Big Picture Weather Pattern:

We have a lot going on over the next three days. Here are the key weather players...

1) Monsoonal moisture will remain over the Southern/Central Rockies on Friday, then will gradually shift east and south over the weekend with a gradual downtick in thunderstorm coverage (lingering moisture/storms will favor CO & NM).

2) A strong cold front will move across the Interior West on Saturday PM/Sunday AM, bringing a taste of fall to many areas.

3) A Pacific low pressure system with impressive moisture levels for August will favor Washington & Northern California with heavy rain, while the Northern Rockies are trending toward lighter and less widespread showers. Snow will fall over some higher peaks and passes as colder air arrives. 

5-Day Temperature Forecast:

Temperatures will be below average near and west of the Continental Divide and slightly above average east of the Rockies. The coolest temperatures will occur in most areas over the weekend with a gradual warming trend expected next week.

Forecast for Fri (Aug 23) to Sun (Aug 25):

It will be an active weekend across most of the West with the heaviest rainfall totals expected in Washington, Northern California, Eastern Utah, and Western Colorado.

Pacific Northwest:

The next storm system will split into two main waves, with the northern wave tracking across Washington and the southern wave tracking across Northern California. Oregon will still see rain showers in this pattern, but not as much compared to areas north and south.

Washington will see its heaviest and most widespread rainfall on Friday and Saturday with the North Cascades seeing the highest totals. Thunderstorms are also possible, with the best chance over NE Washington.

Oregon will see most of its showers on Friday before trending drier on Saturday. Eastern Oregon could also see thunderstorms on Friday. Both Washington and Oregon will see a break in the pattern with drier conditions on Sunday.

Colder air arriving on Friday night and Saturday will also result in snow levels dipping to 6,500 to 7,500 feet.

Northern/Central California:

The northern coast of California will see heavy rainfall from this system on Friday with significant precipitation extending inland toward Mt. Shasta and Lassen Peak from Friday into Saturday.

Lighter rain showers will reach the Tahoe Region and northern/central Sierra Nevada Range on Friday night and Saturday.

Unseasonably cool air arriving with this system will result in snow levels dipping as low as 7,000 feet around Mt. Shasta, and the peaks around Tahoe could also get dusted with snow with snow levels falling to around 8,000 feet on Saturday morning. 

Northern Rockies:

On Friday, monsoonal moisture will favor scattered showers and thunderstorms across Wyoming and Southern Idaho, while Northern Idaho and NW Montana will also see showers and thunderstorms as moisture arrives from the approaching Pacific low pressure system.

On Saturday, a lull in the pattern is expected with just some isolated light showers, mainly over Idaho. Temperatures will drop noticeably as a cold front arrives Saturday night.

On Sunday, a trailing disturbance behind the cold front will bring light to moderate showers to Idaho, Southwest Montana, and Northwest Wyoming. In this cooler airmass, snow levels will dip to 9,000-10,000 feet, resulting in dustings of snow over some of the higher peaks and passes in this region.

Utah:

Significant monsoonal moisture will remain in place on Friday with numerous thunderstorms capable of producing heavy rain and flash flooding. While the Zion region will still see storms and heavy downpours, the focus of the heaviest activity will shift into Eastern Utah, and northward into the Uinta Range.

On Saturday, moisture decreases with thunderstorms more isolated to scattered in nature, and largely confined to Eastern Utah (Canyonlands, Moab, and north to the Uintas).

On Sunday, a disturbance moving through could lead to a relative uptick in storms over Eastern Utah (low confidence), with drier conditions across Western and Northern Utah.

Colorado:

Friday will be an active thunderstorm day with Western Colorado most favored (San Juans, Elks, Grand Mesa, Western Slope). Frequent lightning and heavy rain will be possible, along with the potential for isolated flash flooding.

On Saturday, moisture decreases slightly with thunderstorms more scattered in nature. It still looks like a fairly active day with thunderstorm potential for all mountain ranges, with the Southern San Juan Range seeing the most widespread activity and heaviest rainfall potential.

On Sunday, a disturbance moving through could result in a relative uptick in storms compared to Saturday (favoring the western ranges once again), but confidence is lower. 

Arizona:

Scattered thunderstorms can still be expected during this period, but the pattern will not be as active compared to earlier this week. Thunderstorm activity will favor northern and eastern portions of Arizona, with locally heavy downpours still possible. 

Friday looks like the most active day followed by a downtick on Saturday, and then followed by another slight uptick on Sunday. 

New Mexico:

Significant moisture will push further east into New Mexico during this period compared to earlier in the week. 

On Friday, moisture will continue to favor Northwest New Mexico, then on Saturday, moisture begins to increase eastward toward Taos and Santa Fe with more numerous storms capable of heavy rain. Thunderstorm chances and rainfall potential will also increase to the south across the Sacramento Mountains and Ruidoso area on Saturday.

Sunday looks like the wettest and most active day statewide as moisture levels continue to increase.

Forecast for Mon (Aug 26) to Tue (Aug 27):

On Monday, there may be enough moisture at the tail end of a departing disturbance for some additional showers across Northern Wyoming and Montana (including possible snow up high), but confidence is low on the timing and track of this feature.

On Tuesday, a system will pass across British Columbia and clip the North Cascades and possibly Glacier National Park as it tracks eastward.

Monday looks like an active thunderstorm day across Colorado, New Mexico, and Eastern Arizona, with a downtick on activity possible in Colorado on Tuesday as drier air briefly pushes in from the west/northwest. New Mexico still looks wetter on Tuesday.

Forecast for Wed (Aug 28) to Thu (Aug 29):

A disturbance passing across the Northern Rockies could bring some showers to Montana including Glacier on Wednesday, before drying out on Thursday. Monsoonal moisture will continue to circulate across the Southwest, resulting in scattered afternoon thunderstorms. 

Outlook for Fri (Aug 30) to Tue (Sep 3):

Heading into Labor Day weekend, monsoonal moisture will remain in place over the Southwest with near daily rounds of afternoon thunderstorms expected across the Four Corners states. Some models are hinting at disturbances sliding down the east side of the Divide in the Northern Rockies, which would also result in occasional thunderstorm chances.

Temperatures are expected to warm up throughout the West, reminding us that summer is not quite over yet. In fact, the highest temperature anomalies are expected across the Pacific Northwest (which has recently been very cool and wet). 

Thanks so much for reading and have a great weekend! Next update on Monday (August 26).

Alan Smith