Western US Daily Snow
By Alan Smith, Meteorologist Posted 9 months ago August 14, 2025
Heavy Rain for the Northwest, Monsoon Thunderstorms for the Rockies
Summary
An impressive storm system featuring an atmospheric river will impact Washington and Northwest Oregon from Thu night to Sat, bringing widespread heavy rain at a normally dry time of year. Meanwhile, monsoon moisture will continue to increase across the Southwest and the Rockies following a multi-week hiatus with an uptick in thunderstorms expected.
Short Term Forecast
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5-Day Temperature Outlook:
Temperatures will be below normal near the West Coast and above normal near and east of the Continental Divide over the next 5 days. Temperatures will also be closer to normal over the Southwest as monsoon moisture increases.
Thursday (Aug 14):
Monsoon moisture will increase across Arizona with scattered thunderstorms developing, some of which will produce heavy rain. Vulnerable terrain such as slot canyons, dry washes, and burn scars will be at risk of flash flooding.
To the north, moisture will also increase with scattered thunderstorms developing across the yellow-shaded region over the Central Rockies.
However, dry air in the lower atmosphere will lead to high-based thunderstorms, most of which will produce light to moderate rain at best, with a greater potential for lightning-triggered wildfires.
Friday (Aug 15):
Deeper monsoonal moisture will increase throughout the Four Corners region with more numerous thunderstorms expected in the green-shaded area. Storms will also be more productive with an increased potential for heavy rain, along with an increase in flash flood risk in Southern Utah slot canyons and in any fire burn scars across the area.
In the yellow-shaded zone, a higher percentage of "dry" thunderstorms are expected with light rain and gusty winds, which will continue to result in elevated fire weather concerns.
A powerful low-pressure trough by August standards will also impact the Pacific Northwest with an atmospheric river event expected – something we don't often see this early in the year.
Widespread heavy rain will develop early Friday morning and will continue through Friday night across the Cascades, Olympics, and western lowlands. Favored areas in the Cascades and Olympics will pick up 2-4 inches of rain, while the Puget Sound lowlands, including Seattle and Portland, will see 1-2 inches of rain.
Heavy rain is also expected north of the border across Southern British Columbia.
Rain showers will also pick up across Northeast Washington later on Friday and into Friday night.
Here is a rainfall projection from a blend of weather models across the Northwest that includes Thursday night, Friday, and Saturday totals. Pretty impressive for mid-August!
Saturday (Aug 16):
A broad trough over the Southwest will help to pull richer monsoon moisture northward into Northern Utah and Wyoming, resulting in an uptick in thunderstorm coverage.
Northern Colorado is also looking at another active day, while drier air at the base of the trough will lead to a reduction in thunderstorm activity over Northern Arizona, Southern Utah, and Southwest Colorado.
Southern New Mexico will continue to hang onto higher thunderstorm chances and heavy rainfall potential as deeper monsoon moisture will remain in place here as well.
To the north, rain showers will continue throughout the Northwest on Saturday. The Cascades, Olympics, and western lowlands will see lighter and more intermittent rainfall compared to Friday's washout, while the Inland Northwest will see a higher coverage of showers compared to Friday, with isolated afternoon thunderstorms also possible.
Sunday (Aug 17):
The Southwest trough will begin to lift northward with a combination of Pacific moisture and monsoonal moisture, favoring showers and thunderstorms across Central/Eastern Oregon, Idaho, Northern Wyoming, and Southern Montana. Storms in this region will be capable of producing heavy downpours.
To the south, a drying trend is expected across Utah, Colorado, and Northern Arizona/New Mexico with only some isolated afternoon thunderstorm activity. Deeper moisture will be in place across Southern New Mexico and Southeast Arizona, where thunderstorm coverage will be higher.
Monday (Aug 18):
A relative lull in the pattern is expected across much of the West. Lingering moisture over the Inland Northwest and Northern Rockies will result in isolated to scattered showers and thunderstorms, but coverage and rainfall amounts look much lower compared to prior days.
To the south, deeper monsoon moisture will remain confined to New Mexico and Eastern Arizona, where thunderstorms could produce heavy rain. Colorado and parts of Utah are looking at a more isolated thunderstorm setup, with storms generally producing light rain.
Extended Forecast
August 19-23:
Monsoonal moisture is expected to gradually increase and expand across the Southwest during this period. Initially, moisture looks to favor New Mexico and Colorado, with moisture then expanding westward across Arizona, Utah, and possibly into the Sierra Nevada Range by late in the week.
As a result, look for an uptick in thunderstorm coverage across these regions as the week progresses, with growing potential for heavy rain-producing storms.
Meanwhile, the Northwest will see a drying trend with increased sunshine and more seasonal temperatures.
Temperatures are expected to be above-normal across most of the West next week, though the West Coast and Northwest will hang onto near-normal temperatures (with a gradual warming trend over time).
August 24-28:
Longer-range models remain in good agreement regarding a stronger and more consistent monsoon pattern heading into late August. Moisture is projected to expand throughout the Southwest and the Rockies, potentially extending well into the Northern Rockies at times, as well as across the Sierra Nevada Range.
This could be an active thunderstorm stretch across most of the West, and hopefully, some areas that are currently very dry will be able to pick up some needed rainfall. But keep in mind, we are still 10+ days away, so confidence in the details is low.
Temperatures are generally expected to be warmer-than-normal across most of the West, but localized cooling will be possible in some areas depending on moisture levels and cloud cover.
Thanks so much for reading!
I will not be available to post on Friday, so enjoy the rest of your week and weekend, and check back in for my next post on Monday (August 18).
Alan Smith
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