Forecast Highlights:

• Thunderstorm activity favors the Four Corners states from Wednesday to Friday.
• Flash flooding threat becomes more isolated for dry washes from Wednesday to Friday.
• Wildfires growing across Idaho with smoke for parts of the Northern and Central Rockies.
• Temperatures remain above average across the West with only brief exceptions.
• Thunderstorm activity and rainfall intensity increase across the Four Corners and Eastern MT/WY Rockies this weekend and early next week.

Synopsis:

A ridge of high pressure will be centered over Southern California on Wednesday while the jet stream is set up over the Northern Rockies. Monsoonal moisture is rotating around the northern and eastern fringes of the high from California to Colorado. Meanwhile, two areas of low pressure in the Pacific will move inland late this week, resulting in an increased threat of thunderstorms across Montana. 

Monsoon Update:

Monsoon moisture will stretch from the Sierra to the Colorado and New Mexico Rockies on Wednesday, with the focus of moisture gradually shifting southward and eastward on Thursday and Friday. The focus of thunderstorms will be centered over Colorado and New Mexico on Thursday and Friday, with activity becoming more isolated further west.

The heaviest rainfall over the next three days can be expected across Colorado, New Mexico, and Southern/Eastern Arizona, while the Uintas in Utah could also see some locally heavy rain, especially on Wednesday.

There will still be a flash flooding threat due to heavy rain across the Four Corners states for dry washes, slot canyons, and burn scars, but the threat will be more isolated compared to recent days as widespread heavy rainfall is not expected.

Lightning Outlook:

On Wednesday, thunderstorm activity will be most widespread across the Colorado and New Mexico Rockies, as well as Southern Arizona. Northern Utah could see a fair amount of storm activity as well. Thunderstorms will be more isolated in coverage further west across Nevada and California.

On Thursday, scattered thunderstorms can be expected across Colorado, New Mexico, and Southern Arizona with more isolated activity across Utah and Nevada.

On Friday, look for scattered thunderstorms throughout the Four Corners states with the San Juans, Sangre de Christos, and Mogollon Rim looking like the most active areas.

Also, a trough of low pressure moving across the Northern Rockies will trigger some isolated thunderstorms across Northeast Washington, Northern Idaho, and Western Montana including Glacier National Park. The Canadian Rockies will see some storms as well.

Temperature Outlook:

Temperatures will continue to run above average over the next 5 days from Wednesday to Sunday, with some brief day-to-day exceptions.

The hottest temperatures across the western-most states can be expected on Wednesday and Thursday, while the Rockies and adjacent high plains will see their hottest temperatures on Thursday and Friday, and possibly Saturday for areas east of the Divide.

A cold front will reach the West Coast on Friday and the Northern/Central Rockies on Saturday, which will offer a brief reprieve for some areas. The cold front is then projected to slide down the eastern slopes of the Rockies on Sunday.

Fire and Smoke Outlook:

Hot, dry, and windy conditions early this week have resulted in explosive wildfire activity in Idaho. The Moose Fire near Salmon has burned 12,000 acres in just a couple of days and is producing heavy smoke that is impacting Western Montana, with varying degrees of downstream smoke impacts across Wyoming and Eastern Idaho.

A similar smoke transport pattern is expected in the days to follow with a west/northwest flow aloft in place, and Northern Colorado may see some smoke as well. The Bray Fire over South Central Idaho is also contributing to the smoke.

Outlook for Saturday (7/23) to Tuesday (7/26):

Heading into the weekend, the monsoon is expected to intensify across the Four Corners region, resulting in more widespread thunderstorms with frequent cloud-to-ground lightning and heavy rainfall. The threat for flash flooding will increase over the weekend as well.

Also, a second trough of low pressure will move across the Northwest, resulting in an uptick in showers and thunderstorms across the Northern Rockies. Areas east of the Continental Divide across the Northern and Central Rockies will see a cold front arrive on Sunday, with upslope winds behind the front resulting in more numerous thunderstorms with heavy rain potential.

Here is the European Model's rainfall projection for Saturday and Sunday:

A similar pattern is expected on Monday and Tuesday with an active monsoon continuing across the Southwest, while moisture could extend westward into California. Meanwhile, another cold front is likely to slide down the eastern slopes of the Divide early next week as well, which could lead to more thunderstorms for the Eastern Rockies from Montana to New Mexico.

Outlook for Wednesday (7/27) to Wednesday (8/3):

A tall ridge of high pressure will set up along the West Coast during this period, and a heat wave will be possible across the Pacific Northwest, extending into Northern and Central California as well. Temperatures will be near to below average around the Four Corners, courtesy of an active monsoon.

A strong monsoon is likely during this period with widespread thunderstorms and heavy rain/flash flooding potential expected across the Four Corners states. Some of this moisture could extend into Wyoming, Nevada, and California as well. 

We'll also need to keep an eye for additional "backdoor" cold fronts that could slide down the eastern slopes of the Continental Divide and usher in moisture that could support thunderstorm development and possibly heavy rain.

Thanks so much for reading! Next update on Friday (7/22).

ALAN SMITH