textproduct: Missoula

DISCUSSION

After a persistently mild November, all indications point to a major and abrupt pattern change arriving for Thanksgiving week. This will likely be the first widespread, low-elevation winter impact of the season.

Residents should use the relative calm of the coming week to finalize winter preparations, especially ahead of holiday travel.

KEY MESSAGES:

- Through Wednesday: Light mountain snow with minimal impacts.

- Next Weekend (Nov 22-23): Major pattern change likely, driven by an atmospheric river, bringing heavy mountain precipitation.

- Thanksgiving Week: High confidence in a significantly colder, wintry pattern, with impacts including valley snowfall.

Through Wednesday:

A split-flow pattern continues through midweek, keeping the main storm track deflected. Two weak waves of moisture will pass through:

- Today-Monday: Light showers across central Idaho and northwest Montana. Snow levels will remain high (near 7,000 ft), with only a trace to 2 inches expected on the highest passes.

- Tuesday-Wednesday: A slightly cooler system drops snow levels to 4,500-5,000 ft. Expect minor accumulations (generally 1 inch or less), with the main hazard being localized slick spots on passes during the overnight hours. Daytime travel should remain favorable.

Weekend Pattern Change (Nov 22-23):

Confidence is increasing (60-70% probability) in a major shift as an atmospheric river (AR) targets the Pacific Northwest. This system will be supported by a strong (130+ kt) upper-level jet, creating conditions favorable for widespread rising motion and heavy precipitation.

- Heavy Mountain Precipitation: There is a high potential for heavy precipitation in the mountains. *The NBM 4.3 90th percentile (which often verifies in ARs) shows ~1.00 inch of liquid precipitation (QPF) in 24 hours (ending 5 PM Saturday). *ECMWF Ensemble 90th percentile shows 1-2 inches of liquid equivalent for the mountains along the Idaho/Montana border.

- Snow Levels (Primary Forecast Challenge): *Milder Scenario: A more northward AR track would keep snow levels high, limiting major impacts to areas above the valley floors. *Colder Scenario: A slightly southward track supported by some ensemble members would drop snow levels significantly. The NBM 4.3 10th percentile shows snow levels near 2,900 ft on Saturday across northwest Montana, raising the slight possibility of accumulating wet snow on valley floors.

Thanksgiving Week:

This period carries the highest potential for widespread, high- impact weather. While specific details remain uncertain, guidance favors a deep, cold upper-level trough developing along the West Coast rather than a quick frontal passage.

- Temperatures: Confidence is high in a shift to substantially colder conditions. *The NBM 4.3 shows a 50-60% chance of valley high temperatures staying below freezing on Tuesday, rising to 70-80% by Wednesday. *The moderate probability (30-60%) of lows in the mid-teens and lower expanding across all western Montana valleys by Wednesday morning. *Subzero lows are possible later in the week, especially along the Continental Divide.

- Snow: The large-scale (synoptic) setup strongly favors low- elevation snow. *Ensemble guidance suggests a potential surface low tracking from southern Idaho toward southwest Montana, which is a favorable pattern for significant valley snow, particularly north and west of the low path. *Scenario Uncertainty: If the trough digs farther west, the region could stay somewhat milder with less precipitation. However, a deeper trough could also draw Arctic air south sooner, potentially as early as Monday-Tuesday. *Persistent Wintry Threat: Once the cold air settles, an active northwest flow and an unstable atmosphere may support ongoing snow showers or heavier squalls. This could lead to persistently slick road conditions and sudden reductions in visibility on passes and valley roads.

AVIATION

An upper low lifting through the Great Basin will transport moisture and weak lift across the Northern Rockies through Monday. The first round of light showers will spread south to north this afternoon, followed by additional showers tonight into Monday. The most persistent showers and periodic reductions to MVFR/IFR ceilings and visibility are expected across north- central Idaho and northwest Montana, particularly tonight and Monday morning.

Elsewhere, VFR conditions will largely prevail. Mountain obscurations will be possible at times. Surface winds will remain relatively light across the region through Monday. Patchy valley fog is also a concern for Monday morning where partial clearing occurs tonight.

MSO WATCHES/WARNINGS/ADVISORIES

MT...None. ID...None.