Big Picture Weather Pattern:

A deep trough of low pressure containing well below-average temperatures will move into the Northern Rockies on Monday and Tuesday, resulting in a late-season chill with snow for the mountains. Areas to the south will also see some temporary heat relief along with gusty winds. 

5-Day Precipitation Forecast:

Precipitation will favor the Northern Rockies early this week with additional light showers for the Cascades and Coast Range of British Columbia. Late this week, moisture will increase across the Southwest and Southern Rockies, leading to an uptick in thunderstorm activity.

Southern California Fires:

While northern parts of the Western U.S. are cool and wet early this week, wildfires broke out across Southern California near LA over the weekend. The most notable is the Post Fire near Gorman (north of LA) which started on Saturday and had burned over 14,000 acres as of Sunday.

Here is a view of this fire on our Active Fires Map.

Smoke from this fire and numerous other small fires are resulting in poor air quality across parts of Southern California. Strong winds early this week will also transport some of this smoke into Arizona and Southern Utah.

View → High-Res Smoke (Surface) Map

Forecast for Mon (Jun 17) to Tue (Jun 18):

The storm system on Monday and Tuesday will favor Montana and Idaho for the heaviest rain and snow, while the Cascades will see lighter showers on the backside of the storm. Mostly dry conditions are expected across the Southern Rockies. 

Taking a closer look, many of the higher elevation areas in Montana as well as the Salmon River Mountains in Idaho will pick up over an inch of precipitation (rain and liquid-equivalent snow) on Monday and Tuesday. Southwestern portions of Montana around Bozeman look to see the highest totals.

Snowfall will also be significant across higher-elevation areas of the Northern Rockies, with snow levels dipping below pass level. However, some of the higher snowfall accumulations on this map are likely overdone, as this particular model blend assumes higher snow-liquid ratios than what is typical this late in the year.

OpenSnow point forecasts are projecting 0.71 inches of rain in Bozeman on Monday and another 0.36 inches after 12 a.m. on Monday night/Tuesday morning for a storm total of 1.07 inches. For the higher terrain, our forecast calls for 7-13 inches of total snowfall at Big Sky Resort.

Forecast for Wed (Jun 19) to Thu (Jun 20):

Warmer and drier conditions will gradually take hold across the Northern Rockies, but isolated lighter showers will redevelop both afternoons. Northern Washington and British Columbia will see a few light showers as well. 

On Wednesday, moisture from the Northern Rockies storm will spill southward into the eastern ranges of Colorado and New Mexico, resulting in an uptick in showers and thunderstorms.

On Thursday, a significant surge of moisture from a tropical disturbance in the Gulf of Mexico will reach the Southwest, resulting in more numerous thunderstorms across New Mexico and Southern Colorado with the potential for locally heavy rain.

Forecast for Fri (Jun 21) to Sat (Jun 22):

A weak area of low pressure over the Southwest combined with an uptick in subtropical moisture will aid in thunderstorm development with activity spreading westward into Eastern/Southern Utah and Eastern Arizona.

Locally heavy rain will be possible with some of the highest totals expected across Northern New Mexico and the San Juans in Southwest Colorado.

To the north, a weaker disturbance tracking across Alberta and Montana will result in scattered thunderstorms east of the Continental Divide. The Olympics and Cascades near the West Coast could potentially see a few light showers as well.

Outlook for Sun (Jun 23) to Thu (Jun 27):

Above-average warmth will overtake most of the West once again, with the only exception being the Pacific Northwest (along and west of the Cascades) where temperatures will be near average. Subtropical moisture will linger over the southwest, resulting in additional thunderstorm chances with hit-or-miss rainfall. The Northwest will also see occasional light showers move through. 

Thanks so much for reading! Next update on Wednesday (June 19).

Alan Smith 

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