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Big Picture Weather Pattern:

The strongest and most active monsoon pattern of the season will begin this weekend and will linger through at least Labor Day.

Showers and thunderstorms will become frequent across the Southwest, Sierra, and the Rockies with moisture pushing far to the north by the middle of next week.

Also, a stalled frontal boundary along the eastern side of the Rockies in Colorado/New Mexico will result in low level upslope winds from the east that will transport additional moisture into the Front Range with an increased threat of flash flooding on Sunday-Monday especially. 

5-Day Temperature Forecast:

The Southwest, Central Rockies, and Sierra will see cooler temperatures over the next 5 days, largely as a result of the increase in moisture and cloud cover.

The Pacific Northwest, on the other hand, will experience a heatwave will well above normal temperatures and fire danger will be on the increase as a result of the hot temperatures and low relative humidity.

Saturday (Aug 23):

First, let's start with the Colorado Front Range and Northeast Colorado (along and north of the Palmer Divide).

A much cooler and higher-humidity airmass will be in place on the eastern side of the Continental Divide following Friday's cold front, and the day will likely start out with low clouds/fog in the foothills, along with the potential for scattered showers early in the day.

During the afternoon, thunderstorm development will be dependent on whether or not this cloud cover breaks, allowing for better surface heating. If the sun comes out and the lower atmosphere heats up enough, then stronger thunderstorms with heavy rain will be possible.

If the heavy cloud cover holds, then expect a cooler day with scattered showers, limited lightning potential, and lighter rainfall rates. Also, orographic lift will likely lead to greater shower/thunderstorm coverage over the Front Range foothills versus the Denver metro area. 

South of the Palmer Divide, including Colorado Springs, Pikes Peak, and the Sangre de Cristo Range (CO/NM), fewer early clouds and warmer temperatures should allow stronger thunderstorms to develop, some of which could produce large hail and heavy rainfall along with the potential for flash flooding. 

The Rockies along/west of the Divide to the Southwest to the Sierra Nevada Range will see an active day of showers and thunderstorms, especially in the afternoon and evening, as monsoonal moisture increases. Storms will be capable of heavy rain and localized flash flooding, especially in slot canyons, dry washes, and fire burn scars.

Sunday (Aug 24):

An active day is expected as monsoon moisture will increase across the board for the Rockies and the Sierra, as well as canyon country. Upslope flow will also increase east of the Divide in Colorado and New Mexico, supporting an active thunderstorm day there as well.

A slight chance of showers and thunderstorms will exist during the morning hours, then more widespread showers and thunderstorms are expected after noon. The intensity of thunderstorms will be dependent on cloud cover and daytime heating, but even if stronger storms do not materialize, clusters of showers with soaking rains will likely develop in some areas.

The threat of flash flooding will also increase throughout the region, especially in slot canyons, dry washes, and burn scars, though steep terrain with poor drainage will have a flooding threat as well.

Also, a few severe thunderstorms could develop east of the Continental Divide in Colorado with large hail possible.

Monday (Aug 25):

On the east side of the Divide in Colorado, as well as Northern New Mexico and Southern Wyoming, another surge of low-level moisture from the east will set the stage for an active day with showers and thunderstorms. A higher threat of flash flooding is also expected in the Front Range Mountains and foothills.

West of the Divide, it's shaping up to be an active day as well across a large portion of the west with numerous showers and thunderstorms expected. Morning showers and storms are also possible across Utah and Colorado.

Any stronger storms that develop across the green shaded region will have the potential to produce heavy rain, with flash flooding also possible. 

Tuesday (Aug 26):

Get ready for another active day across much of the West, from the Sierra to the Great Basin to the Rockies. Two subtle shortwave troughs will aid in transporting abundant moisture further north and areas with the highest projected rainfall totals include Southern Idaho, Western Wyoming, Northern Utah, Western/Central Colorado, and Northern New Mexico.

However, all areas in the green-shaded regions could see an active day with showers and thunderstorms capable of producing heavy rain, and flash flooding will remain a threat in slot canyons, burn scars, and vulnerable terrain.

The Colorado Front Range east of the Divide does not look quite as active with upslope flow weakening somewhat, but enough moisture will linger that scattered thunderstorms can still be expected with heavy rain possible. The higher threat of flash flooding looks to shift southward into the Pikes Peak Region and Sangres, however.

Wednesday (Aug 27):

Another active day can be expected across a large portion of the West with numerous showers and thunderstorms along with pockets of heavy rain.

The latest AI version of the European Model is honing in on the Northern Rockies (Idaho & Wyoming) along with Arizona for the heaviest rainfall, while other models are painting a broader picture with significant rainfall possible across a larger portion of the West.

Also, an area of low pressure over Oregon could transport moisture far enough north for scattered thunderstorms in the Cascades of Northern Oregon and Washington. Some of these storms could produce heavy rain, but given the preceding heatwave, there will also be a risk of dry thunderstorms and lightning-triggered fires in an areas that receive little rain.

Outlook for August 28 to September 1:

Heading into Labor Day weekend, monsoonal moisture will linger across the Southwest and the Rockies, but should gradually decrease over time compared to prior days with perhaps more sunshine and better morning weather windows.

However, this looks to be more of a consistent monsoon pattern than we have seen this summer, with daily rounds of afternoon thunderstorms possible across much of the Southwest and the Rockies, with storms capable of producing heavy rain.

Enough moisture also looks to remain over the Sierra Nevada Range for thunderstorm chances to continue.

Also, a trough is projected to move across the Pacific Northwest over Labor Day weekend, which could introduce a chance of showers and thunderstorms, though confidence in the timing and details is low.

Temperatures are expected to remain below-average across the Rockies and the Southwest thanks to the abundance of monsoonal moisture, while the Northwest will hang onto above-average temperatures even with shower chances eventually arriving later in the weekend.

That being said, a cooling trend should occur over time across the Northwest following a heat wave to start this period.

Outlook for September 2-6:

For the week after Labor Day, a fairly active pattern looks to continue with monsoonal moisture over the Southwest and Utah/Colorado, resulting in routine afternoon thunderstorm chances, while additional disturbances could also move across the Northwest and Northern Rockies with showers and thunderstorms possible. 

No major temperature discrepancies from "average" are expected at this time across most of the West, except across the Northern Rockies, where above-average warmth is expected to continue.

Keep in mind that as we head into September, "average" temperatures will start to decline at a faster rate as we transition from summer to fall.

Thanks so much for reading and have a great weekend! Next update on Monday (August 25).

Alan Smith