Forecast Summary May 26 to June 1:

Here is a quick summary of what to expect across the West, before we get into day-to-day details...

• California & Sierra Nevada Range – Active pattern with daily rain showers from Tuesday to Friday with much-needed moisture expected. The High Sierra will also receive snow.
• Pacific Northwest – Frequent rain chances along and east of the Cascade Crest, including thunderstorms across the Inland Northwest. Severe thunderstorms are also possible for the Inland NW on Thursday.
• Northern Rockies – Active thunderstorm pattern from Tuesday to Friday, with strong thunderstorms possible. More widespread significant precipitation is possible Saturday-Sunday.
• Colorado – Thunderstorm and shower chances favoring the Front Range and areas east of the Divide all of this week. Large hail possible across the Front Range on Friday.
• Utah – Central & eastern Utah favored for thunderstorms on Tuesday. Drying trend mid-week, followed by shower & thunderstorm chances across Northern Utah this weekend.
• Southwest – Seasonally dry pattern with shower and thunderstorm activity favoring the eastern ranges of New Mexico. 

Tuesday (May 26):

A slow-moving cut-off low is located over Northern California on Tuesday, and this will lead to active weather from Northern California into the Interior Northwest. Winds blow counterclockwise around low pressure centers, and upper-level winds follow the black lines in the image below, which can be used to approximate the direction that showers and thunderstorms will travel. 

Rainfall on Tuesday and Tuesday night will favor Northern California, including Tahoe, as well as Central and Western Nevada, and northward into the Inland Northwest. A secondary focal point of rainfall is expected over the Sange de Cristos in Southern Colorado and Northern New Mexico. 

An unstable atmosphere will be in place across the Interior West, well to the east of the cut-off low, resulting in an active afternoon for thunderstorms across the Rockies.

Early to mid-afternoon thunderstorm activity will favor central and eastern portions of Utah, as well as Northwest Wyoming, with more isolated activity across the Wasatch and across most of Colorado.

Within Colorado, southerly low-level flow will transport enough Gulf moisture for isolated thunderstorms over the Front Range Mountains and Sangre de Cristo and Wet Mountains, but storms will likely stay anchored to the higher terrain with lower (but non-zero) chances across the Denver urban corridor.

As a shortwave disturbance tracks from south to north across the Interior West, thunderstorm chances will ramp up across Montana late in the afternoon, along and east of the Divide from Yellowstone to Glacier, and northward into the Southern Canadian Rockies.

Isolated instances of severe weather (1" diameter hail or 58+ mph wind gusts) also couldn't be ruled out in this region. 

Wednesday (May 27):

The cut-off low will only move slightly, setting up near the Sierra crest in California/Nevada. This pattern will bring more beneficial rain to California, while to the east, southerly flow and unstable air will contribute to thunderstorms across the Northern Rockies.

Rainfall will favor Northern California, Nevada, and Southern Oregon, while a secondary uptick is expected near and east of the Divide in Colorado. Thunderstorm chances are limited across these regions due to a cooler and more stable atmosphere.

In fact, it will be cold enough for snow to fall across the higher elevations of the Sierra, with significant amounts possible over the High Sierra and Eastern Sierra, including Mammoth.

Thunderstorm activity will favor the Northern Rockies again on Wednesday, though rainfall will generally be lighter and more variable compared to Tuesday.

According to our StormNet Model, the greatest risk of lightning is expected around McCall, Idaho, and along the eastern side of Glacier Park and the Rocky Mountain Front in Montana.

Thursday (May 28):

The low pressure center will remain nearly stationary over California with showers continuing across California and Nevada. However, the deepest moisture will push northward into the Pacific Northwest (along/east of the Cascades, especially), while to the east of the low, dry air will push into the Rockies from the south.

The southern and central Oregon Cascades are favored for the heaviest rainfall in this pattern, with a notable drying trend across most of the Rockies. However, low-level moisture from the Gulf will continue to result in isolated to scattered showers and thunderstorms along the eastern slopes of the Colorado Rockies. 

Thunderstorm activity on Thursday afternoon will favor the Inland Northwest, with StormNet showing a strong signal for lightning from Central and Eastern Oregon into the Idaho Panhandle and Northwest Montana. Also, the eastern slopes of the Rockies in Colorado and Northern New Mexico are favored for lightning activity. 

On Thursday night, StormNet is picking up on a strong signal for overnight thunderstorm activity across northern portions of the Inland Northwest with organized lines/clusters of thunderstorms possible, especially across Washington (east of the Cascade crest), Northern Idaho, Northwest Montana, and Interior BC/Alberta. 

Our StormNet Model is also picking up on a severe weather threat across the Inland Northwest on Thursday evening, with stronger storms capable of producing large hail (1" diameter) and damaging wind gusts (58+ mph).

Friday (May 29):

The low will finally pick up some movement and will make forward progress into Nevada and Utah on Friday.

The Sierra and Northern California will hang onto some rain chances, but the better moisture will shift eastward with shower and thunderstorm chances favoring the Interior Northwest and Northern and Central Rockies.

Lightning activity looks to favor the Northern Rockies from Central Idaho and Montana, as well as the eastern slopes of the Rockies and adjacent plains from Wyoming to Colorado, where rich low-level Gulf moisture will increase from the southeast. 

We are also picking up on a large hail threat along the Colorado Front Range and Southeast Wyoming on Friday afternoon.

Saturday (May 30) to Sunday (May 31):

Heading into the weekend, the low pressure center is projected to swing northeastward and track slowly across the Northern Rockies. 

This storm track would favor significant precipitation across the Northern Rockies, with the potential for some areas to receive 1-3 inches. Further south, shower/thunderstorm chances should favor northern portions of Utah and Colorado, with more isolated activity further south.

The 72-hour precipitation projection below represents the average of 50 simulations of the European AIFS Model.

Keep in mind that this forecast is subject to change depending on the exact track/movement of the upper low. Cut-off upper lows can be challenging to forecast more than a few days in advance, though the European AIFS Model does a reasonably good job at handling these patterns.

Also, I wouldn't be surprised if portions of the Northern Rockies pick up some high-elevation snowfall in this pattern, though we'll have a better idea about this later in the week. 

Total Precipitation May 26 to June 1:

Overall, this is shaping up to be a very productive pattern in terms of moisture across the Sierra, Interior Northwest, and Northern Rockies, while in Colorado, rain and thunderstorm chances will favor the Front Range and eastern slopes of the Rockies this week. 

Here is our PEAKS Model total precipitation forecast through Monday, June 1:

June 1st to 7th:

The first week of June looks to favor warmer temperatures across most of the West, but with subtle troughs favoring shower and thunderstorm chances across the Interior Northwest and Northern Rockies, as well as New Mexico and Colorado.

The finer details are highly uncertain this far out, but in general, a seasonally unsettled early summer pattern (think afternoon thunderstorms) looks to take hold, along with more summer-like temperatures. 

Thanks so much for reading! Next update on Friday (May 29).

Alan Smith