Forecast Highlights:

• Scattered thunderstorms across the Sierra, Great Basin, and Intermountain West on Wednesday, becoming more widespread and stronger on Thursday and Friday.
• Heavy rain and isolated flash flooding remain possible under stronger thunderstorms across the West for the remainder of the week.
• Fire activity and smoke conditions decreasing for now across California and the Great Basin, while fire activity remains high across the Inland Northwest and Northern Rockies.
• Cooler temperatures for the Pacific Northwest with showers across Western Washington on Wednesday night/Thursday morning, followed by a return to dry conditions.

Synopsis:

The North American Monsoon will continue to dominate the weather pattern across the West for the foreseeable future. On Wednesday, abundant moisture will remain in place from California to the Central Rockies though thunderstorm coverage will be less compared to recent days.

However, monsoonal moisture will further increase across most of the West (Washington and Nothern Idaho/Montana excluded) with another uptick in thunderstorm activity expected along with heavy rainfall potential.

Monsoon Update:

Abundant monsoonal moisture will remain in place across a large portion of the West on Wednesday, though the risk for heavy rain and flash flooding will be somewhat lower compared to recent days as the atmosphere stabilizes a bit. A trough of low pressure will also move across British Columbia on Wednesday night and Thursday, with some light showers expected across Western Washington.

On Thursday, a similar pattern will remain in place but monsoonal moisture will further increase and instability will also increase, resulting in a growing potential for heavy rainfall. 

On Friday, another trough of low pressure will move across the Northwest and into the Northern Rockies, and southwest winds associated with the trough will usher in anomalously high amounts of moisture into the Great Basin, Wasatch, and Greater Yellowstone Region with a greater potential for heavy rain and isolated flash flooding expected across these regions. The heavy rain threat will persist across the Sierra, Great Basin, and Four Corners as well.

Here is the 3-day rainfall projection from the European Model, which is projecting a wet signal across a large portion of the U.S. The Sierra, Great Basin, Northern/Central Utah, Western Colorado/New Mexico, and Greater Yellowstone Region could all see locally heavy rainfall, especially on Thursday and Friday.

Lightning Outlook:

Lightning activity on Wednesday will be most widespread across Southern Colorado, Northern New Mexico, and Southern Arizona, while Southwest Utah, Nevada, and the Sierra Nevada Range will be quite active as well. Thunderstorms will also extend northward into the Cascades of Northern California and Oregon, with more isolated activity across the Central and Northern Rockies.

An uptick in lightning activity is expected across most of the West on Thursday, with the West Coast, Pacific Northwest and Northern Idaho/Montana being the only areas that look to remain storm-free.

Another active thunderstorm day is expected across the West on Friday, though coverage should decrease a little bit across the Sierra compared to prior days as the focus of monsoonal moisture begins to shift eastward as a trough moves across the Northern Rockies. 

Across Colorado, Utah, and Wyoming, Friday looks like the most active thunderstorm day over the next 3 days.

Temperature Outlook:

Temperatures will be somewhat moderated across the West over the next 5 days with no major heatwaves expected. A couple of cold fronts moving across the Northwest and Northern Rockies over the second half of the week will signal the end of the recent heatwave this region experienced, while monsoonal moisture will also keep temperatures from getting too hot (relative to average) across the Great Basin and Southwest.

Fire and Smoke Outlook:

The recent surge of monsoonal moisture has led to a reduction in fire behavior across California and to some extent Idaho, at least for the time being. As a result, areas of the Intermountain West that were smoky earlier this week will see much lighter smoke levels over the coming days.

North of the monsoonal moisture core, fire activity has expanded across Southern British Columbia, Northwest Montana, and around the Black Hills, and these regions and areas downwind could see moderate to locally heavy smoke from time to time over the next few days.

Outlook for Saturday (8/6) to Tuesday (8/9):

Saturday is looking somewhat similar to Friday in terms of thunderstorm coverage and intensity with numerous storms expected across the Sierra, Great Basin, and Rockies from Wyoming to New Mexico.

On Sunday, the flow of monsoonal moisture will shift southward a bit with the Four Corners region being the most favored. Moisture should also decrease somewhat across California and Nevada on Sunday, though isolated thunderstorms will remain possible.

The Front Range of Colorado and Southern Wyoming and adjacent should also see an uptick in moisture and thunderstorm coverage on Saturday and Sunday.

On Monday and Tuesday, an active monsoon pattern will remain in place though uncertainty exists regarding which areas will be the most favored. The Four Corners region looks to be in the best position, but the Sierra, Great Basin, Northern Utah, and Front Range of Colorado will be "wild cards" early next week as well.

Outlook for Wednesday (8/10) to Wednesday (8/17):

Temperatures will be above-average across most of the West during this period, except for the Four Corners and Great Basin where temperatures will be near to slightly below average. 

 

An active monsoon pattern is expected to continue across the West during this period with the Sierra, Great Basin, and parts of the Northern Rockies being favored for above-average rainfall in addition to the Four Corners. 

Thanks so much for reading! Next update on Friday (8/5).

ALAN SMITH