textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Areas of fog and low clouds in valleys this morning and again on Monday

- Renewed flooding and wind concerns Tuesday-Wednesday

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

High pressure will persist today over the Northern Rockies. Expect dry conditions, mild temperatures, and light winds today and tonight. Valley fog has already formed in the Flathead Valley early this morning, and will be in the forecast again for Monday. Upper level flow will flatten out and become westerly this afternoon. A strong low pressure circulation off the coast of British Columbia will start to push moisture onshore in the form of an atmospheric river. That Pacific moisture will reach the Northern Rockies, northwest Montana in particular, on Monday morning, kicking off a week of wet and potentially impactful weather.

Precipitation will become more widespread Monday afternoon as snow levels rise above 7,000 feet. Winds will pick up on Monday night as an upper level jet streak moves overhead. 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 continues this morning. Best chances for fog will be at KGPI, where decreased visibility and lowered ceilings are already present early this morning. Cloud cover will persist over northwest Montana, while aviators should expect decreasing clouds along and south of about Interstate 90. Cloud cover increases again tonight ahead of more unsettled weather overnight and into Monday morning.

HYDROLOGY

A prolonged atmospheric river event will impact Western Montana and North-Central Idaho from Monday through Wednesday, bringing moderate to heavy precipitation. Snow levels will 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, and Sanders counties 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.