Forecast Highlights:

• Monsoon moisture will result in a heavy rain and flash flooding threat across New Mexico on Monday, with moisture expanding north and west as the week progresses.
• Heavy rain and flooding concerns across Montana on Monday in response to a low-pressure system moving across the Northern Rockies.
• Well below-average temperatures across the Northern Rockies Monday with a little bit of snow for the higher elevations of Montana and Wyoming.
• Thunderstorm and heavy rain potential expands into Colorado, Utah, Arizona, Nevada, Wyoming, and even California later this week.
• Much drier and sunnier weather pattern ahead for the Pacific Northwest... finally!

Synopsis:

A trough of low pressure moving across the Northern Rockies will result in widespread heavy rainfall across Montana on Monday along with some high-elevation snow.

Meanwhile, the North American Monsoon began this weekend and will persist through the upcoming week. Rain and thunderstorms associated with the monsoon will be suppressed southward into New Mexico on Monday thanks to the trough passing over the Northern Rockies, but moisture will expand northward and westward as the week progresses.

An area of low pressure will also develop over Southern California, and this will help to draw in some monsoonal moisture into the SoCal Mountains and Sierra Nevada Range with increasing rainfall and thunderstorm potential.

Recent Rainfall:

We're coming out of yet another active weekend across the Western U.S. with significant rain totals for much of the Northwest and the Southwest. New Mexico and Southwest Colorado in particular received some much-needed moisture, but not without the burn scar flooding and mudslide concern.

Heavy rain across Montana has also resulted in some localized flooding and runoff issues, including portions of Glacier National Park.

Forecast for Monday:

The heaviest rainfall on Monday will occur along and east of the Continental Divide in Montana, including the eastern side of Glacier National Park. Moderate rainfall is also likely across Idaho, Montana, and Northern Utah, while moisture associated with the monsoon will become suppressed further south into New Mexico.

Higher elevations of Montana and Wyoming will receive some snow as well thanks to the colder airmass that has settled into place with temperatures running well below average for this time of year. However, snow levels will still be higher compared to this time last week when Glacier National Park was hit with heavy snow. 

Flooding and excessive runoff will be a concern across both New Mexico and Montana on Monday. New Mexico will see more of a localized flash flooding threat, whereas stream levels are already high across Montana and widespread heavy rain will continue to stress rivers and steep/unstable terrain. 

Forecast for Tuesday:

Conditions will begin to dry out over the Northern Rockies on Tuesday aside from some early morning sprinkles associated with the departing system from Monday. Meanwhile, the monsoon over the southwest will begin to expand northward again with heavy rain expected across a good portion of New Mexico, while thunderstorm chances will also return to the Sangre de Christo's in Southern Colorado.

A storm system passing north of the Canadian border could bring some showers to the Northern Washington Cascades on Tuesday night with heavier showers remaining confined to areas north of the border in British Columbia.

The flooding and excessive runoff threat will be highest over New Mexico on Tuesday, while Southern Colorado and Eastern Arizona will also see a minor flooding threat. Keep in mind that burn scars from recent fires in these regions will be most susceptible to flooding and runoff issues.

Forecast for Wednesday:

An area of low pressure setting up over California will help to draw in some monsoonal moisture into Southern and Central California, resulting in a chance of showers and thunderstorms throughout the Sierra and the SoCal Mountains with locally heavy rain possible.

Monsoon moisture will begin to expand a little westward overall with increasing thunderstorm chances across Arizona and Southern Utah as well, while activity will remain most widespread over New Mexico. We could also see an uptick in thunderstorms across the Southern San Juans in Colorado while Northern Colorado should largely stay dry with only isolated storms if anything. 

The Washington Cascades will see some light showers on Wednesday as well from a storm system passing north of the border, but overall impacts across the Pacific Northwest (south of the Canadian border) will be low.

The highest threat for flash flooding will shift into Northwest New Mexico on Wednesday with a minor threat across Southern Colorado, Eastern Arizona, and even the SoCal Mountains. 

Outlook for Thursday (6/23) to Friday (6/24):

An active pattern can be expected for much of the West late this week as monsoon moisture continues to expand northward and westward. A noticeable uptick in thunderstorms can be expected across Colorado, Arizona, Utah, Nevada, California, Wyoming, and Southwest Montana. 

Glacier National Park could also see some light showers late this week from a storm system moving across Montana, while precipitation amounts north of the border around Banff, Lake Louise, and Jasper could be quite heavy. 

Meanwhile, the Pacific Northwest will head into a warmer and drier pattern, which will be a welcome change coming out of a very cool and wet spring. 

Outlook for Saturday (6/25) to Sunday (6/26):

A similar pattern can be expected this weekend with an active monsoon in place, resulting in widespread thunderstorms along with heavy rain/flash flooding potential across Colorado, New Mexico, Arizona, and Utah in particular.

Along the periphery of the monsoon, we should also see enough moisture for showers and thunderstorms to be common across the Sierra, Wasatch, Tetons, and Wind Rivers through the weekend. 

The Pacific Northwest will continue to see nice weather with dry, sunny conditions and warmer temperatures. 

Outlook for Monday (6/27) to Monday (7/4):

Next week, a classic active monsoon pattern is likely with frequent stormy conditions across the Four Corners along with the potential for heavy rainfall. Great news for this drought-stricken area, though flash flooding and especially burn scar flooding will be the downsides, as will lightning danger.

Cooler than average temperatures are also likely across the Four Corners in response to the amount of moisture and associated cloud cover.

A strengthening ridge of high pressure across the Western U.S. will result in warmer than average temperatures elsewhere, including the Pacific Northwest for the first time in a long time. 

A strong wet signal is expected across the Four Corners region next week, which is no surprise given the early (and strong) monsoonal pattern. Drier conditions are expected across the Northwest and Northern Rockies next week.

Thanks so much for reading! Next update on Wednesday (6/22).

ALAN SMITH

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