textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Current: Freezing fog and low stratus continue to create slick spots and reduced visibility in valleys of Western Montana.

- Advisories have been issued for tonight through Friday morning commute. Snow likely at onset, though impacts will be mostly related to mixed precipitation types and black ice.

- A lull for most on Saturday, followed by another round of mixed precip into Sunday.

Freezing fog and stratus continue to plague Western Montana valleys. High pressure aloft is reinforcing valley cold pools.

Winter Weather Advisories go into effect later this evening. While some snow accumulation is expected, the decision was driven by the higher-impact potential for mixed precipitation and formation of black ice. Expect travel issues for the Friday morning commute. Please slow down and allow for extra travel time.

A significant dry layer at lower levels (dewpoints well-below freezing) will promote widespread evaporational cooling at onset, leading to an initial period of snowfall. Don't let this catch you off guard, as warm air advection begins to try overriding the valley cold pools, a transition to mixed precipitation is likely.

Valley surfaces temperatures have mostly been in the teens and 20s for the last couple days leading to much colder subsurface and road temperatures. Any transition to liquid (or even wet snow) will freeze on contact, or with compaction/frictional heating by tires, creating dangerous conditions for widespread black ice formation.

The deeper cold pools that are much more resistant to scouring by west-southwesterly flow, as in this case, may keep all or most precipitation as snow and overproduce on snow/ice amounts in relation to model guidance (Seeley Lake, HWY200 east of Bonner, Lemhi County, & US-12 Lochsa/Selway River corridor).

In the Flathead and Mission valleys, there may end up being a split (with the lake as the dividing line). Higher confidence in a mixed p-type in the Mission Valley with the northern Flathead potentially able to keep (all or mostly) snow as a dominant p-type.

A relative lull in active weather arrives for the end of Friday into Saturday, though residual moisture and cold valley floors will likely support yet another round of fog and stratus. This appears to be the least problematic travel period of the weekend.

Precipitation returns for Sunday. We are closely monitoring another bout of moisture arriving out of the southwest and mixed precipitation types with potential for slick travel.

In the extended, ensemble clusters favor (70%+) a return to broad troughing across the Northwest and Northern Rockies later next week. While intensity and further details such as temperatures (any arctic influence?) are still TBD, a return to active, cool weather is far more likely than a ridging scenario.

AVIATION

A weak shortwave approaching from the southwest will transport moisture across the Northern Rockies, resulting in gradually lowering ceilings through tonight. Precipitation is expected to develop over Lemhi County and southern Idaho County by mid afternoon, spreading northward through the evening.

Due to lingering valley inversions and initially low dewpoints, precipitation may begin as light snow in many valleys before transitioning to a light wintry mix overnight as the column saturates. While overall liquid equivalents remain low for terminal sites, there is a potential for black ice formation due to the wintry mix, especially at KMSO and KHRF. KGPI will likely stay all snow into Friday morning. Widespread MVFR and IFR conditions are anticipated at terminal sites during periods of precipitation.

Precipitation will diminish across western Montana valleys Friday morning; however, terrain obscurations and light precipitation will likely persist over higher elevations of western Montana and all areas of north-central Idaho. Areas of fog and low stratus will be a concern Friday night as partial clearing occurs.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Winter Weather Advisory from 5 PM this afternoon to noon MST Friday for Bitterroot/Sapphire Mountains...Lower Clark Fork Region.

Winter Weather Advisory from 5 PM this afternoon to 2 PM MST Friday for Kootenai/Cabinet Region.

Winter Weather Advisory from 7 PM this evening to noon MST Friday for Flathead/Mission Valleys...Missoula/Bitterroot Valleys...Potomac/Seeley Lake Region.

ID...Winter Weather Advisory from 4 PM this afternoon to 11 AM PST Friday for Southern Clearwater Mountains.

Winter Weather Advisory until noon MST Friday for Eastern Lemhi County...Western Lemhi County.