Quick Summary by Region:

• Northern Rockies – A low pressure system will bring substantial rain to Montana, Alberta, and parts of Wyoming this weekend, with snow also expected over the higher mountain ranges. 
  
  
• NorCal to PNW – Showers on Friday will give way to a drying trend over the weekend with temperatures on the cooler side of average.
  
  
• Utah – Rain showers favoring Northern Utah on Friday and Saturday, including some overnight snow for the Wasatch and Uinta Ranges.
  
  
• Colorado – Relatively quiet pattern through the weekend with isolated to scattered showers and thunderstorms favoring the northern ranges, Front Range, and eastern plains.
  
  
• Southwest – Scattered thunderstorms for Arizona & Southern Utah on Friday with cooler temps, followed by drier and warmer conditions this weekend.

Next, we will take a look at the day-to-day details from Friday to Monday, followed by expected precipitation totals through Monday. 

Friday (May 29):

A cut-off low pressure system will move from California into Southern Nevada, with southerly flow bringing moisture into Utah. 

To the north, a secondary area of low pressure that has pulled away from the main low will contribute to showers across the Pacific Northwest, and thunderstorms from Central Idaho into Montana. 

StormNet is favoring midday thunderstorm development over portions of the Northern and Eastern Rockies, as well as over Northern Arizona.

By mid to late afternoon on Friday, a more robust thunderstorm environment is expected across West Central Montana with a higher threat of lightning extending from Missoula to Helena and Great Falls. 

Saturday (May 30):

The low pressure center will track northeastward into Southwest Wyoming, with moisture favoring Northern Utah, Wyoming, Montana, and Eastern Idaho. Cooler air will also overspread this region.

Lightning chances will be reduced across the Northern Rockies due to the cooler and more stable airmass, with just plain rain (and for higher elevations, snow) expected.

Just east of the Rockies, thunderstorms are likely across the high plains of Montana and Wyoming as well as the Black Hills, while the eastern slopes of the Beartooths and Bighorns could possibly see some lightning as well.

Sunday (May 31):

The low pressure center will continue to lift northward into Montana, with widespread precipitation favoring Northern Wyoming, Montana, and Alberta. A drying trend is expected further south in Utah.

Thunderstorm activity will remain limited across most of the West, but we could see some pockets of isolated thunderstorm activity around the Tetons and over the Northern Colorado Rockies, with higher chances further east on the plains. 

Monday (June 1):

The main low pressure area will continue its northward journey and will be located near the Canadian border by midday Monday. The best moisture will gradually lift northward as well, favoring Northern Montana and Alberta, with a drying trend expected across Wyoming and Southern Montana.

To the south, a shortwave will move into Colorado from the southwest, though moisture will be lacking west of the Continental Divide.

East of the Divide, low-level easterly upslope flow will increase, transporting Gulf moisture toward the Front Range. This will result in a better chance of thunderstorms east of the Divide, though the greatest coverage is currently expected to stay confined to the plains. 

StormNet favors a low to moderate threat of thunderstorms east of the Continental Divide and along the I-25 corridor on Monday, with a higher threat across the Eastern Colorado plains. 

Total Precipitation Forecast (Friday to Monday):

Measurable rainfall is expected across the PNW and Northern and Central Rockies, but the highest totals are expected across Montana and Alberta, and to a lesser extent Northern Wyoming, Eastern Idaho, and Eastern British Columbia. 

Many areas in the Northern Rockies are looking at 1-3 inches of rain (or liquid equivalent precipitation) through the weekend, especially along and east of the Divide in Montana and Alberta. 

One of the wettest areas this weekend will be Glacier National Park. Our multi-model forecast chart for Many Glacier shows 2-4 inches of rain over the next 5 days, with our PEAKS Model average indicating around 3 inches.

Snow:

As cooler air sneaks into the picture with this system, snow will also fall across the higher ranges of the Northern Rockies and Northern Utah. Snow levels will dip to around 9,500 feet in the Wasatch and Uintas, and to 9,000 feet from the Tetons to the Beartooths. Several inches of snow are possible in these areas.

Check out our snow level forecast range chart for Beartooth Pass (10,947 ft.), near the Wyoming/Montana border.

Tuesday (June 2) to Sunday (June 7):

Next week, an upper-level trough is projected to move into the Pacific Northwest, resulting in cooler temperatures and increased rainfall potential across the Cascades (WA, OR) and Northern Rockies (Northern ID/MT and BC/AB). 

Meanwhile, a low pressure area is also projected to set up over the Southwest and pull in some subtropical moisture into New Mexico and Colorado, leading to an uptick in thunderstorm chances along with measurable rain potential. This pattern would likely favor the Sangres, San Juans, Sawatch, and Front Range.

In between these two features, weak high pressure and drier air will favor a significant warm-up across the Central Rockies (UT, WY, Southern MT) to above-average levels for early June. California will also see a warm-up, but only to slightly above average levels. 

Thanks so much for reading and have a great weekend! Next update on Monday.

Alan Smith