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Forecast Summary:

• Late-season heatwave will impact the Pacific Northwest with hot and dry conditions and high fire danger persisting into next week.

• Monsoonal moisture will be confined to areas east of the Continental Divide on Saturday, before focusing on New Mexico on Sunday and Monday, then shifting westward into Arizona and California by Tuesday–Wednesday.

• Colorado will see scattered thunderstorms on Saturday, favoring the Front Range and Sangres, then only isolated storms over the higher peaks are expected from Sunday to Wednesday.

• Wildfire smoke will affect air quality over portions of California, the Northwest, and the Rockies, with elevated fire danger in the Northwest due to heat and low relative humidity.

• Medium-range outlook (Sep 4–8) favors hot and dry conditions over the Northwest, while an uptick in monsoon thunderstorm activity is possible across the Four Corners.

Big Picture Weather Pattern:

A tall ridge of high pressure will build over the Northwest, resulting in very hot temperatures for so late in the season across Washington and Oregon. 

Monsoonal moisture will be confined to areas east of the Continental Divide on Saturday, including the Colorado Front Range, before focusing over New Mexico on Sunday and Monday, and eventually spreading westward into Arizona and California on Tuesday and Wednesday. 

5-Day Temperature Outlook:

Temperatures will generally be above normal west of the Continental Divide and below normal east of the Divide over the next 5 days. Anomalous warmth is expected over the Pacific Northwest as a heatwave takes hold with well-above-normal temperatures. 

Wildfire Smoke:

While the monsoon surge this week brought beneficial rain to many parts of the West, areas from the Pacific Northwest to Central California have seen an uptick in wildfire activity recently due to hot and dry conditions, along with some dry thunderstorms and lightning strikes on the edge of the monsoon moisture plume.

Smoke will impact portions of California, the Northwest, and the Rockies this weekend and early next week as a result. Fire danger will also remain high over the Northwest due to the heatwave (high temperatures and low relative humidity).

High-Res Smoke Forecast - Next 48 Hours:

Low-Res Air Quality Forecast - Next 5 Days:

Saturday (Aug 30):

Moisture will primarily be focused along and east of the Continental Divide with scattered showers and thunderstorms favoring the Bighorns, Colorado Front Range, and Sangres. 

Stronger storm cells will be capable of producing heavy rainfall, and there is some risk of flash flooding –  especially in New Mexico, as a cold front arrives with increasing moisture and upslope flow, while a shortwave passes overhead. 

West of the Divide in Colorado, only isolated thunderstorms are expected with light and spotty rain.

Sunday (Aug 31):

A routine late-season monsoon pattern will set up over New Mexico and Southeast Arizona with scattered afternoon thunderstorms. Stronger storms will be capable of producing heavy rain and localized flash flooding.

Colorado will be on the fringe of the monsoonal moisture and will only see some isolated afternoon thunderstorms over the higher terrain with light and spotty rainfall. 

In our last forecast, we mentioned the potential for showers and thunderstorms over Western Washington this weekend. However, models have since backed off on any moisture reaching the PNW, with hot and dry conditions now expected to prevail. 

Monday (September 1):

Labor Day Monday will feature a very similar setup to Sunday, with scattered thunderstorms favoring New Mexico and Southeast Arizona. Colorado will only see some isolated activity favoring the higher peaks.

Tuesday (September 2):

A shortwave trough will set up near the Bay Area and will help to pull in a healthy late-season surge of monsoonal moisture westward into Arizona and California.

This will result in an uptick in thunderstorm activity across the Sierra Nevada Range, as well as the SoCal Mountains and Southern and Central Arizona. Stronger storms will be capable of producing heavy rain and localized flash flooding.

Limited moisture will remain over Northern New Mexico and Colorado, where isolated to widely scattered thunderstorms can be expected. Most storms will produce only light rain, but stronger storms could produce locally heavy downpours. 

Wednesday (September 3):

The pattern looks similar to Tuesday, with moisture and thunderstorm chances favoring Arizona, the SoCal Mountains, and the Sierra Nevada Range. 

More isolated thunderstorm activity is favored over the higher terrain of Colorado and New Mexico, with storms producing light rain and gusty winds. 

September 4-8:

A tall ridge of high pressure is projected to remain over the Pacific Northwest, which will support hot and dry conditions.

However, underneath the ridge models are beginning to hint at a late-season uptick in monsoonal moisture, possibly aided by a tropical system in the Eastern Pacific.

Arizona looks to be the most favored at this time, though the entire Four Corners region could potentially see an uptick in thunderstorm coverage, while the Sierra Nevada Range and the Northern Rockies (mainly Wyoming) could see some thunderstorms as well. 

Temperatures are expected to remain well above normal across the Pacific Northwest as a late-season heatwave continues, while the Southwest is favored for below-normal temperatures due to the uptick in moisture expected. 

September 9-13:

Most models are projecting the high pressure ridge over the West Coast to eventually weaken, possibly opening the door for Pacific disturbances to sneak into the Northwest. Meanwhile, a fairly active late-season monsoon is expected to continue over the Four Corners states.

Temperatures are projected to moderate a bit over the Northwest, but with above-average anomalies still expected (just not as extreme as before). Temperatures are also expected to stay relatively cool over the Four Corners region.

Thanks so much for reading and have a great weekend! Next update on Tuesday (September 2).

Alan Smith