A major flip of the weather pattern will allow for cold Canadian air to stream into the Intermountain West bringing the first snow of the season to portions of the Rockies.
The West has overall been very warm the last month or two while the East has had some cooler bouts of weather. Temperatures will take a tumble as the overall weather pattern across the nation flips due to a upper level trough of low pressure digging into the Northwest. The jet stream will dig further south. This in turn causes ridging in the East where above average temperatures will become common for the rest of September.
But does it really mean accumulating snow for parts of Idaho, Montana and Wyoming? Yes it does. In fact higher elevations of the Rocky Mountains might see 3-6″ of snow with isolated areas seeing even more. Only 40s and 50s will be likely for highs on Friday and Saturday with lows in the 30s or colder at night. That means temperatures 30 to 40 degrees colder than compared to the beginning of the week!
A low pressure system will move in and start out as rain. As the temperatures fall the rain will change to snow and then it will start to accumulate. A few areas will see lows reach into the 20s making snowfall accumulation pretty easy.
The first winter weather headlines are out for portions of western Montana, eastern Idaho plus northern and western Wyoming. It’s always good to remember that snow accumulations can vary a lot from spot to spot. While 6,000 feet and higher should see snow, the mountain peeks will likely see a greater amount; likely 1-2 feet.
— NWS Billings (@NWSBillings) September 13, 2017