Big Picture Weather Pattern:

A ridge of high pressure is centered near the New Mexico/Texas/Mexico border region on Monday and subtropical moisture circulating around the periphery of the ridge is flowing into Arizona. This moisture will expand northward and eastward into Utah, Colorado, and New Mexico on Wednesday-Thursday.

A trough of low pressure will also move into the Pacific Northwest on Wednesday and Thursday, bringing wet weather to the Cascades and Northern Rockies including Glacier Park to Banff, along with thunderstorms east of the Cascade crest.

5-Day Precipitation Forecast:

There will be two favored regions for rainfall over the next 5 days. The first will be the Four Corners region where the North American Monsoon is becoming established, and the second will be the Cascades and Northern Rockies as a system moves through mid-week. 

Forecast for Mon (Jun 24) to Tue (Jun 25):

Scattered afternoon thunderstorms will develop across the higher terrain of Arizona with locally heavy rain possible along with a flash flooding threat in vulnerable terrain. Storms will be more isolated in Utah, Colorado, and New Mexico but some heavy rainers will be possible in these areas as well, including near slot canyons.

The Central and Eastern Sierra will also see some isolated thunderstorms.

In Arizona, thunderstorm coverage will be greatest over the Mogollon Rim, San Francisco Peaks, Grand Canyon, and the southeastern ranges near/east of Tucson.

Monday's forecast for Flagstaff indicates a typical thunderstorm setup, with the highest rain and lightning chances during the afternoon hours. 

Forecast for Wed (Jun 26) to Thu (Jun 27):

The pattern will turn more active mid-week as a trough of low pressure moves into the Northwest. Moisture arriving with this trough will bring rain showers to the Olympics, Cascades, and Puget Sound, with showers and thunderstorms from the east side of the Cascades into the Northern Rockies (Northern Idaho and Montana especially).

The Canadian Rockies and both the interior and coastal ranges of BC will see quite of bit of rainfall and thunderstorm activity from this system.

Southerly flow ahead of the trough will also help to draw in monsoonal moisture further north into Utah and Colorado, and to a lesser extent Wyoming. Moisture will also expand eastward into New Mexico. The result will be an uptick in thunderstorm activity with locally heavy rain across these regions.

Forecast for Fri (Jun 28) to Sat (Jun 29):

Most areas will see a drying trend on the backside of the mid-week low pressure trough. The exception will be Colorado, New Mexico, and Eastern Arizona where lingering moisture will result in scattered thunderstorms, favoring the Sangre de Christo Range. 

Outlook for Sun (Jun 30) to Thu (Jul 4):

An active early-season monsoon is expected to reload across the Southwest during this period with above-average moisture levels for the beginning of July. This will result in a good chance of thunderstorms across the Four Corners region along with temperature moderation compared to recent weeks.

Across the Northwest, low pressure troughs will continue to move through somewhat regularly, resulting in occasional showers for the North Cascades and thunderstorms east of the Cascade crest into the far Northern Rockies of Idaho, Montana, and Canada. 

Thanks so much for reading! Next update on Wednesday (June 26).

Alan Smith