textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Clearing skies and sub-freezing temperatures tonight will create a risk for black ice on roads that remain wet or slushy from today's scattered showers.

- A Pacific moisture plume will bring 4 to 8 inches of snow to mountain passes Sunday into Monday, while valleys transition from morning snow to mixed precipitation Sunday afternoon.

- A passing cold front may bring slushy accumulations to some valleys Monday morning A weak disturbance combined with afternoon heating will trigger scattered showers today, primarily north of I-90. While valley accumulations will be negligible, brief bursts of snow over mountain passes may cause rapid visibility drops and slick surfaces. Clearing skies tonight will allow temperatures to fall well below freezing; expect black ice on any surfaces remaining wet or slushy from today's activity.

A shifting moisture plume (remnants of a California atmospheric river) arrives Sunday morning as southwest flow increases ahead of a deepening West Coast trough. Precipitation begins in Lemhi County/southwest Montana early Sunday, reaching Missoula by mid- morning and northwest Montana by late afternoon.

Key Impacts & Uncertainties:

-Snow Levels: A warm front will lift snow levels to 4,000 to 6,000 feet through Sunday. While most valleys start as snow, a transition to rain or a rain/snow mix is expected from south to north. Confidence is low regarding the exact timing of this transition; if cold air lingers, snow may persist in valleys into the early afternoon.

-Travel: Sunday daytime valley impacts remain low due to solar radiation and marginal temperatures. However, wet roads may refreeze Sunday night. A cold front Monday morning will drop snow levels again, potentially bringing a quick slushy accumulation to valley floors.

-Mountains: Moderate snow (4 to 8") is expected for Lolo, Lookout, Lost Trail, Marias, and Williams Creek passes through Monday. Frontal passage Monday will also bring west-southwest gusts of 20 to 35 mph.

The pattern remains active and relatively mild Tuesday through the rest of the holiday week. While no major winter storms are currently forecast, recurring mountain snow and nightly valley refreeze (black ice) will maintain minor travel impacts.

Note on Christmas Eve: Ensemble guidance remains split on a potential Arctic front crossing the Continental Divide into northwest Montana. Only around 20% of models depict this scenario; however, if it materializes, it would significantly increase snow and wind impacts. We will continue to monitor this low-probability, higher- impact scenario.

AVIATION

Showers continue along the ID/MT border and through northwest Montana, obscuring terrain and causing periodic lowered ceilings and reduced visibility. Convective snow showers will increase with a shortwave trough this afternoon into the evening hours. Continued terrain obscurations and brief visibility and ceiling reductions will linger into the evening. Southwest wind gusts of 15-25 kts also return to most valleys by late this morning.

MSO WATCHES/WARNINGS/ADVISORIES

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