Forecast Highlights:

• Strong thunderstorms with heavy rain will favor Arizona, Utah, and Wyoming from Friday to Sunday, including all of canyon country, the Wasatch Range, the Wind River Range, and the Tetons/Yellowstone.
• Scattered thunderstorms with light/moderate rain for Idaho, Montana, and Washington along with Northern/Western Colorado & New Mexico (localized downpours possible in CO).
• Elevated flash flood threat for slot canyons, dry washes, and mountainous terrain in Arizona and Southern Utah, as well as the mountainous terrain in Northern Utah and Western Wyoming. 
• Minimal smoke transport for most of the West, at least for the time being, while a mix of wet and dry thunderstorms will lead to elevated fire danger across the Interior Northwest on Friday. 
• Thunderstorm chances increase across Eastern Colorado & New Mexico early next week, and also across Nevada and California including the Sierra Nevada Range. 

Synopsis:

A trough of low pressure located off of the Washington Coast is resulting in a southerly flow across the Interior West on Friday that is transporting abundant monsoonal moisture from Arizona northward into Utah and Wyoming. A bit of this moisture has also extended northward into the Inland Northwest, which will interact with the jet stream to result in scattered thunderstorms. 

A similar pattern will hold through Saturday, then on Sunday, the trough of low pressure will sweep inland and move across Canada. This will help to usher in drier air across the Northwest and Northern Rockies, while the focus of monsoonal moisture remains across the Southwest and the Central Rockies. A weak area of low pressure is also expected to develop across Arizona, which could lead to enhanced thunderstorm coverage.

Monsoon Update:

A ridge of high pressure centered near the Colorado/Kansas border will keep the eastern ranges of Colorado & New Mexico on the drier side, while south/southwest flow associated with the low-pressure system off the Washington/BC Coast will transport significant monsoonal moisture into Arizona, Utah, and Wyoming.

All of these areas will see an increased threat of heavy rain, excessive runoff, and flash flooding in vulnerable areas on Friday and Saturday.

Further north, monsoonal moisture will interact with the jet stream to result in scattered thunderstorms throughout the Interior Northwest and Northern Rockies, as well as the Washington Cascades. Thunderstorms for these northerly areas will be a mix of wet and dry storms, but brief downpours will be possible under stronger storms.

On Sunday, the jet stream will shift inland into the Northern Rockies with drier air spreading into Washington, Northern Idaho, and Montana.

Eastern Idaho and Northern Wyoming will see a relative decrease in moisture as well compared to prior days, while the most significant moisture levels (and heavy rain threat) will exist across Arizona, Nevada, Utah, and Southern/Central Wyoming. The northern-most ranges of Colorado could see some heavy rain as well.

Total rainfall from Friday through Sunday will be heaviest across large swaths of Arizona, Utah, and Wyoming, while far northern Colorado could also see some decent amounts. Rainfall will be spottier across the Northwest and Northern Rockies, as well as Central Colorado and New Mexico. Rain chances will also be marginal across the Sierra in California.

Lightning Outlook:

Lightning activity will be extensive not only across Arizona, Utah, and Wyoming on Friday, but also across the Inland Northwest and Northern Rockies, including Eastern Washington & Oregon, Northern/Central Idaho, and Western/Central Montana. While the Northwest will see a mix of wet and dry thunderstorms (dry storms mean less than a tenth of an inch of rain), there will be some potential for lightning-triggered fires. 

Thunderstorm coverage will be somewhat limited across Southern Colorado and Northern New Mexico, with a better chance of storms across Northern and Western Colorado, along with Southwest New Mexico.

On Saturday, a similar setup is expected with extensive lightning across Arizona, Utah, Wyoming, Montana, and Eastern Idaho. Northern and Western Colorado will also see scattered thunderstorms. Across the Inland Northwest, thunderstorms are unlikely for Eastern Washington/Oregon and Western Idaho as most of the activity shifts eastward into Montana. 

Across the Far West, the SoCal Mountains and Southern/Eastern Nevada will see quite a bit of lightning activity as well, while the Central and Northern Sierra will only see a marginal threat for thunderstorms.

Thunderstorm coverage will begin to decrease across Montana and Idaho as drier air moves in, while Arizona, Utah, and Wyoming will see another active lightning day. Thunderstorm coverage will pick up a bit across Western and Central Colorado as well. Nevada and SoCal will also see thunderstorms, with only a slight chance across the Sierra.

Temperature Outlook:

On Friday and Saturday, extensive monsoonal moisture will have a moderating effect on temperatures across the Southwest as well as Utah and Western Wyoming where temperatures will be near to below average. The Pacific Northwest west of the Cascade Crest will also see cooler than average temperatures.

Hotter than average temperatures will prevail into the early part of the weekend across the Northern Rockies as well as Eastern Colorado and New Mexico. 

Early next week, the pattern will change as a tall ridge of high pressure sets up near the West Coast. This will result in a transition to well-above-average temperatures across California, Oregon, Washington, Idaho, and Northwest Montana. 

Meanwhile, a cold front will slip down the eastern side of the Continental Divide, ushering in a cooler airmass into eastern portions of Wyoming and Colorado. An active monsoon will continue to have a moderating effect on temperatures across Utah and Arizona as well. 

Fire and Smoke Outlook:

No news is good news! While we are near the climatological peak of fire season, smoke conditions are very quiet across the West for the time being thanks to the uptick in monsoonal moisture and associated relative humidity. Only the McKinney Fire in Nothern California is expected to have somewhat of an uptick in smoke conditions on Friday.

Heading further out in time, fire behavior from ongoing fires across Idaho, Montana, and BC will likely pick up again once monsoon moisture decreases late this weekend and early next week and temperatures heat up. 

We'll also have to keep an eye on the Pacific Northwest, where extensive lightning activity has occurred over the past few days but with only spotty rainfall. A lot of small fires have been reported across Washington and Oregon this week due to lightning, but it's unknown yet if any of these fires will flare up into anything larger. 

Outlook for Monday (8/15) to Thursday (8/18):

On Monday and Tuesday, a tall ridge of high pressure will build along the West Coast, while monsoonal moisture will shift southward and westward with a noticeable uptick in thunderstorm coverage and rain potential expected across the Sierra Nevada Range.

Northern Utah and Western Wyoming will see drier conditions with only isolated thunderstorms, while Arizona and Southern Utah will remain in a very active thunderstorm pattern.

Colorado and New Mexico will also see an uptick in moisture and thunderstorm chances statewide. This includes the Front Range and areas east of the Continental Divide, where upslope winds behind a cold front will support better thunderstorm chances along with the potential for heavy rainfall.

On Wednesday and Thursday, a weak trough is projected to move into the Northwest U.S., and this will likely draw moisture northward into Oregon and Idaho where an uptick in thunderstorms is expected. 

The Front Range and areas east of the Divide in should see a drying trend on Wednesday/Thursday, while New Mexico, Western Colorado, Arizona, Utah, Nevada, and California should remain in an active pattern with daily rounds of thunderstorms along with the potential for heavy rain.

Outlook for Friday (8/19) to Friday (8/26):

Temperatures are expected to be above average along the West Coast and across the Northern Rockies during this period, and near to below average across the Southwest.

The monsoon looks to remain active heading into late August as well with wetter than average conditions expected across the Southwest, especially in Arizona and New Mexico. 

Thanks so much for reading and have a great weekend! Next update on Tuesday (8/16).

ALAN SMITH