Forecast Highlights:

• Late summer heatwave for much of the West this weekend over the holiday weekend with record highs expected from the West Coast to the Great Basin and Northern Rockies.
• Monsoon has weakened significantly with only some isolated higher-terrain thunderstorms across the Southwest and Southern Rockies for most of this week.
• Slight uptick in thunderstorms for Colorado on Wednesday favoring areas near the Divide as a disturbance slides in from the northwest.
• Isolated dry thunderstorms for the Sierra and Cascades from Monday to Wednesday as a weak disturbances moves from south to north along the West Coast.
• Fire danger will gradually increase this week as hot and dry conditions take hold for an extended period of time. 

Big Picture:

A dominant ridge of high pressure is setting up over the Western U.S. and will remain in place for the next 1-2 weeks. This pattern will result in a late summer heatwave across much of the West with reduced thunderstorm activity compared to recent weeks.

Over the next couple of days, the only notable weather "feature" aside from high pressure will be weak disturbance located over Northern California that will migrate to the north, resulting in some isolated "dry" thunderstorms (< 0.1" of rain) across the Sierra and Cascades.

Monsoon Update:

Over the weekend, a trough of low pressure moving across the West scoured out most of the monsoonal moisture in place across the Rockies. The monsoon is now operating in a much weaker state, as is typical for this time of year, with lesser amounts of moisture confined to the Southwest U.S.

For the next couple of weeks or more, we'll continue to see a weaker monsoon across Arizona, New Mexico, and Southern Colorado with occasional brief intrusions further north.

We are expecting the pattern of endless thunderstorm days and heavy rain/flash flooding potential to be over for the summer, but an occasional active day with widespread storms and heavy rain potential is possible moving forward, especially if any tropical storm remnants from the Eastern Pacific migrate northward into the Southwest U.S. 

Rain and Thunderstorm Outlook for Monday:

Monday is shaping up to be a quiet day across the West with just some isolated thunderstorms across New Mexico, with a very low threat across Colorado, Arizona, and Utah.

The Lake Tahoe area and Northern California will also see some isolated dry thunderstorms due to the mentioned weak disturbance moving across the area. 

Rain and Thunderstorm Outlook for Tuesday:

Thunderstorm coverage will increase across the southern half of New Mexico on Tuesday, while Colorado and Eastern Arizona could see some isolated thunderstorms as well. The West Coast disturbance could also trigger some dry thunderstorms across Oregon and Idaho as it moves northward.

Rain and Thunderstorm Outlook for Wednesday:

A subtle disturbance sliding in from the northwest will result in an uptick in thunderstorm chances across Colorado on Wednesday, mainly near and east of the Continental Divide including the adjacent plains. Heavy rain is not expected with this feature, but active cloud-to-ground lightning will be possible.

Elsewhere, thunderstorm activity will be more isolated across New Mexico and Arizona. Also, isolated dry thunderstorms are expected to develop across Washington and Northern Idaho, including the Cascades, as a weak disturbance along the West Coast continues to move northward. 

Temperature Outlook:

Over the next five days, temperatures will be well above average throughout the West with record highs possible for many areas. If you're looking to escape the heat, then head to the higher elevations!

If there is one positive about this heatwave, it's that it's occurring late enough in the summer when the sun angle is weaker, the nights longer, and the mornings cooler compared to mid-summer when the heat would be more intense. 

Fire and Smoke Outlook:

The overall fire situation hasn't changed much since last week with most of the activity confined to the Northwest and California. Heavy smoke remains confined to areas near the fires, with occasional light to moderate smoke drifting further downwind from the fires.

The California coastline is also seeing some light smoke on Monday, though the latest models indicated this will be short-lived. 

Heading further out, we do run some risk of seeing an uptick in fire activity over the next week due to the extended spell of hot and dry conditions ahead. Also, dry thunderstorms across the Sierra and Cascades could factor in early this week.

The good news is that winds should be fairly light under this pattern for most of this week. However, this weekend and next week, minor disturbances rounding the top of the high-pressure ridge could potentially result in stronger winds at times across the Western U.S. which will be something to keep an eye on. 

Outlook for Thursday (9/1) to Friday (9/2):

Late this week, the West Coast and Northwest will see further drying with no thunderstorms expected. Meanwhile, a weak monsoon will keep isolated thunderstorm chances going on Thursday and Friday across Arizona, New Mexico, and Southern/Western Colorado. Of the two days, Friday looks a bit more active than Thursday. 

Outlook for Labor Day Weekend (Saturday 9/3 to Monday 9/5):

The late summer heatwave will be the dominant weather pattern across the West over the holiday weekend. The core of the heat is expected across Central and Northern California, where numerous record highs are likely to be broken.

In terms of rain and thunderstorm chances, Arizona, New Mexico, and Southern/Western Colorado will continue to be favored for daily rounds of isolated thunderstorms over the higher terrain in particular. 

However, monsoonal moisture is also projected to spread northwest into Southern Utah, Nevada, and into the Sierra Nevada Range of California on Sunday and Monday, resulting in increasing thunderstorm chances.

Another feature to keep an eye on is a weak trough of low pressure that will move into British Columbia on Saturday. Models are in poor agreement on whether or not this feature will extend far enough south to bring showers to the Olympics and Cascades in Washington on Friday night and Saturday, or if it stays far enough north for these areas to remain dry. 

Outlook for Tuesday (9/6) to Monday (9/12):

Summer is not planning to let go easily this year as well-above-average warmth is likely to persist across most of the West beyond Labor Day and into mid-September.

The best chances of showers and thunderstorms are expected across the Southwest during this period, primarily over Southern Arizona and Southeast California, which could be aided by tropical activity in the Eastern Pacific.

Drier conditions are expected the further north you go next week with only some occasional isolated thunderstorm activity. 

Thanks so much for reading! Next update on Wednesday (8/31).

ALAN SMITH