textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Renewed Flooding Threat: Another atmospheric river returns today through Wednesday. Rain falling on saturated soils will likely cause rapid rises on streams and creeks, with rockslides possible in steep terrain.

- High winds tonight in northwest Montana

- Strong Cold Front Wednesday: There will be a sharp transition from rain to snow on Wednesday. This brings a threat of damaging winds and bursts of snow or snow squalls.

An atmospheric river is set to impact the Northern Rockies starting this morning. This system is again warmer and wetter than typical December storms, driving snow levels above 7000-8000 feet by this afternoon. Light precipitation begins in northwest Montana this morning, spreading south and intensifying by tonight. Precipitation will be in the form of rain for all but the highest terrain. Strong winds will accompany precipitation tonight as an upper level jet streak moves into the northwest U.S. Expect widespread gusts to about 40 mph for northwest Montana and the Camas Prairie of Idaho.

Precipitation will continue on Tuesday. Snow levels will creep down ahead of a strong low pressure system approaching from the west. Valley locations should still expect rain.

The big change comes on Wednesday. A potent Pacific cold front will move through the Northern Rockies. Timing for the frontal passage is looking to be in the morning. Temperatures will plunge along and behind the front, causing rain to change rapidly to snow and wet surfaces to freeze. Winds will shift to the west- northwest and could gust 45-60 mph in the valleys and even higher in the mountains, posing a risk of downed trees in saturated soil. Snow accumulations over mountain passes and bursts of snow or snow squalls will cause periods of travel impacts. If you have travel plans for Wednesday, please continue to check the forecast and road reports before departing.

Ensemble solutions are in good agreement to keep low pressure over the Pacific north of the border, and high pressure further south off the coast of California. The upper level jet will stay concentrated over the the Pacific Northwest on Thursday and Friday. This configuration will maintain the Pacific moisture feed. Temperatures will remain cooler behind Wednesday's cold front, so snow levels should stay at valley floors for Thursday, then creep back up on Friday.

AVIATION

Precipitation will move into the Northern Rockies this morning. Expect lowered ceilings and widespread precipitation by midday. Some showers will result in decreased visibility. High winds will impact northwest Montana tonight. Southwesterly gusts to 35 knots will be likely at KGPI (about an 80% chance), and there will be about a 20% chance for 45 knots. Low-level wind shear will also be present this afternoon through the nighttime hours.

HYDROLOGY

Saturated soils across Lincoln, Mineral, Sanders, and Flathead counties will result in efficient hydrologic response from incoming precipitation. High resolution ensemble guidance mean 24-hour rainfall totals are at about 1.5 inches by Tuesday morning in the higher terrain of the Cabinets. High end rainfall amounts (90th percentile) of about 3 to 4 inches would be capable of putting small streams over their banks tonight. Saturated soils will also be prone to rock and mudslides across northwest Montana.

MSO WATCHES/WARNINGS/ADVISORIES

MT...High Wind Watch from late Tuesday night through Wednesday afternoon for Bitterroot/Sapphire Mountains... Butte/Blackfoot Region...Flathead/Mission Valleys... Kootenai/Cabinet Region...Lower Clark Fork Region... Missoula/Bitterroot Valleys...Potomac/Seeley Lake Region.

Wind Advisory from 5 PM this afternoon to 3 AM MST Tuesday for Kootenai/Cabinet Region...Lower Clark Fork Region.

Wind Advisory from 7 PM this evening to 5 AM MST Tuesday for Flathead/Mission Valleys...Potomac/Seeley Lake Region... West Glacier Region.

Winter Storm Watch from Tuesday evening through Wednesday evening for West Glacier Region.

ID...High Wind Watch from Tuesday evening through Wednesday afternoon for Lower Hells Canyon/Salmon River Region...Northern Clearwater Mountains...Orofino/Grangeville Region... Southern Clearwater Mountains.

High Wind Watch from late Tuesday night through Wednesday afternoon for Eastern Lemhi County...Western Lemhi County.