textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- A warm front will bring accumulating snow tonight through Monday morning. Expect generally one half to 3 inches in the valleys, but it is common for these types of weather systems to produce higher amounts in favored locations. A wintry mix is also possible.

- A strong ridge of high pressure will build later this week, sending temperatures soaring 15 to 20 degrees above normal by Friday.

- The rapid late-week warming will quickly melt the fresh valley snow, which may lead to urban runoff issues and ponding of water in low-lying areas and poorly drained streets.

Colder air is currently in place across the region. While some models suggest temperatures will warm up quickly, fresh snow cover means that warming will likely be reduced in most places

As a warm front pushes into the region late this evening, it will lift abundant moisture up and over our existing cold, dry airmass. As precipitation begins, evaporational cooling will quickly lock in freezing temperatures for the valleys. Combined with northwesterly flow aloft, this creates a classic setup for accumulating snow in the narrow valleys of north-central Idaho and west-central Montana, including Missoula.

Standard model guidance tends to under-forecast snow amounts in these setups, generally showing less than an inch. However, given the environment, higher amounts of 2 to 4 inches are entirely possible. Commuters should plan for winter driving conditions and extra travel time Monday morning, as roads will likely be snow- covered and slick. Additionally, as snow levels rapidly rise on Monday, pockets of freezing rain are possible, which would further complicate travel.

Further north toward Kalispell, the forecast is trickier. The deepest moisture is expected to remain further south. While the Flathead Valley will likely see some snow, amounts should be light, generally an inch or less, unless the moisture shield drifts further north than anticipated.

Behind Monday's system, a significant pattern change takes hold as a strong ridge of high pressure builds across the western US. While the Northern Rockies will sit on the northern periphery of this ridge, atmospheric heights will climb well above average, driving a major warming trend.

By Thursday and Friday, temperatures will run 15 to 20 degrees above normal, pushing afternoon highs into the upper 50s to upper 60s for many valley locations. Despite the warmth, Pacific moisture will continue to stream into northwest Montana. This will keep showers in the forecast for the entire week, primarily in the mountains, with snow levels rising remarkably high to between 7,000 and 8,000 feet.

Urban Runoff Potential: With this rapid transition from freezing temperatures and fresh snow to well- above-average warmth, the newly deposited valley snowpack will melt rapidly. Expect localized ponding of water in low-lying areas, poor drainage zones, and urban roadways from Tuesday into the weekend.

Next Weekend: The ridge axis is expected to shift eastward by Sunday, opening the door for the next Pacific trough to approach from the northwest. This will likely bring our next chance for widespread precipitation and cooler temperatures, though the exact timing remains uncertain.

AVIATION

Mid and upper-level clouds will be on the increase through the rest of today as the next weather system moves in overnight tonight. All elevations will be cold enough for light snow starting after 16/0300Z this evening. Light to moderate snow will continue through the night and into Monday morning for all aviation sites in western Montana. Mid-level warming may be just enough to where light snow will transition to a light freezing rain for the valleys along I-90, encompassing KMSO. While the confidence is low, at around a 15% probability, it is still worth mentioning. Visibility down to 1SM and low ceilings can be expected at least through 16/1500Z tomorrow morning for KMSO, KHRF and KBTM. Widespread mountain obscurations will be present most of the day Monday.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Winter Weather Advisory from 9 PM this evening to noon MDT Monday for Bitterroot/Sapphire Mountains...Lower Clark Fork Region.

Winter Weather Advisory from midnight tonight to noon MDT Monday for Missoula/Bitterroot Valleys...Potomac/Seeley Lake Region.

ID...Winter Weather Advisory from 8 PM this evening to 11 AM PDT Monday for Southern Clearwater Mountains.