Forecast Highlights:

• Heavy rain and flash flood threat across New Mexico, Eastern Arizona, and Southwest Colorado on Wednesday as an active monsoon pattern remains in place.
• Scattered to numerous thunderstorms across the Southwest on Wednesday, expanding in coverage into Utah, Arizona, Colorado, Wyoming, Nevada, and California on Thursday and Friday.
• A mix of wet and dry thunderstorms across the Sierra Nevada and SoCal Mountains late this week, with the potential for lightning-triggered fires under dry thunderstorms.
• Heavy rain/flash flood threat increases this weekend across most of Colorado and New Mexico. 
• Warm, dry, and sunny pattern this weekend for the Pacific Northwest.

Synopsis:

Monsoon moisture will increase across New Mexico and the Southwest U.S. during the middle part of this week, while an area of low pressure centered over Southern California will help to draw some of this moisture northward as well. As a result, thunderstorm chances will be on the increase across a large portion of the West over the coming days. A trough of low pressure will also track along the U.S./Canada border, but moisture with this feature will be limited south of the border. 

Late in the week, the area of low pressure over California will track across the Central Rockies. This will aid in thunderstorm development across the Southwest and the Rockies late this week.

Temperatures late this week and into early next week will trend cooler than average across the Four Corners region due to the active monsoon and associated moisture and cloud cover, while the rest of the West including the West Coast and Northern Rockies will see a trend toward above-average warmth. 

Forecast for Wednesday:

Thunderstorms will develop throughout the Southwest on Wednesday from New Mexico/Colorado to California. The heaviest and most widespread rainfall will occur across Central and Western New Mexico along with Southwest Colorado, including the San Juan Range. 

The increase in moisture across California will result in a mix of wet and dry thunderstorms, which could be a good and bad thing as any moisture is welcome, but in areas where dry thunderstorms develop, lightning-triggered wildfires will be a possibility. 

Further north, a low pressure system will move across British Columbia with widespread showers expected north of the border, while some light showers are expected south of the border across the Washington Cascades and possibly far Northern Idaho on Wednesday night.

The threat for flash flooding and excessive runoff will be highest across Northwest New Mexico and Southwest Colorado on Wednesday. NOAA's Weather Prediction Center has highlighted much of this region under a Slight (level 2 out of 4) risk for excessive rainfall. 

Forecast for Thursday:

Scattered to numerous showers and thunderstorms can be expected across the Sierra, Great Basin and Southern/Central Rockies with more isolated showers/thunderstorms developing across Northwest Wyoming and Montana as fringe-level monsoonal moisture interacts with a trough moving across the Northern Rockies. 

Thunderstorms on Thursday should be a bit faster moving compared to Wednesday and the flash flooding threat will be lower as a result, though isolated areas of flash flooding couldn't be ruled out. Lightning activity also looks more intense on Thursday as instability is expected to increase across much of the West. 

Forecast for Friday:

Thunderstorm activity will become more isolated over the Sierra on Friday, while the Southern and Central Rockies will see another round of scattered to numerous thunderstorms capable of frequent cloud-to-ground lightning. The flash flood threat looks low overall but heavy downpours will be possible under stronger thunderstorms.

Isolated showers and thunderstorms will be possible further north over Montana, while the Pacific Northwest will see the beginning of a welcome warm and dry spell following an unseasonably cold and wet spring. 

Outlook for Saturday - Sunday:

The weekend is shaping up to be very wet and stormy across the Southern Rockies, especially in Colorado and New Mexico. A backdoor cold front will work its way into Eastern Colorado and Eastern New Mexico, resulting in increasing moisture via upslope winds along the eastern slopes of the Continental Divide.

At the same time, an uptick in monsoonal moisture via southwest flow is also expected across New Mexico and Colorado. As a result, numerous showers and thunderstorms capable of heavy rain and frequent lightning will be possible across a large portion of Colorado and New Mexico both east and west of the Divide.

The heaviest rainfall for this weekend is currently projected across Northern New Mexico and Southern Colorado, while the Front Range of Colorado could potentially see heavy rain-producing storms as well with localized flash flooding a possibility, especially over burn scars.

Elsewhere, scattered to numerous thunderstorms are also likely across portions of Arizona and Utah with more isolated activity across California, Nevada, and Wyoming. 

Outlook for Monday (6/27) to Tuesday (6/28):

The threat of heavy rain and flash flooding should relax a bit early next compared to the weekend at least. However, the monsoonal flow will remain in place across the Four Corners region with scattered to numerous thunderstorms expected across large portions of Colorado, New Mexico, Arizona, and Utah, while portions of the Great Basin and Northern Rockies will see more isolated activity.

Outlook for Wednesday (6/29) to Wednesday (7/6):

A similar pattern is likely to remain in place for the end of June and beginning of July with a ridge of high pressure in place over the Western U.S., while an active monsoon will persist across the Four Corners.

This will result in above-average temperatures for the West Coast and Northern Rockies, and below-average temperatures across the Southwest.

The Four Corners region will continue to be favored for wet conditions and near-daily rounds of thunderstorms, with drier conditions prevailing across the Northwest and Northern Rockies. 

Thanks so much for reading! Next update on Friday (6/24).

ALAN SMITH

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