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Forecast Summary:

• A slow-moving Pacific trough will dominate the West this week, bringing widespread showers and thunderstorms from California and Oregon into the Rockies.

• Monsoonal moisture will surge north into Western Colorado and New Mexico, producing heavy rain, flash flooding risks, and some severe storms with hail and damaging winds.

• High Sierra peaks will see light snow above 10,000–11,000 feet midweek, with snow also dusting areas above treeline in the Rockies late in the week.

• Cooler and wetter conditions will help reduce wildfire activity and smoke impacts, though not eliminate them completely.

• Weekend Outlook – The main trough shifts east with lingering showers across the Rockies, before another system reaches the Northwest on Sunday with increasing rain chances.

• Longer Range (Sep 15–24) – The West stays unsettled with mild temperatures and occasional showers over the Northwest, while a late-season monsoon surge is possible over the Southwest next week. 

Big Picture Weather Pattern:

We have an active pattern ahead over the next several days as a slow-moving Pacific trough works its way across the West. This will lead to frequent showers and thunderstorms across California, Oregon, and into the Northern Rockies.

To the south, southerly flow at the leading edge of the trough will pull monsoonal moisture northward into Western Colorado and Northern New Mexico, where heavy rain is expected along with the potential for flash flooding and even localized severe thunderstorms with strong wind gusts and large hail. 

As for wildfires and smoke, the overall trend will be for decreasing fire behavior and decreasing smoke as cooler and wetter conditions take hold, but that doesn't mean the issue will go away completely in all areas.

5-Day Temperature Forecast:

Temperatures will be well below normal across California, with the High Sierra peaks expected to see some snow, while an overall cooling trend will take hold over the remainder of the West. Washington and areas near the Canadian border will see continued above-normal temperatures, but cooler compared to recent days/weeks.

Wednesday (Sep 10):

Sierra/Northwest – A deep and slow-moving trough is located over the West with a center of low pressure aloft over Northern California. Pacific moisture streaming into the area will contribute to numerous showers and thunderstorms from Northern California to Oregon and Western Idaho.

Most areas in the green-shaded region will see anywhere from 0.1 to 0.5 inches of rain, but locally higher amounts of an inch or more could occur in Northern California and Oregon, with a risk of flash flooding in recent fire burn scars.

The cooler airmass will also result in snow levels dipping to 10,000-11,000 feet in the Sierra Nevada Range with light snow dusting the higher peaks. The cooler/wetter pattern should also help with the wildfire and smoke situation across this area. 

Rockies – Ahead of the trough, monsoon moisture will increase across Northern New Mexico, Western Colorado, and Eastern Utah, resulting in scattered afternoon thunderstorms. Most storms will produce light to moderate rain, but locally heavy downpours are possible in the San Juan Range in Colorado. 

Thursday (Sep 11):

California to Northern Rockies – Scattered showers can be expected off-and-on throughout the day with thunderstorms also possible in the afternoon. Most areas in the green-shaded region will pick up an additional 0.05 to 0.30 inches of rain, but isolated higher amounts will be possible, especially in Idaho and Montana, where stronger afternoon thunderstorms are possible.

Snow levels will remain in the 10,000-11,000-foot range over the Sierra with light snow for the higher peaks, while snow levels will be higher to the east over the Northern Rockies, where temperatures will be warmer.

Central/Southern Rockies – Increased "lift" from the approaching trough, along with high levels of monsoonal moisture for this late in the year, will contribute to numerous showers and thunderstorms across Southwest and Western Colorado, along with Northern New Mexico and the eastern borders of Utah and Arizona.

The San Juans, Elks, and Grand Mesa will see rainfall totals ranging from 0.4 to 1.2 inches with isolated higher amounts possible, which could lead to some localized runoff and flash flooding concerns.

Also, strong to severe thunderstorms will be possible during the afternoon across Western Colorado with the potential for 0.5-inch to 1-inch diameter hail along with wind gusts of 60 mph or higher.

Friday (Sep 12):

Colorado/Southern Rockies – Another active day is expected across Western Colorado with deep moisture in place as the trough slowly progresses eastward. Numerous showers and thunderstorms can be expected west of the Divide.

The San Juans in Colorado and the Sangres and surrounding areas in Northern New Mexico will take the brunt of this system with widespread rain totals of 0.50 to 1.25 inches, with isolated higher amounts under stronger thunderstorms.

Flash flooding will also be a greater concern across this region, especially with saturated soils from prior days' rainfall. Also, strong to severe thunderstorms are possible in the afternoon with 0.5 to 1-inch diameter hail and 60 mph wind gusts possible. 

Other western ranges of Colorado (Elks, Sawatch, Grand Mesa), as well as the La Sals in Utah, could also see locally heavy rain with some risk of flash flooding in areas with poor drainage. 

Northern Rockies – Scattered showers and thunderstorms can be expected once again as this region remains under the influence of a trough. Most areas will pick up an additional 0.05 to 0.25 inches of rain, but isolated heavier amounts are possible, especially with thunderstorms.

Sierra to Great Basin – Lingering moisture will result in isolated showers and thunderstorms, but coverage and rainfall amounts will be lower compared to prior days. Also, temperatures will begin to warm up with snow levels creeping higher in the High Sierra.

Saturday (Sep 13):

Rockies – The trough will push eastward on Saturday with lingering moisture resulting in abundant cloud cover and scattered showers (AM and PM) throughout the Rockies. Rainfall rates will be lighter compared to prior days, but conditions are still expected to be wet across much of this region, especially in Colorado and New Mexico, due to rain from prior days.

Temperatures will also be cooler on Saturday throughout the Rockies with snow levels ranging from 11,000-12,000 feet, meaning that higher peaks and many areas above treeline will get dusted with snow. 

Sunday (Sep 14):

Pacific Northwest – The next trough will reach the Northwest on Sunday with showers developing across Washington, Oregon, BC, Northern Idaho, and Northwest Montana. Thunderstorms are also possible east of the Cascades on Sunday afternoon.

Rockies – An overall drying trend is expected, though lingering moisture could lead to stubborn clouds and isolated light showers over some of the higher peaks. Snow levels look to remain around 11,000-12,000 feet. The lightning threat looks low at this time, though it couldn't be ruled out if there is enough solar heating in the afternoon. 

September 15-19:

A generally unsettled pattern is expected next week, though no major fall storm systems are expected at this time.

Frequent weak disturbances are projected to track across the Pacific Northwest and Northern Rockies, which will favor a scattered showers-type pattern with possibly some afternoon thunderstorms.

A late-season monsoon moisture surge is possible across the Southwest, which could lead to an uptick in showers and thunderstorms across Arizona, New Mexico, and southern portions of Utah and Colorado. This moisture surge may be enhanced by tropical activity over the Eastern Pacific, though confidence is low on how this will play out. 

Temperatures are generally expected to be warmer than average over the Pacific Northwest, and potentially cooler than average over the Southwest (depending on how the monsoon surge plays out). Keep in mind that "average" temperatures are quickly decreasing at this time of year.

September 20-24:

Overall, the pattern looks to remain mildly unsettled across the West with no major fall storm systems projected at this time. However, that could always change since this is 10+ days out.

For now, the pattern looks to favor additional weak disturbances across the Pacific Northwest and Northern Rockies while a low-grade monsoon may persist across the Southwest.

Thanks so much for reading this summer! This will be my last post of the season as we gear up for the winter season.

Be sure to follow us throughout the winter season, as we have Expert Daily Snow forecasters for specific regions, snow and weather forecasts for ski resorts and any location/elevation in the world, and numerous high-resolution weather maps.

We also have some exciting new features we are planning to release this winter, so stay tuned for that as well.

Thanks again!

Alan Smith