textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Slick Travel Conditions: Trapped cold air in the valleys will lead to a messy mix of snow and wintry mix through this morning. Expect slick condition at times including the threat for black ice on roads and overpasses.

- Unsettled Holiday Week: A series of disturbances will keep the region wet and unsettled through Christmas. A stronger cold front on Friday will likely transition all valley precipitation back to snow.

- New Year`s Eve Outlook: A potential atmospheric river could bring milder, wetter, and windier conditions to end the year.

A complex weather pattern continues across the Northern Rockies as moist Pacific air overrides stubborn, cold air trapped in the valleys. This setup will create a variety of precipitation types and travel hazards through the early part of the week, followed by a series of storms leading into the Christmas holiday.

The primary concern today is the persistent "cold pool" in the valleys. While weather models often try to wash this cold air out too quickly, local observations show it remains firmly in place. This means that many valleys, including the Salmon, Missoula, northern Bitterroot, and Flathead Valleys, will deal with snow and a wintry mix.

Winter Weather Advisories remain in effect through this morning to account for slushy accumulations and the development of black ice on cold pavement. By Tuesday, another pulse of moisture arrives. This will bring more light snow to the mountain passes and a likely wintry mix to the valleys Tuesday night. Temperatures will remain marginal, making the difference between a wet road and an icy road very narrow.

Wednesday through Sunday: The pattern remains very active through the Christmas holiday. A deep "dip in the jet stream" (trough) over the eastern Pacific will eject several waves of moisture into the region.

Christmas Eve/Day: We are watching for a "wedge" of Arctic air to potentially spill over the Continental Divide. If this happens, areas near Marias Pass and Essex could see a sharp drop in temperatures and an increase in snow. For the rest of the area, expect periodic rain and snow showers to continue through the holiday.

Friday Front: A more robust cold front is forecast to sweep through on Friday. This will bring a much cleaner transition to snow for the valley floors as snow levels lower.

Next Weekend: Conditions look to turn cooler with persistent mountain snow showers, though the heavy, organized precipitation may take a brief break before a potential "Atmospheric River" targets the region for New Years Eve.

AVIATION

The main concern is the persistence of low ceilings and mixed precipitation types. With the forecast trending colder, terminals such as KSMN, KGPI, and KMSO are more likely to see periods of snow or wintry mix rather than a clean switch to rain. Expect IFR/LIFR conditions to dominate as the low levels remain saturated. Mountain obscuration will be widespread. Low- Level Wind Shear (LLWS) may be present at times as strong westerly flow aloft overrides the trapped valley cold pools after 1200Z this morning across southwest Montana and along the Divide. Expect more consistent improvements to visibility and ceiling heights after around 1800Z, along with valley wind gusts of 12-22 knots through the afternoon.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Winter Weather Advisory until 11 AM MST this morning for Butte/Blackfoot Region...Flathead/Mission Valleys... Kootenai/Cabinet Region...Lower Clark Fork Region... Missoula/Bitterroot Valleys...Potomac/Seeley Lake Region... West Glacier Region.

Winter Weather Advisory until 9 AM MST this morning for Bitterroot/Sapphire Mountains.

ID...Winter Weather Advisory until 10 AM PST this morning for Northern Clearwater Mountains...Southern Clearwater Mountains.

Winter Weather Advisory until 9 AM MST this morning for Eastern Lemhi County...Western Lemhi County.