Forecast Highlights:

• Monsoon season begins with numerous showers/thunderstorms across Colorado, New Mexico, and Eastern Arizona from Friday to Monday with locally heavy rain and flash flooding possible.
• Severe thunderstorms possible in Glacier National Park and Northwest Montana on Friday with 2-inch diameter hail possible.
• Moderate to heavy showers across the Cascades and Northern Rockies Friday to Monday with localized flooding possible in Idaho and Montana.
• Strong winds throughout the Intermountain West on Saturday and Sunday with high fire danger across Nevada and parts of Utah and Arizona. 
• Varying levels of smoke from Arizona/New Mexico fires can be expected across the Southern and Central Rockies this weekend.

Synopsis:

A slow-moving trough of low pressure located near the West Coast will gradually work its way across the Western U.S. from Friday through Monday. Significant weather impacts can be expected across the Northern Rockies where several waves of moisture and energy will be arriving, though flood-ravaged Yellowstone largely looks to be spared.

Further south, the North American Monsoon will get going this weekend as strong southerly flow ahead of the trough carries moisture northward into New Mexico and Colorado, and to lesser degrees in Arizona, Utah, and Wyoming. Strong winds near the leading edge of the trough can be expected across a large portion of the West throughout the weekend.

The surge of monsoonal moisture northward into the U.S. will also be aided by Pacific Hurricane Blas, which is currently located off the West Coast of Mexico. This hurricane will track due west away from the coast, but residual tropical moisture from the hurricane will still become entrained into the southerfly flow, which will carry it northward into New Mexico and Colorado.

Fire and Smoke Outlook:

Strong winds from Friday through Sunday will result in critical fire danger across the Great Basin, and to a lesser extent areas further east. The brown-shaded area indicates a more "conditional" fire risk, where lightning associated with dry thunderstorms could potentially ignite new fires.

Southerly winds will continue to transport smoke from Arizona and New Mexico fires into Utah, Colorado, and Wyoming on Friday.

On Saturday, the flow pattern will remain similar but smoke levels should decrease overall compared to Friday as rainfall associated with the emerging monsoon becomes more widespread.

Forecast for Friday:

The dominant weather pattern for the weekend will get underway on Friday with moderate to heavy showers developing across the Cascades and Northern Rockies, while Colorado and New Mexico will also see their first notable uptick in showers and thunderstorms as the monsoon pattern takes hold.

The Central Rockies (Northern Utah, Wyoming, Southern Montana) will largely be caught in between these two main weather features over the weekend with some showers/thunderstorms but less rainfall compared to other areas. This is good for Yellowstone, who needs a break from significant rainfall. 

The most "interesting" weather on Friday may end up happening in Northwest Montana, including Glacier National Park, where the atmosphere will become very unstable on Friday afternoon while wind shear (increasing winds with height) will also increase.

Both of these factors along with abundant moisture and energy arriving from the Pacific will lead to an elevated risk for strong to severe thunderstorms. The Black Hills of Northeast Wyoming/Western South Dakota will also be in a highly unstable environment on Friday afternoon.

NOAA's Storm Prediction Center has highlighted Glacier National Park and Northwest Montana as being under a Slight (level 2 out of 5) Risk for Severe Weather, which is significant for this region.

A fairly substantial threat for large and damaging hail will exist with any stronger thunderstorms that develop in the Glacier National Park area. The yellow "hatched" risk area on the map below will be under an elevated risk for 2" diameter (or larger) hail, which is something we typically see more over the plains than in the mountains. 

In addition, severe thunderstorms over Northwest Montana could produce damaging wind gusts in excess of 60 mph, and while unlikely, an isolated tornado couldn't be ruled out either. 

Further south across New Mexico, Colorado, and Eastern Arizona, the main threat from thunderstorms on Friday will be lightning as well as heavy rainfall, with areas of excessive runoff and flash flooding possible under stronger thunderstorms.

Forecast for Saturday:

Thunderstorm coverage should increase a bit more across New Mexico, Colorado, and Eastern Arizona and Utah with the San Juan Range expected to be favored for the most widespread and heaviest rain totals. Showers will also continue across the Northwest and Northern Rockies, with a chance of thunderstorms as well. Shower activity will extend as far south into the Tahoe area of California.

Heavy rain and localized flash flooding will be possible across the Southern Rockies, while Northern Idaho and Montana will also see a flooding risk due to heavy showers while rivers and streams are already stressed from recent precipitation and snowmelt. 

Forecast for Sunday:

The mentioned low pressure system will track further inland across the Rockies and a cold front will also move through, resulting in an uptick in shower and thunderstorm activity across Southwest and West Central Montana and to a lesser extent Wyoming. Numerous showers and thunderstorms can be expected across Colorado and New Mexico as well. 

Heavy rain totals are currently projected across portions of Southwest and West Central Montana and this will result in another uptick in flooding concerns across the region, though Yellowstone should miss out on the heaviest rainfall at least. Heavy rain and flash flooding will remain possible across the Southern Rockies as well.

Outlook for Monday (6/20) to Tuesday (6/21):

Showers will continue across the Northern Rockies and Northern Cascades on Monday post-cold front with potentially heavy rainfall possible near and east of the Continental Divide in Northern and Central Montana, including Glacier National Park. This region will need to be monitored for possible flooding issues.

Some high elevation snow will also be possible across the Northern Rockies on Monday, but snow levels will be higher compared to last weekend, and most outdoor activities outside with the exception of alpine climbing should not be significantly impacted. 

The Northern Rockies and the Cascades will start to dry out on Tuesday, while additional rounds of thunderstorms can be expected across the Southern Rockies, although the focus will shift southward a bit favoring New Mexico more so than Colorado.

Outlook for Wednesday (6/22) to Thursday (6/23):

Overall, the monsoon pattern will weaken just a bit during this timeframe but will still remain in place across the Southern Rockies and may expand a bit further west toward Zion, Grand Canyon, Flagstaff, etc. Colorado and New Mexico should continue to see daily rounds of storms, but perhaps more isolated in coverage compared to prior days. 

The Northwest and Northern Rockies will generally see a drier pattern during this period, though a disturbance may clip the Northern Cascades and far Northern Rockies around Glacier. 

Outlook for Friday (6/24) to Friday (7/1):

A ridge of high pressure will build over the South Central and Southwest U.S. during this period, resulting in a trend toward hotter than average temperatures across a good portion fo the West, except for the Pacific Northwest where temperatures will be close to average (but still much warmer compared to prior weeks). 

The monsoon will continue across the Four Corners region with wet signals showing up across Colorado and New Mexico, while Arizona should also get into a wetter pattern with hopefully some much-needed moisture around Flagstaff where the Pipeline Fire is burning. 

The Northwest and Northern Rockies are looking much drier during this period as the storm track shifts northward into Canada, but there are some hints that Glacier could get clipped by disturbances, resulting in periods of shower/thunderstorm activity. 

Thanks so much for reading and have a great weekend! Next update on Monday (6/20).

ALAN SMITH