Big Picture Weather Pattern:

A few different weather features will be factors across the West this weekend...

1) Northern Rockies – On Friday, a departing low pressure system will contribute to numerous showers and thunderstorms from Northern Montana into the Canadian Rockies (including Glacier and Banff).

2) Pacific Northwest – A deep trough of low pressure will bring impressive moisture to this region (for mid-August) on Saturday, resulting in numerous showers and thunderstorms along/west of the Cascade crest and in NE Washington, with locally heavy rainfall. Lingering showers/storms are also possible on Sunday.

3) Southwest to Central Rockies – Monsoonal moisture will surge back into this region on Saturday and Sunday, resulting in an uptick in thunderstorm chances with locally heavy downpours. New Mexico see less moisture than other areas, however, due to high pressure deflecting most of the moisture to the west and north.

5-Day Temperature Forecast:

Temperatures will remain cooler than average near the West Coast with a trend toward warmer than average temperatures across the Rockies.

There is only a slight warm signal for most areas in the Rockies (thanks in part to moisture and afternoon thunderstorms), while New Mexico is expected to be well above average with drier conditions and more sunshine expected. 

Forecast for Fri (Aug 16) to Sun (Aug 18):

Rainfall through the weekend will favor the Pacific Northwest (west of the Cascade crest to the coast), and Northwest California will see some of this moisture as well. The Rockies will also turn more active this weekend with scattered thunderstorms producing locally heavy rain. New Mexico will only see isolated thunderstorm activity in this pattern. 

Let's dive into the details by region...

Pacific Northwest:

Mostly dry conditions are expected on Friday, then numerous showers and thunderstorms will develop on Saturday, favoring the west side of the Cascades and the mountain ranges of Northeast Washington and Northern Idaho.

Locally heavy rain is possible with showers and storms, and there is also a flash flood watch in effect for some areas due to the potential for burn scar flooding. Rain totals will be highly variable across the region, but the Olympic Peninsula should see some of the heaviest and most widespread rain. 

Most areas will start to dry out on Sunday, but Washington could hang onto some shower and thunderstorm chances with lighter rainfall. 

Arizona:

Mostly dry conditions are expected on Friday except for some isolated thunderstorms over Southern Arizona. On Saturday and Sunday, monsoonal moisture will surge northward into Arizona, primarily favoring northern portions of the state with more isolated activity for the southern half of the state. 

Heavy rain will be possible with thunderstorms, and there is at least a slight risk of flash flooding in vulnerable terrain statewide. Northern portions of the state, including the Flagstaff area and the Grand Canyon, are favored for the highest rainfall totals this weekend. 

Utah:

Friday will be dry, then moisture will arrive from the south on Saturday and Sunday with a good chance of thunderstorms statewide on both days. Heavy rain and frequent lightning will be possible with thunderstorms, and there will be a risk of flash flooding in slot canyons and dry washes of Southern Utah.

Colorado:

Friday looks like a rare near-zero thunderstorm chance day over the higher peaks of Colorado.

On Saturday, a little bit of moisture will sneak into Western Colorado with isolated thunderstorm chances returning to the picture west of the Divide. 

On Sunday, a significant uptick in moisture is expected with more widespread thunderstorms expected statewide. The Central Ranges (Elks and Sawatch especially) look favored for the heaviest rainfall, but heavy downpours and frequent lightning are possible in all mountain ranges. 

Northern Rockies:

On Friday, showers and thunderstorms are expected across Northwest Montana in association with a low-pressure system moving through. Glacier National Park and the Flathead Lake region will be favored for significant rainfall.

Saturday looks dry for most areas, then on Sunday, monsoonal moisture will arrive from the south with an uptick in thunderstorms expected across Wyoming, Southern/Central Idaho, and Southwest Montana.

Forecast for Mon (Aug 19) to Tue (Aug 20):

The Northwest will dry out mostly, but some isolated showers could linger over the Cascades and Olympics into Monday. A consistent monsoon pattern will remain in place over the Rockies with daily rounds of thunderstorms, some of which will produce locally heavy rain. New Mexico looks drier overall, though northern portions of the state should see a bit of an uptick in activity. 

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

Not much change to the pattern is expected with daily thunderstorms continuing to favor the Rockies, though New Mexico may see more of an uptick compared to previous days.

Also, another low pressure system will approach the Pacific Northwest, and there is a chance that areas along and west of the Cascades pick up more rain, but confidence is low at this time. 

Fire and Smoke Outlook:

The cooler and wetter pattern is helping to scale back fire intensity and smoke transport across the Pacific Northwest. However, fires burning in Idaho are not getting the same level of cooler/wetter conditions, and fires are burning more intensely there as a result. 

Winds blowing from the southwest will transport smoke from these Idaho fires into portions of Western Montana at times, with general hazy conditions otherwise expected across interior portions of the Northwest. 

Outlook for Fri (Aug 23) to Tue (Aug 27):

A similar pattern is expected with "troughiness" and cooler temperatures continuing along the West Coast, while another wet system or two (by August standards) could track across the Northwest and Far Northern Rockies with showers and thunderstorms possible.

The monsoon will continue at a moderate intensity from the Southwest into the Central Rockies with typical afternoon thunderstorms expected. Temperatures will be running slightly above average for late August in most of this region, while temperature anomalies will be higher over Central/Southern New Mexico. 

Thanks so much for reading and have a great weekend! Next update on Monday (August 19).

Alan Smith