textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Areas of fog and low clouds in valleys each morning through Monday

- Renewed Hydro + Wind concerns Tuesday-Wednesday

- Pass level travel impacts Wednesday as snow levels to valley floors

Through Sunday Night: High pressure ridging will dominate bringing stable conditions, light winds, mild temperatures, and partly cloudy skies. Valley fog is expected to develop during the late night and morning hours through Monday. Travelers should be alert for locally dense fog, which may cause sudden reductions in visibility.

Monday through Wednesday: An atmospheric river will introduce lower cloud cover late Sunday, with light showers possible along the Idaho/Montana border as early as Monday morning. Precipitation will become more widespread Monday afternoon as snow levels rise above 7,000 feet. By Tuesday, snow levels are expected to settle around 5,000 feet in Northwest Montana and 6,000 feet south of I-90.

A robust surge of moisture arrives Tuesday night into Wednesday morning ahead of a strong cold front.

-Precipitation Totals Monday-Wednesday: Expect 1-2 inches in the valleys of Northwest Montana and 2-4 inches in the mountains. Areas south of I-90 will likely see about half these amounts.

-Hydrologic Concerns: Although forecast precipitation totals are lower than the previous mid-week atmospheric river event, there is a continued risk for river rises, minor flooding, and debris flows/mudslides, particularly across Northwest Montana.

Strong to potentially damaging winds are also a primary concern with the frontal passage Tuesday night into Wednesday.

-Impacts: Given the saturated and unfrozen soils in many valleys, the risk for treefalls blocking roads and causing significant power outages is elevated.

-Wind Speeds: Early guidance indicates mean wind gusts of 45-55 mph in many valleys, which is sufficient to produce the impacts described above.

Behind the Wednesday morning front, snow levels will drop to valley floors. Winter travel impacts are likely at pass levels due to slick roads and reduced visibility. While valley accumulations are less certain, slick roadways may develop under heavier post-frontal showers.

Extended Outlook (Thursday into Christmas Week): Another atmospheric river will follow closely, impacting the region Thursday and Friday. Forecast confidence is lower regarding the moisture plume's trajectory (current clusters are split 50/50 between a direct hit overhead versus a track south of the Northern Rockies). Regardless of the exact track, wintry road impacts at pass levels are likely, potentially even at valley floors.

The overall active pattern persists heading into Christmas week, with atmospheric rivers impacting the West Coast and periodic arctic air banking against the Continental Divide. At the very least, this will result in periodic pass-level snow impacts and a wintry mix in the valleys.

AVIATION

High pressure ridging dominates into Sunday with VFR conditions for most of the area into this evening. Stable conditions, light wind and mostly clear skies overnight will be a decent setup for valley fog development especially from I-90 northward including KMSO and KGPI. Once any fog breaks up on Sunday morning, VFR conditions will once again be present.

HYDROLOGY

A prolonged Atmospheric River is expected to impact Western Montana and North-Central Idaho from Monday through Wednesday, bringing moderate to heavy precipitation. Snow levels are anticipated to start around 7,000 feet on Monday before falling to valley floors behind a cold front early Wednesday. The most likely rainfall scenario forecasts 1 to 2 inches for the valleys of Lincoln, Mineral, Sanders in western Montana and Clearwater, and Idaho counties in Idaho. 2 to 4 inches expected in the mountains of northwest Montana and north-central Idaho. Areas further south and east in western Montana will likely see 1 inch or less. A high- end scenario could bring totals 1 to 2 inches higher than the estimates given above.

Due to already saturated ground, particularly in northwest Montana, this rainfall increases the risk of rock and landslides, stream and river rises, and minor flooding. Continue to monitor the latest forecast updates as this system approaches.

MSO WATCHES/WARNINGS/ADVISORIES

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