooding.

Moisture will also increase across the Pacific Northwest in response to a trough near the West Coast, with isolated to scattered showers and thunderstorms expected. Most areas in the Northwest will see light to moderate rain, but stronger thunderstorms could produce locally heavy downpours. 

Sunday (Sep 7):

Another active day is expected across the Rockies with monsoon moisture in place. Some models are hinting at the possibility of morning showers and thunderstorms in some areas as well, likely in response to subtle shortwave troughs. But confidence is low this far out.

Shower and thunderstorm coverage will still be highest in the afternoon in most areas, with stronger storms or training of storms capable of producing heavy rain and localized flash flooding. 

Outlook for September 8-12:

Next week, a fall-like low-pressure trough is projected to slowly move across the West, bringing increased rain shower potential to the Northwest and the Northern Rockies, along with cooler temperatures. Some of the higher peaks could also get dusted with snow in this pattern.

To the south, the monsoon is expected to weaken, though the Four Corners region may hang onto some afternoon thunderstorm chances at times. 

Temperatures are projected to be below normal across the Far West with a gradual cooling trend over the Central Rockies. Temperatures are projected to be warmer than normal east of the Continental Divide.

Outlook for September 13-17:

Confidence in the pattern decreases during this period, though longer-range models are showing some subtle troughing signals, which would indicate an unsettled weather pattern with at least a chance of showers across the Northwest and the Rockies.

However, it's too early to know whether or not there will be any significant weather systems.

In terms of temperature, the European AI Ensemble Model is projecting below normal temperatures across much of the Western U.S., with above-normal temperatures returning to the Northwest.

Other models do not have the same cool signal pronounced over the West Central U.S., so there is plenty of uncertainty this far out.

Thanks so much for reading! Next update on Friday (September 5).

Alan Smith