Following a dry week across the Northern Rockies, the pattern will turn much more active over the weekend and into the first half of next week as a series of Pacific troughs move into the Western U.S. This pattern will actually be rather complex with a variety of impacts across the region as warm Pacific air from the southwest battles with cold arctic air pushing southward into Montana.
The approaching trough of low pressure is still located well off the coast of California this morning, but significant moisture ahead of this trough can be seen streaming across the western states from south to north in this morning’s water vapor satellite image (source: NOAA).
The southerly source of this Pacific moisture will result in wet/dense snowfall at the start of this storm cycle, along with mixed precipitation or rain in many of the valleys. Moisture will arrive for most areas west of the Continental Divide during the day on Saturday, with the heaviest precipitation expected across Idaho and Western Wyoming Saturday night. Warm air in place will keep this a rain/snow mix for areas such as Jackson Hole and Ketchum/Sun Valley with significant travel impacts likely limited to mountain passes and areas above the valley floors.
The next image shows the approaching trough of low pressure by 5pm Saturday, as projected by the GFS Model (image source: tropicaltidbits.com).
More snow is expected region-wide from Sunday through Wednesday as a second trough of low pressure moves across the area early next week. Also, an arctic cold front will push into Western Montana on Sunday morning, before stalling along the Continental Divide. This will result in much colder temperatures and higher road impacts due to snow across the Bozeman area on Sunday.
A variety of impacts can be expected over the next several days as follows, and are subject to change given the complexity of this pattern.
Missoula: Cold air will push in from the north/northeast on Sunday morning, combining with Pacific moisture to result in the potential for respectable snowfall amounts and travel impacts. This area is in a tough spot, however, as arctic air sometimes doesn’t push as far west as the models show, in which case snowfall could be less than forecasted. However, the prime times for travel impacts (assuming meaningful snow does occur) will be Sunday morning and Sunday night/Monday morning.
Bozeman: Snow will arrive Sunday morning and continue off and on for the next few days with arctic air in place, clashing with Pacific moisture. Temperatures will fall well below freezing behind the arctic front Sunday morning, and therefore roads will quickly become snowpacked on Sunday with continued travel impacts expected through Wednesday. Snowfall amounts Sunday-Monday will likely be in the 3-6″ range, with additional accumulations likely on Tuesday and Wednesday.
Jackson Hole: Borderline temperatures will result in a rain/snow mix Saturday and Saturday night. Colder air aloft should allow for mostly snow in the valley on Sunday, but temperatures will still be warm enough that travel impacts should be limited in the valley (unless heavy snowfall rates overcome borderline temps Sunday morning).
We should gradually see colder temperatures filter in Sunday night and Monday, with better snow accumulation potential in the valley along with travel impacts. The cooling trend will continue as periods of snow persist on Tuesday and into Wednesday, with each day seeing progressively higher travel impacts. Travel conditions over Teton Pass will be very difficult throughout the period as significant snowfall is expected from Saturday through Wednesday.
Ketchum/Hailey: Similar to Jackson Hole, warm temperatures will keep travel impacts limited across the valley areas on Saturday and into the first part of the day Sunday, before gradually colder temperatures filter in by Sunday evening with heavy snowfall and more significant travel impacts likely. Monday and Tuesday will see continued periods of lighter snowfall, but with colder air arriving travel conditions will remain difficult.
Idaho Falls/Pocatello: This will be mostly a rain event over the weekend due to warm temperatures, but we could see some light snow impact the area by Tuesday-Wednesday next week as the last trough passes through the area along with colder temperatures.