Forecast Highlights:

• Monsoonal moisture results in daily thunderstorms across the Southwest and Central/Southern Rockies through the end of this week and into the weekend.
• A trough of low pressure over the Northwest will lead to daily thunderstorms across the Cascades, Inland NW, and Northern Rockies from Wednesday to Saturday with heavy rain possible across Montana.
• Isolated flash flooding is possible across the West this week though widespread flooding is not expected. Dry washes, slot canyons, burn scars, previously saturated areas, and steep/rocky drainages will be most susceptible. 
• Monsoon will weaken somewhat next week but near daily rounds of isolated to scattered thunderstorms are expected across the Southwest and Southern/Central Rockies.

Big Picture:

A slow-moving trough of low pressure will be located over Washington on Wednesday, which will be undercutting a tall ridge of high pressure extending northward into Canada and Alaska where more anomalous warmth is expected compared to the U.S. This area of low pressure is transporting subtropical Pacific moisture into the Northwest and Northern Rockies.

Meanwhile, monsoonal moisture is rotating clockwise around a center of high pressure over the Four Corners, with moisture favoring Colorado, New Mexico, Arizona, SoCal, Utah, and Wyoming.

Monsoon Update:

Rainfall associated with the monsoon will be most abundant across New Mexico and Arizona on Wednesday, with some decent rains expected across Socal, Eastern Nevada, Utah, and Wyoming as well. Colorado will see more spotty rainfall, favoring the San Juans and Continental Divide.

While not monsoon-related per say, a low pressure system over the Pacific Northwest has transported an abundance of subtropical Pacific moisture into the Northwest U.S. This will result in the potential for heavy rain producing thunderstorms across Western Montana in particular.

On Thursday, a similar pattern is expected overall in terms of moisture, with the one exception being Colorado who will see an uptick in monsoonal moisture compared to Wednesday.

On Friday, the trough of low pressure will finally peak up some speed and move across the Northern Rockies (just north of the Canadian border).

Westerly winds and "energy" associated with the trough will interact with abundant moisture to produce more widespread showers and thunderstorms across both the Northern Rockies and Southwest/Southern Rockies.

In terms of heavy rainfall, the threat should decrease somewhat across Montana, while the Four Corners region will see an increased risk of heavy rain and localized flash flooding.

Here are the projected 3-day rain totals across the West from Wednesday through Friday. The heaviest rainfall is expected across Northwest Montana, Arizona, New Mexico, and Southwest Colorado. 

Lightning Outlook:

On Wednesday, significant lightning activity can be expected across Eastern Washington and Northern Idaho, with a "maximum" in lightning activity also expected across Arizona, SoCal, Southern Nevada, and Southwest Utah.

Scattered thunderstorms with cloud-to-ground lightning can be expected throughout the Rockies as well, with a few storms also possible across the Cascades along with the Southern Sierra Nevada.

On Thursday, thunderstorm and lightning activity will be most widespread across the Four Corners region, especially Arizona, Nevada, Utah, Western Colorado, and Northwest New Mexico. SoCal will also see thunderstorms with only a slight chance further north in the Sierra.

Across the northern tier, thunderstorms can also be expected on Thursday across the Cascades and Northern Rockies with Western Montana looking like the most active region.

On Friday, an active day of thunderstorms can be expected across the Southwest and throughout the Rockies. As always in a persistent thunderstorm pattern like this, be sure to plan your high elevation hikes and peak bagging excursions for early in the day and keep a close watch on the skies.

Temperature Outlook:

The warmest temperature anomalies (i.e. temperature relative to average) can be expected well north of the Canadian border with more active weather having a moderating effect on temperatures across the Western U.S.

Generally speaking, temperatures will still be above average across the western-most states over the next 5 days and below average across Arizona and New Mexico. Across the Rockies, temperatures will be near to slightly above average.

Fire and Smoke Outlook:

Overall, there has been little change to the fire and smoke situation with isolated medium to large fires across California and the Northwest U.S. while smoke remains near and just downwind of these fires.

The only noticeable change since my last post is that fire and smoke activity has increased a bit across Southwest British Columbia, and skies across Western Washington may be a little hazier at times as a result. 

Outlook for Saturday (8/27) to Tuesday (8/30):

The trough moving across the West on Friday will attempt to scour out at least some of the monsoonal moisture in its wake, but not entirely.

There will still be plenty of lingering moisture heading into the weekend, and a second trough will move across the Rockies on Saturday and Sunday which will result in scattered showers and thunderstorms across the Southwest and the Rockies. The Cascades could see a few light showers as well.

Rainfall over the weekend should be lighter compared to recent days, but cloud-to-ground lightning will remain a hazard and winds could also be quite strong across the Rockies on both Saturday and Sunday. 

On Monday and Tuesday, the monsoon should weaken a bit, but isolated thunderstorms will remain possible each day across the Southwest and Central/Southern Rockies (as far north as Wyoming). Rainfall should be on the lighter side in this pattern for most areas compared to recent weeks.

The one exception will be areas east of the Divide in New Mexico, Colorado, and Southern Wyoming where a backdoor cold front will arrive on Monday or Tuesday, with upslope easterly winds behind the front supporting the potential for more numerous thunderstorms capable of producing moderate to heavy rain. 

Outlook for Wednesday (8/31) to Wednesday (9/7):

While we often look forward to cooler temperatures heading into September, summer will live on during the first week of September this year with well-above-average warmth expected for most of the West.

The monsoon will continue in a weaker state heading into early September with most of the action expected across Arizona, New Mexico, and Southern Colorado. Drier conditions are expected further north into the Central Rockies and beyond, though isolated thunderstorms may be possible from time to time. 

Thanks so much for reading! Next update on Friday (8/26).

ALAN SMITH