textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Strong cold front Tuesday, bringing colder temperatures, widespread snow, and risk for snow squalls.

- Arctic intrusion north of I-90 and along the Divide Tuesday night into Wednesday morning.

- Snow showers along with colder temperatures Wednesday into Friday.

Mild temperatures continue today with valley highs in the 40s and 50s ahead of an approaching cold front. Increasing southerly flow will produce breezy conditions this afternoon, specifically across the Camas Prairie, where gusts may reach 40 mph. The primary concern transitions to a potent cold front and deepening surface low tracking from the Great Basin into southwest Montana tonight. This system will bring rapidly falling snow levels and intense snowfall rates. High-resolution guidance indicates a 60 to 90% probability of snowfall rates exceeding 1 inch per hour with the cold front passage between midnight and 9 a.m. Tuesday for much of North-Central Idaho and Western Montana. Travelers should prepare for snow squalls Tuesday morning in southwest Montana and Lemhi County, particularly along the I-90 and I-15 corridors near Butte, where sharp temperature drops will coincide with the heaviest precipitation.

Widespread valley snow is expected, with National Blend of Models (NBM) probabilities for >1 inch exceeding 50% for all valley locations. Mountain totals through 48 hours are forecast between 4 and 10 inches above 4,000 feet. A localized mesoscale snowband may enhance totals Tuesday morning for the Flathead Valley, Seeley Lake, and Potomac areas due to favorable upslope flow. Numerous winter weather advisories remain in place.

By Tuesday night, an arctic air mass will spill over the Continental Divide. While the westward extent of the coldest air remains uncertain, there is high confidence in temperatures dropping into the single digits and teens by Wednesday morning in northwest Montana and along the Continental Divide, with sub-zero wind chills likely along US Highway 2 and MacDonald Pass.

An unsettled west-northwest flow will persist through the week, maintaining snow levels at valley floors and generating daily snow showers. The most consistent accumulations will focus along the Montana/Idaho border. On Wednesday, a mid-level circulation will target Idaho and Clearwater counties, bringing a 50 to 80% chance of at least an inch of new snow. Residents and travelers should anticipate variable road conditions and rapid visibility reductions through the weekend as this broad troughing remains overhead.

AVIATION

VFR to MVFR conditions will prevail through this afternoon across most terminals, though localized showers will produce lowered ceilings and obscurations of the terrain. Strengthening southerly flow will create breezy surface conditions and potential low-level wind shear, particularly in the vicinity of the Camas Prairie where gusts to 35 kt are possible. Conditions will deteriorate rapidly after 17/0000Z as a potent cold front enters northwest Montana and tracks southeast. Expect a transition from rain to heavy snow with an abrupt wind shift to the west- northwest. High-resolution guidance indicates a 60 to 90% probability of snowfall rates exceeding 1 inch per hour as the front passes between 17/0300Z and 17/1600Z for Western Montana and North-Central Idaho terminals (KGPI, KMSO, KBTM, KSMN, KHRF). This will lead to frequent LIFR visibilities and vertical obscurations. Of particular concern is the potential for snow squalls and rapid pressure rises Tuesday morning near KBTM and south into Lemhi County, which may produce localized whiteout conditions and severe turbulence along the frontal boundary.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Winter Weather Advisory from 11 PM this evening to 5 PM MST Tuesday for Bitterroot/Sapphire Mountains.

Winter Weather Advisory from 2 AM to 5 PM MST Tuesday for Butte/Blackfoot Region...Flathead/Mission Valleys... Kootenai/Cabinet Region...Potomac/Seeley Lake Region...West Glacier Region.

Winter Weather Advisory from 8 PM this evening to 5 PM MST Tuesday for Lower Clark Fork Region.

ID...Winter Weather Advisory from 10 PM this evening to 4 PM PST Tuesday for Northern Clearwater Mountains...Southern Clearwater Mountains.

Winter Weather Advisory from 7 PM this evening to 10 AM PST Tuesday for Lower Hells Canyon/Salmon River Region... Orofino/Grangeville Region.

Winter Weather Advisory from 2 AM to 5 PM MST Tuesday for Eastern Lemhi County...Western Lemhi County.