Forecast Highlights:

• Heavy rain and flash flood risk across the Southwest and Southern Rockies on Monday as an active monsoon persists.
• Daily rounds of thunderstorms for the Southwest and Southern Rockies this week, but coverage becomes less widespread mid-week and faster storm motions will reduce the flood threat. 
• Isolated thunderstorms across the Northern Rockies, mainly during the middle of the week.
• Early week heat across the West Coast including the Pacific Northwest on Monday, followed by much cooler temperatures. 
• Showers across the North Cascades from Tuesday to Thursday. 
• Fourth of July weekend will be more active for most of the West. 

Synopsis:

An active monsoon will remain in place over the Southwest U.S. early this week with a continued threat for locally heavy rains and flash flooding. A trough of low pressure will then move across the Pacific Northwest on Tuesday along with a cold front, ushering in a cooler airmass across the West Coast.

Showers and thunderstorms will also develop across the Northwest and the Northern Rockies during the middle part of the week, while thunderstorm coverage will become less widespread across the Southern Rockies and Southwest with stronger winds aloft and faster storm motions limiting the flash flooding threat compared to recent days. 

Forecast for Monday:

An active monsoonal flow will remain in place across the Southern Rockies with New Mexico and Southern Colorado being favored for the most widespread thunderstorms, while coverage will be more isolated across Arizona and Southern Utah. Light showers will also reach the Washington Cascades during the overnight hours as a cold front approaches.

The highest potential for excessive runoff and flash flooding will be across New Mexico, but Eastern Arizona and Southern Colorado could see some heavy rain as well. Despite thunderstorm coverage being more isolated, slot canyons of Southern Utah and Northern Arizona will be under a flash flooding risk as well. 

Forecast for Tuesday:

A trough will move into the Northwest on Tuesday, resulting in showers across the Olympic and North Cascade Ranges, while isolated thunderstorms will develop across the Northern Rockies.

Moisture levels will decrease slightly across the Southwest and Southern Rockies, but scattered thunderstorms will still develop. Stronger winds in the upper atmosphere will result in faster-moving storms, which should limit (but not eliminate) the flash flooding threat. Storms on Tuesday will also be capable of frequent cloud-to-ground lightning throughout the Rockies.

Forecast for Wednesday:

Light showers will continue across the North Cascades and Northern Idaho/Montana around Glacier National Park. Scattered thunderstorms will develop once again across the Southern Rockies and Southwest, while areas along/east of the Divide in Wyoming and Southern Montana including the Big Horns and Beartooths will also see some thunderstorms. 

Outlook for Thursday - Friday:

A broad but relatively flat ridge of high pressure over the Southwest and South Central U.S. will allow for monsoonal moisture to increase across the Four Corners states. This will result in an uptick in thunderstorm chances along with increasing potential for heavy rain and flash flooding.

Shower and thunderstorm coverage will be more spotty across the Northwest and the Northern Rockies while California should stay dry.

Outlook for Saturday (7/2) - Sunday (7/4):

The weather pattern will become more active over the Fourth of July holiday weekend as a trough of low pressure deepens along the West Coast. Pacific moisture associated with this trough will lead to increasing shower chances across the Cascades and Pacific Northwest.

In addition, southwest flow ahead of the trough will transport monsoonal moisture into parts of the Northern Rockies, including Idaho, Wyoming, and Southwest Montana. Abundant monsoonal moisture will also continue to rotate around a center of high pressure near the U.S./Mexico border, leading to a continued active thunderstorm pattern across the Four Corners states.

In other words, an active thunderstorm pattern is likely throughout the Rockies over the holiday weekend, so be sure to plan your outdoor adventures accordingly.

Outlook for Tuesday (7/5) to Monday (7/11):

The pattern should start to relax somewhat beyond the Fourth of July as the trough over the West Coast eventually weakens. Still, cooler temperatures associated with this trough are expected to linger across the West Coast for most of the week with below-average temperatures from Washington to California, while above-average temperatures are expected across the Central and Northern Rockies. 

The monsoon will continue across the Southwest, with perhaps some gradual weakening compared to the Fourth of July weekend. Wetter conditions are also expected further north in Canada, while the Northern Cascades and Glacier NP could see some unsettled weather at times. 

Thanks so much for reading! Next update on Wednesday (6/29).

ALAN SMITH