Forecast Highlights:

• Thunderstorms with locally heavy rain for the Four Corners states on Friday, with coverage decreasing somewhat on Saturday. A more significant drying trend is then expected from Sunday into early next week.
• Showers and thunderstorms for the Cascades and Northern Rockies on Friday and Saturday as back-to-back low pressure systems impact the area.
• Significant drying trend throughout the West from Sunday through next week as the monsoon weakens with reduced thunderstorm potential.
• Brief cool-down for the Northwest and Northern Rockies this weekend as a cold front moves through.
• Well above average warmth expected next week for much of the West, and in particular, the Northwest. 

Big Picture:

The North American Monsoon has been very active this summer, and while flooding has caused problems in some areas, overall the rainfall this summer has been very welcome across the drought-stricken West and has also helped to keep our fire (and smoke) season at bay.

Starting this weekend, we are going to see a transitional weather pattern as our active monsoon pattern (which has been ongoing for more than 2 months) will weaken significantly. This will not be the end of the monsoon per say, but rather just a transition into a late summer/early fall "weak" monsoon pattern. This means that thunderstorms will be less common moving forward, instead of a near-daily occurrence. 

First, we do have a couple of active days to get through (especially on Friday) as back-to-back low pressure troughs move across the Western U.S. The first trough will interact with monsoonal moisture to result in an active thunderstorm day across the Four Corners region.

On Saturday and Sunday, a stronger and "deeper" trough will move across the West. This trough will support scattered showers and thunderstorms across the Northwest, Rockies, and Southwest on Saturday, but a strong jet stream and drier air on the backside of the trough will help to scour out most of the monsoonal moisture on Sunday. 

The trough will also result in gusty winds across the higher terrain throughout the West this weekend.

Monsoon Update:

On Friday, monsoonal moisture will remain well-established across the Four Corners states, and this will support an active thunderstorm day across the mountains and canyons with locally heavy rain and isolated flash flooding possible.

A trough of low pressure moving across Montana will also result in scattered thunderstorms, though rainfall will be lighter compared to previous days. Scattered showers will also develop over the Washington Cascades as a second trough of low pressure approaches.

On Saturday, monsoonal moisture will start to decrease overall but enough will remain for scattered thunderstorms to develop again across the Southwest and the Rockies with locally heavy rain possible, though less likely compared to Friday. 

As the second trough moves across the West along with a stronger cold front, Pacific moisture and energy will support more numerous showers and thunderstorms across Western and Central Montana, and to a lesser extent, Idaho and Northern Wyoming. Scattered showers will remain possible across the Pacific Northwest as well.

With cooler air arriving on Saturday afternoon and Saturday night, some of the higher peaks of Montana (especially the Beartooths) could be dusted with snow.

On Sunday, we'll see a significant drying trend throughout the West, even the Four Corners, as the jet stream dips southward and scours out the significant monsoonal moisture that has been in place for weeks on end.

Rainfall over the next three days will favor the Four Corners as well as Montana, British Columbia, and Alberta, with nearly all of this rain across the West occurring on Friday and Saturday before drying out on Sunday. The one exception is the eastern-most ranges of Montana which could see some lingering showers early on Sunday.

Lightning Outlook:

Thunderstorm activity will be fairly widespread across the Rockies and the Southwest on Friday. A few isolated storms are possible across Southern Oregon, NorCal, and the Sierra Nevada Range as well.

On Saturday, the Four Corners states and Southwest Montana will be the most favored for thunderstorms

On Sunday, the Northern Rockies are looking storm-free while Colorado, New Mexico, and Arizona will still see some afternoon storms, with the Continental Divide in Colorado being most favored.

These storms will likely be much "drier" than what we've been accustomed to lately, with lightning posing a greater risk compared to heavy rain. 

Temperature Outlook:

The most notable temperature feature heading into the weekend will be a cold front moving across the Northwest and the Northern/Central Rockies. This will be a brief cooldown but will give these areas a quick taste of fall.

On Monday and Tuesday, a ridge of high pressure will quickly rebuild along the West Coast and will extend well northward up into Canada. This will result in a significant warm-up with well-above-average temperatures across the Northwest and "moderately" above average temperatures across California and the Southwest.

Areas east of the Continental Divide will see more moderated temperatures early next week following a cold front that slides down the eastern side of the Divide. 

Fire and Smoke Outlook:

Fires continue to burn across the Northwest, Northern Rockies, and California, but smoke transport remains low overall with smoky conditions confined to areas near and just downwind of fires.

Gusty winds are expected across much of the West this weekend, which could exacerbate some of the ongoing fires, but overall, the abundance of monsoonal moisture this summer has resulted in a relatively quiet fire season.

Outlook for Monday (8/29) to Thursday (9/1):

The early part of next week is looking quiet across the West. This map below for Monday-Tuesday rainfall is the driest outlook we've seen in quite some time! Higher elevation areas near the Continental Divide in Colorado and New Mexico, as well as portion of Arizona, will be most favored for spotty/weak afternoon thunderstorms.

Some models were hinting at an uptick in moisture/storm chances near the Front Range for Monday/Tuesday initially, but most models have backed off on this with better moisture expected to remain further east.

During the middle part of next week, a relatively quiet pattern is expected to continue with just some very isolated thunderstorm activity possible across the higher terrain of Colorado, New Mexico, and Arizona.

Temperatures will also be quite warm across the West and it will feel more like mid-summer than the end of August//beginning of September.

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

Temperatures are looking very summer-like across the West for the first week of September. No big early fall cold fronts are on the horizon just yet. The greatest temperature anomalies (relative to average) are expected from California into the Great Basin and Central Rockies.

In terms of moisture, we'll be keeping an eye on New Mexico and Arizona where monsoonal moisture is projected to hang on a bit with better thunderstorm potential compared to areas further north. This would be a fairly typical pattern for early September. 

Meanwhile, this particular model is projecting a drier outlook for the Pacific Northwest, but some models are hinting at a low pressure system that could dip just far enough south to bring some rain to the PNW and Cascades over Labor Day weekend.

Thanks so much for reading and have a great weekend! Next update on Monday (8/29).

ALAN SMITH

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