Forecast Highlights:

• Monsoonal moisture will result in near daily t-storm chances across the Four Corners states + Wyoming and Nevada this week with active cloud-to-ground lightning being the main hazard.
• Weak area of low pressure over the Northwest U.S. will result in near daily t-storm chances across portions of the Cascades, Inland Northwest, & Northern Rockies with the heaviest rain expected in Montana.
• Isolated flash flood potential can be expected for slot canyons and previously saturated areas across the Rockies this week, but overall the flooding threat will be much lower compared to the previous week.
• Fire activity and smoke transport remains limited and confined to isolated pockets across the Northwest and Northern Rockies.
• Monsoon looks to weaken and become suppressed further south starting next week with above-average warmth expected for most of the West.

Big Picture:

The large-scale weather pattern is taking on an unusual appearance early this week. A tall ridge of high pressure has set up over the West and is extending northward all the way into the Arctic Circle. However, a weak area of low pressure is undercutting the ridge and setting up/stalling over the Pacific Northwest.

This area of low pressure has transported subtropical moisture from the South Pacific into the Northwest U.S. and will result in an active thunderstorm pattern across the Cascades, Inland Northwest, and Northern Rockies.

A weak area of low pressure is also moving slowly across Colorado early this week and will eventually work its way into Kansas, with monsoonal moisture circulating around the low favoring the high country of Colorado and New Mexico.

Monsoon Update:

On Monday, monsoonal moisture via westerly winds aloft will contribute to afternoon thunderstorms across Eastern Idaho, Western Wyoming, and Northern Utah, while storm motions will take on more of a north-to-south orientation across Colorado and New Mexico.

Monsoon moisture will linger across Arizona and Utah as well, but storm activity is expected to be less compared to recent days with a relative break in the heavy rainfall threat expected. That doesn't mean you should totally let down your guard in canyon areas, though. 

Also, an abundance of Pacific moisture with subtropical origins is working its way into the Northwest, which will lead to an uptick in thunderstorm activity on Monday and over the days to follow.

Tuesday's pattern will be similar to Monday's, with the main difference being that moisture will begin to increase from east to west into Arizona with a better chance of rain (including isolated heavy rain) expected.

On Wednesday, moisture will continue to increase across Arizona, as well as Southern California, Southern Nevada, and Utah. The Greater Yellowstone region of Eastern Idaho, Western Wyoming, and Southwest Montana will see an increase in moisture as well.

Further north, the area of low pressure will remain over Washington with moisture circulating around the low in Northern Idaho, Northwest Montana, British Columbia, and over the WA Cascades. All of these areas will be favored for thunderstorms, with the potential for heavy rain across Western Montana (and possibly Northern Idaho) on both Tuesday and Wednesday.

Lightning Outlook:

Lightning activity is expected to be greatest across Idaho, far Northern utah, Western Wyoming, and Southwest Montana on Monday, while moderate activity can also be expected across Central Colorado and New Mexico. Lightning will be more isolated across Arizona, Southern Utah, and Nevada, while a few stray storms are also possible over the Sierra Nevada Range.

Also, notice the uptick in lightning activity expected across Northern Washington as well as a large portion of BC.

On Tuesday, the North Cascades of Washington, as well as Montana and Central/Eastern Wyoming will be favored for the most widespread thunderstorms. Southern/Central Arizona will also see an uptick in lightning.

Colorado and New Mexico will see more isolated thunderstorms favoring the higher terrain, while thunderstorm activity should be sparse across Utah.

On Wednesday, lightning activity looks to increase across much of the West. The Cascades will see some thunderstorms, while further east, the Inland Northwest and Northern Rockies will see more abundant lightning activity.

Heading south, lightning activity will also pick up across Arizona, SoCal, Nevada, and Utah while Colorado and New Mexico will see a slight uptick as well compared to Tuesday. The Sierra Nevada Range in California (excluding far southern portions) will be the best bet if you're looking for thunderstorm-free conditions on Wednesday. 

Temperature Outlook:

Temperatures will generally be warmer than average this week across the West Coast states and Great Basin, due to the ridge of high pressure set up across Western North America. Temperatures will be closer to average across Montana, Wyoming, and Northern Colorado, while temperatures will be below average across Arizona, New Mexico, and Southern Colorado.

Fire and Smoke Outlook:

Fire season continues to be relatively quiet across the West compared to the past couple of summers. There are a handful of moderate to large fires burning across the Northwest and Northern Rockies, but heavy smoke is generally confined to areas near and just downstream of these fires with only occasional light smoke expected elsewhere.

The one area of high fire danger due to weather this week can be expected across the Inland Northwest on Monday due to the threat of "dry" thunderstorms (storms that produce less than a tenth of an inch of rain under the heaviest part of the storm). However, this area will see increasing humidity and a better chance of wet vs. dry thunderstorms from Tuesday on.

The rest of the West is generally expected to see low to moderate fire weather concerns for the remainder of the week. While I know that not every area has been spared, overall it sure has been nice to see a break in the widespread smoke this summer across the West after what we dealt with last summer!

Outlook for Thursday (8/25) to Sunday (8/28):

On Thursday and Friday, the pattern will remain fairly similar to previous days with an active monsoon across the Four Corners states. This will result in daily rounds of thunderstorms with isolated areas of heavy rain possible. Moisture will extend just far enough west for the Sierra to see some isolated thunderstorm activity as well.

Further north, lingering low pressure will continue to result in thunderstorms across the Northern Rockies on Thursday and Friday with locally heavy rain possible across Montana in particular. A secondary trough of low pressure could also produce some lighter showers/thunderstorms across the Cascades.

Heading into the weekend, the pattern will start to shift a little bit. A trough of low pressure moving across the Northern Rockies will initially bring another round of showers/thunderstorms to Montana on Saturday with drier air expected to work into the Northern Rockies on Sunday (drier air should arrive a day earlier, across the PNW, on Saturday).

Saturday could be another active day for thunderstorms across the Four Corners states. However, in the wake of the trough moving across the Northern Rockies, monsoonal moisture should start to decrease and become suppressed further south on Sunday with the focus of thunderstorms expected across Arizona/New Mexico while Utah and colorado should see more isolated activity.

Keep in mind that Saturday and Sunday's outlook is subject to change depending on the timing/track of the Northern Rockies trough. 

Outlook for Monday (8/29) to Monday (9/5):

A ridge of high pressure is expected to dominate the weather pattern next week, resulting in above-average temperatures for nearly the entire West. As a result, summer-like warmth is currently expected to continue right into Labor Day weekend (with the obvious caveat that this is still almost 2 weeks away). 

Believe it or not, we're also seeing our first signs of a weakening monsoon on the heels of what has been a very active monsoon season. The bulk of the moisture is expected to be suppressed further south into New Mexico and Southeast Arizona, and thunderstorm activity should be most widespread across this region as a result.

Colorado and Utah are likely to see a much drier pattern during this period, though isolated afternoon thunderstorms will remain possible at times. Drier conditions are also expected further north. 

Thanks so much for reading! Next update on Wednesday (8/24).

ALAN SMITH