Big Picture:

An active pattern will take hold across the Rockies this week with increasing potential and coverage of showers and thunderstorms. Frequent lightning and heavy rainfall will be possible at times, along with the potential for localized flash flooding.

The subtropical high pressure center that has been located over the Southwest in recent weeks is shifting eastward into the Southern Great Plains. This is opening the door for deeper monsoonal moisture to stream into the Western U.S. unobstructed.

At the same time, a slow-moving area of low pressure near the Baja Peninsula will work its way northward over the course of the week, helping to direct the moisture plume into the Central and Northern Rockies while moisture will decrease across the Southwest by later in the week. 

The amount of water vapor in the atmosphere from Northern Utah into Southern Montana will approach 200% of average, or 3 standard deviations above average, by the middle of this week to give an idea of the significance of this pattern. 

In addition to wetter conditions, temperatures will also be much cooler across the West Central U.S. this week, thanks in part to increased moisture and cloud cover. 

Here are projected 5-day temperature anomalies from Monday through Friday:

Fire and Smoke Update:

A handful of large fires continue to burn across the Western U.S. and Western Canada. The most notable fire in recent days has been the York Fire south of Las Vegas. This fire has burned 77,000 acres of mostly grass and brush since last Friday and could send some light smoke into portions of the West Central U.S. over the next few days.

Further north, varying degrees of light smoke can also be expected at times from fires in Oregon, Idaho, Montana, Washington, and Canada. Locally heavy smoke can be expected near and just downstream of these fires. 

Forecast for Monday Afternoon and Monday Night:

Showers and thunderstorms will be most numerous across Arizona, Eastern Nevada, Utah, Colorado, and Wyoming with some isolated scattered storms also possible across Southern California and eastward across New Mexico.

Locally rainfall will be possible along with a risk of flash flooding in slot canyons and dry washes. However, the highest potential of heavy rainfall will be across the Colorado Front Range and Southern Wyoming where localized flash flooding is also possible.

Forecast for Tuesday:

Showers and thunderstorms will be most numerous across Nevada, Utah, and Colorado with locally heavy rainfall possible along with frequent lightning. The highest risk of flash flooding will be across the Colorado Front Range.

Thunderstorms will be more scattered in coverage in Northern Arizona, New Mexico, and Wyoming. The Sierra Nevada Range and SoCal Mountains should see some action as well with lighter rain amounts.

Forecast for Wednesday:

Moisture will begin to increase northward with scattered to numerous showers and thunderstorms across Nevada, Utah, Colorado, Idaho, and Wyoming. Locally heavy rainfall will be possible throughout the region, with the highest threat of heavy rain and flash flooding existing across Northern Utah, Eastern Idaho, Western Wyoming, and the Colorado Front Range.

Along the periphery of the moisture, isolated thunderstorms will remain possible across the Sierra Nevada Range, Northern Arizona, and New Mexico. At the northern end, Southern Montana will also start to get in on the action with scattered thunderstorms and locally heavy rain possible.

Forecast for Thursday to Friday:

Significant moisture will remain in place across the Northern Rockies this week with a gradual northern and eastern push into Montana. The highest threat of flash flooding will exist across the southern ranges of Montana, while heavy rain will remain possible in Idaho and Wyoming.

Thunderstorms will become more isolated to scattered in coverage across Colorado, Utah, and Nevada, while some moisture should also sneak into Oregon with scattered thunderstorms possible. Further south, conditions will turn much drier in Arizona while New Mexico could see isolated storms.

Outlook for Saturday (Aug 5) to Thursday (Aug 10):

The pattern this weekend and next week will feature lingering monsoonal moisture across the Northern Rockies with additional thunderstorms across Idaho, Montana, and Wyoming. The threat of heavy rainfall should decrease, though. Disturbances moving into the PNW could also bring light showers to the Cascades.

Meanwhile, a strong (by August standards) jet stream is expected to set up across the Central Rockies next week, which will help to suppress deeper monsoonal moisture well south of the Mexico border, resulting in drier conditions across the Southwest.

Above-average temperatures are expected across the Southwest and along the West Coast. Below-average temperatures are expected to persist across the Central and Northern Rockies. 

Thanks so much for reading! Next update on Wednesday (August 2).

Alan Smith