Cold air from Canada has invaded the whole state of Montana's north-central region, and it will gradually move into the state's southwest region over the next day or two.
The cold weather pattern and snow showers are expected to continue over the holiday weekend. Additional colder air is expected to arrive on Sunday and continue through the middle of the week.
West to southwesterly flow continues above into Saturday night, but Arctic air stays entrenched across most of central and northern Montana for the remainder of today and into Saturday night. Snowfall continues to be restricted to a few sites along the Divide and in southern Madison and Gallatin counties, among other places.
The Winter Weather Advisory in this area has been lifted early due to a decrease in snowfall in the West Yellowstone portion of the region. The passage of a fast-moving shortwave over the region late tonight into Saturday morning will bring an increase in snow intensity along the Divide as well as a period of light snow to the rest of the forecast area.
As this shortwave delivers lift and moisture over the mountains and interacts with the frontal surface boundary, forecast soundings indicate a dendritic growth zone that is deepening throughout the course of the forecast.
Snowfall rates of up to 1 inch per hour may be expected in the vicinity of Highway 2 from East Glacier Park to Marias Pass, which includes the region of East Glacier Park.
Marias Pass is expected to get an 8-inch or more of snow until Saturday evening, with a 70 percent probability of receiving more. The Winter Storm Watch has been downgraded to a Winter Weather Advisory, which will remain in force until Saturday evening.
For the light snow predicted across the rest of the region, no winter weather products are being developed at this time. A fast dusting to as much as 2 inches of snow may be expected tonight into Saturday early, though.
According to the most recent probabilistic advice over the Hi-Line and south into Fergus county, there is a 40 percent probability of snowfall exceeding one inch, with lesser odds for other lower elevation places. In addition to the snow, patchy fog may develop over the plains, which may contribute to any slick roads and reduced visibility that may result.
This light snow occurrence prompted the issuance of a Special Weather Statement to address the possible consequences. The weather begins to improve by Saturday afternoon, however, the low temperatures will persist throughout the weekend.
Then, when the next trough approaches from the west, the southwesterly flow aloft starts to strengthen Saturday night into early Sunday. Streams of moisture will pass across the cold air mass, delivering another light to moderate snow event to a large amount of the predicted region.
This storm is likely to provide snowfall quantities largely in the 1 to the 3-inch range for central and north-central regions, with greater amounts forecast across southern Madison and Gallatin counties in particular.
North-central Montana is being pushed into the area by a much colder airmass as the main trough sweeps across the region. Temperatures in north-central Montana are expected to drop to -20 degrees Fahrenheit Sunday night, while temperatures in the southwest will only reach single digits.
This dangerously cold wave is expected to last until the middle of next week. By Thursday or Friday next week, downsloping westerly Chinook winds will begin to erode some of the cold air from Southwest Montana and the Rocky Mountain Front, allowing for a return to normal temperatures.
These gusts may result in blowing snow concerns, necessitating the usage of high wind products in some instances. With the approach of another weather system to finish out the month, there is a chance for snow to return.