Originally posted September 25, 2019.
Updated September 26, 2019.
The northern Rocky Mountains are about to get a harsh blow from Mother Nature. A potential blizzard-like storm could shut down roads and knock out power in many high-elevation areas. The National Weather Service (NWS) has already issued a Winter Storm Warning and a Winter Storm Watch for parts of Montana and Idaho, including portions of I-90 and I-15. However, these alerts may be expanded again to include additional counties. Shippers may want to plan now in order to get ahead of the storm.
The set-up
A strong Canadian cold front will likely move through north-central and central Montana on Friday, September 27, lingering over southwestern Montana the next day. During this time, a low pressure center will move southward into the Pacific Northwest, sending plenty of moisture into the northern Rockies. This will likely bring rain showers and mountain snow showers to the region Friday into Friday night, with a slight chance of afternoon thunderstorms.
Temperatures will gradually begin to cool with highs in the 40s and 50s across northwestern Montana by Friday, lows in the mid-20s to mid-30s. The cold air will continue to push south behind the front on Saturday, causing more widespread accumulating snow in the mountains of northwestern Montana, rain in the southwestern valleys, and a mix of rain and snow in the northern plains. The coldest air over the plains will initially be along the Rocky Mountain Front, making that area most likely to see accumulating wet snow on the plains.
Snowfall amounts
Parts of north-central Montana, including the Rocky Mountain Front and adjacent plains are areas of focus for the Winter Storm Warning, which for now is in place from Friday evening through Sunday afternoon, September 29. Total snow accumulations in these areas are likely to reach 15 to 36 inches on the plains, with higher amounts in the mountains. Wind gusts of 40 mph across the Rocky Mountain Front and west over the Continental Divide will create periods of blizzard/white-out conditions. Adding insult to injury – the storm may produce record or near-record cold temperatures in the teens and 20s, along with wind chills of zero to 10° above zero.
The nearby city of Great Falls has been added to the Winter Storm Watch area. Jason Anglin, a meteorologist with the NWS in Great Falls, told FreightWaves on September 25 that he was expecting that city to receive six to eight inches of total snow from this storm. Anglin also noted that his outlook could change, and it did. The forecast now calls for Great Falls to get much more – possibly around 12 inches based on the latest online NWS forecast. The historical average three-day total for this time of year has been less than half an inch.
Besides treacherous road conditions, this early season storm has the potential to cause other extreme impacts such as widespread loss of electricity due to downed power lines. In addition, widespread significant tree damage is possible with heavy, wet snow and strong winds weighing down trees that have a lot of foliage.
Making history?
According to the NWS, this early season winter-like storm has the potential to set a new benchmark for snow accumulations, cold temperatures, and resulting impacts for parts of the Northern Rockies and the Rocky Mountain Front. A similar storm in 1934 produced prolific amounts of snow in late September over north-central Montana.
Anglin said the 1934 storm dropped a three-day snowfall total of 13.2 inches in Great Falls (elevation 3,675 feet) and 28.0 inches in Conrad (elevation 3,517 feet), about 60 miles to the northwest. He also mentioned that the last significant September snowstorm to hit Great Falls was in 1988, when 8.4 inches came down over a three-day period. Anglin explained that if a storm of this magnitude were to pan out, it would likely happen early or late in the season because this is when the polar jet stream – the winds aloft that steer weather systems – is most active.
Other areas of Big Sky country – Lewistown, Helena, MacDonald Pass, Rogers Pass, West Yellowstone, Butte, the Bitteroot Range, Bad Rock Canyon, Essex, Highway 83 from Bigfork to SwanLake, Marias Pass, Polebridge and I-90 east from Missoula to Bearmouth – could be blanketed with anywhere from six to 25 inches of snow, depending on elevation. Winds in all of these spots could gust as high as 40 mph, leading to additional areas of blowing snow and little/no visibility.
Snowfall totals as high as nine inches are forecast for portions of Lemhi County in eastern Idaho. This mainly affects the Williams Creek Summit area.
Precipitation should gradually decrease across the region next Monday, September 30 into Tuesday, October 1 as the low pressure center moves farther to the south away from the area.
NWS forecast confidence is moderate at this time regarding total snowfall accumulations, wind strength, associated impacts, and the timing of the onset and end of this winter storm. Look for updates on the FreightWaves social media accounts.