Big Picture:

A ridge of high pressure is located over Arkansas with a trough of low pressure off the California Coast. South/southwest winds in between these features are allowing monsoonal moisture to flow into the Southwest U.S. and Southern Rockies.

Also, another tropical storm is playing a role in this monsoonal moisture surge. Tropical Storm Harold recently made landfall in Texas and the remnants of this system will work its way into the Southwest U.S. This will add additional moisture to the atmosphere and increase the threat of thunderstorms and heavy rainfall. 

Meanwhile, the effects of Hilary are waning across the West, though lingering moisture will become entrained into a trough of low pressure moving across Western Canada. As a result, Eastern BC and Western Alberta will see some beneficial rainfall to help with fires. 

Temperatures will remain below average across much of the West on Wednesday, with above-average warmth confined to areas near and east of the Continental Divide.

Below-average temperatures will persist near the West Coast through Thursday, then will warm up heading into the weekend. By Sunday, temperatures will be well above average across the Pacific Northwest, with near to slightly below average temps from California to the Central Rockies.

Fire and Smoke Update:

An increase in relative humidity, and for some areas, rainfall have led to a reduction in fire behavior in recent days across the Northwest and Western Canada. This trend will continue through the middle part of this week. Fire behavior may ramp up again in some areas this weekend as hotter and drier conditions return. 

More Details → Pacific Northwest and Western Canada Wildfire Update 

Areas of smoke can still be expected across portions of the Northwest and Northern Rockies over the next few days, but at a larger scale, smoke will be lighter and less widespread compared to the previous 7-10 days.

More smoke maps:

• Smoke (Surface) Map → Regular | U.S. Lower 48 States & Canada
• Smoke (Surface) Map → High Resolution | U.S. Lower 48 States

Forecast for Wednesday:

Scattered to numerous showers and thunderstorms can be expected across Utah, Arizona, and Southern/Western New Mexico with more isolated to scattered activity in Colorado. Heavy rain is expected across Southwest New Mexico in association with the remnants of Tropical Storm Harold.

Flash flooding is also possible in slot canyons and dry washes throughout the Southwest.

Further north, lingering moisture from Hilary will result in scattered showers/thunderstorms across portions of the Northern Rockies. A trough of low pressure moving into Canada will result in more numerous showers and thunderstorms and heavier rain across Central and Eastern BC.

Forecast for Thursday:

An area of low pressure associated with post-tropical storm Harold will continue to work its way into the Four Corners region with numerous showers and thunderstorms along with heavy rainfall expected across Western New Mexico, Northern/Central Arizona, Southern Utah, and Southwest Colorado including the San Juan Range.

Stronger thunderstorms will be capable of producing frequent lightning and heavy rain, and flash flooding is also possible with the greatest risk expected in Southern Utah.

Meanwhile, a trough of low pressure located off of the West Coast will transport some moisture into Central Oregon and portions of Northern California and Southern Washington via southerly winds. The result will be scattered showers and thunderstorms developing, especially near and just east of the Cascades.

Forecast for Friday:

An area of low pressure associated with Harold will work its way northward/eastward into the Utah/Wyoming/Colorado vicinity. This feature will help to focus shower and thunderstorm activity across these three states with frequent lightning and heavy rainfall possible while activity will become more scattered to isolated in Arizona and New Mexico.

More widespread heavy rain is currently projected to develop along the Colorado Front Range and adjacent plains as well as Southeast Wyoming. This could potentially result in flash flooding concerns along the eastern slopes of the Continental Divide.

Scattered showers and thunderstorms can also be expected across the Northwest in association with the trough of low pressure located offshore.

Forecast for Saturday to Sunday:

Additional showers and thunderstorms can be expected across the Central Rockies this weekend, with activity favoring areas near the Continental Divide in Wyoming and Colorado. Heavy rain will remain possible across these regions with the potential for flash flooding. 

Shower and thunderstorm activity will generally be more isolated to scattered outside of Colorado and Wyoming.

Outlook for Monday (Aug 28) to Friday (Sep 1):

A trough of low pressure is expected to move into the Pacific Northwest during the middle portion of this period, which has the potential to bring more widespread showers, favoring the Washington Cascades.

An active monsoon is expected to remain over the Southwest and Southern Rockies as well with near-daily rounds of thunderstorms expected. The Front Range of Colorado should dry out a bit compared to days prior, though.

Despite the active pattern, temperatures are expected to be warmer than average throughout the West during this period. The PNW will see some briefly cooler temps mid-week but will warm back up again toward the end of this period. Overall, we are not seeing any early signs of fall as we close out August. 

Thanks so much for reading! Next update on Friday (August 25).

Alan Smith