Big Picture Weather Pattern:

A powerful trough of low pressure (more reminiscent of an autumn or cool season system) is bearing down on the Pacific Northwest and moisture will spread into Oregon and Washington over the next 2 days with rain favoring the West Coast to the Cascades.

After a brief early week lull, monsoonal moisture will also surge northward back into the U.S. from Mexico, favoring Arizona, Utah, and Colorado. Widespread thunderstorms with heavy rain can be expected across this region, and there will also be an increased threat of flash flooding. 

5-Day Temperature Outlook:

Temperatures will be well below average across the West Coast states over the next 5 days. The cooler air will also spread eastward over time, reaching the Central Rockies this weekend. Many areas will see their first taste of fall. 

Forecast for Wed (Aug 21) to Thu (Aug 22):

The two favored regions for rainfall will be the Pacific Northwest and the Southwest/Southern Rockies. There will also be scattered thunderstorms in Montana on Wednesday, with activity favoring the eastern-most mountain ranges. 

Pacific Northwest:

On Wednesday, scattered showers will develop over the West Coast, Olympics, and Cascades with more widespread rain developing over the Olympics on Wednesday night.

On Thursday, widespread rain will develop over the Oregon Cascades and Southern Washington Cascades with lighter showers for the North Cascades. The Olympics will continue to see showers on Thursday as well. To the south (not pictured), lighter (but still welcome) showers will extend into Northern California.

The Olympic Peninsula, NW Oregon, and the Southern Cascades from roughly Mt. Rainier to Crater Lake will see the highest rainfall totals during this 2-day stretch. Snow levels will range from 9,000 to 10,000 feet across most of this region.

Arizona:

A significant monsoon moisture surge will result in widespread thunderstorms with the potential for very heavy rainfall rates. Heavy rain and flash flooding potential will exist statewide, but northern parts of the state are favored for the heaviest rainfall (and highest flash flooding risk), including the Flagstaff/Sedona region and the Grand Canyon.

This active pattern will begin on Wednesday, but Thursday looks like the wettest day overall with more widespread thunderstorms. Also, overnight activity is possible on both Wednesday and Thursday nights.

Utah:

A moisture surge into Southern Utah will result in numerous thunderstorms with a heightened risk of flash flooding. Southwest Utah (Zion region) is favored for the heaviest rainfall, but Southeast Utah will see heavy downpours as well.

The northern extent of this moisture will reach the Central/Southern Wasatch Range near SLC/Park City and the Uinta Range, with lighter amounts (locally heavy downpours still possible) for these northern areas. 

The uptick in thunderstorm activity and flooding potential will begin in Southern Utah on Wednesday, while Thursday will be the wetter of two days with more widespread thunderstorms. Overnight and early morning showers/thunderstorms are also possible during this period.

Colorado:

Both Wednesday and Thursday will be active with numerous mountain thunderstorms expected statewide. Storms will be capable of frequent lightning and heavy rain, so make sure you are prepared if you have peak bagging or high-elevation hiking plans. 

While the highest coverage will be during the afternoon hours, there is also a chance of overnight and early morning shower/thunderstorm activity, so keep an extra close eye on the skies if you are heading above treeline early in the day. 

New Mexico:

Thunderstorm activity will favor the northwest part of the state and areas along the western border with Arizona, while the Sangre de Christo Range including Taos will be less active with only isolated thunderstorms expected. Heavy downpours will be possible with thunderstorms (mainly west of the Sangres).

Forecast for Fri (Aug 23) to Sat (Aug 24):

The active pattern will continue into the weekend thanks to the Pacific trough and the ongoing monsoon moisture surge.

1) Periods of rain will continue across the Pacific Northwest, with rainfall shifting inland to Eastern Oregon, Idaho, and Western Montana. There is still some uncertainty in the exact track of this system, but rain could reach as far south as Tahoe and the Tetons/Yellowstone will also be right on the edge.

2) A cold front will also move across the Northwest and Northern Rockies, and snow is a very real possibility across the higher terrain of the Cascades, Northern Rockies, and even the Sierra (including the higher peaks around Tahoe) if the precipitation makes it that far south. 

3) The Southern Rockies will see another active day on Friday with widespread thunderstorms and heavy rain potential in Utah, Colorado, and NW New Mexico, while Arizona will see less coverage compared to prior days. Saturday looks drier across Utah as a cold front pushes in (slight chance of showers/storms), while Colorado will continue to see thunderstorms though with less coverage compared to prior days. 

Forecast for Sun (Aug 25) to Mon (Aug 26):

Confidence in the forecast decreases for this period, as models are in poor agreement on how quickly the low pressure system exits the West.

For now, there is a decent chance that moisture will linger over the Northern Rockies on Sunday with some models projecting heavy rain across portions of Idaho and Montana, including Glacier National Park. High-elevation snow will also be possible across the Northern Rockies.

To the south, lingering monsoonal moisture will favor Colorado, New Mexico, and Southeast Arizona with scattered thunderstorms possible while most of Utah looks drier. This looks like more of a routine afternoon thunderstorm setup, with a reduced threat of heavy rain and flash flooding. 

Overall, the Pacific Northwest will see a drying trend. There is a chance that some light shower activity will sneak back into the Cascades (most likely on Monday), but confidence is low at this time.

Outlook for Tue (Aug 27) to Sat (Aug 31):

The pattern should calm down a bit next week, but it won't completely dry out. A lower-grade monsoonal pattern will continue across the Southwest with near-daily rounds of afternoon thunderstorms. Moisture looks to favor Colorado and New Mexico the most in this pattern. 

Also, a cold front is expected to sneak into the Northern Rockies around mid-week, with enough moisture to result in a chance of showers from the North/East Cascades into portions of Idaho and Montana. 

Temperatures will rebound to above-average values for late August across most of the West, while the Northern Rockies will be a little cooler than average due to the mid-week cold front.

Thanks so much for reading! Next update on Friday (August 23).

Alan Smith