textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Gusty west winds Friday afternoon will create critical fire weather conditions across southwest Montana, choppy conditions at area lakes, and hazardous crosswinds for high- profile vehicles on I-90 from Deer Lodge to Butte, and I-15 south of Butte.

- Strong northeast winds will develop Saturday, causing choppy waves that could pose a danger to docks and boats on the southwestern portion of Flathead Lake. Isolated power outages are possible across northwest Montana through Saturday night.

- The first winter storm of the season is expected to bring wet, heavy snow, cold temperatures, and gusty northeast winds to the mountains and high valleys of west-central, southwest, and central Montana late Saturday through Sunday night. This heavy snow could damage trees and power lines, and will significantly impact backcountry recreation, hunters, and travel.

It be a breezy day across the Northern Rockies. Southwest winds will increase to 15 to 25 mph, bringing a 1 to 2 foot chop to Flathead Lake. Stronger winds of 20 to 30 mph with gusts up to 45 mph are possible from Lemhi County to southwest Montana. In addition to dangerous crosswinds on north-south highways, the dry and windy conditions will create critical fire weather conditions across southwest Montana.

On Saturday, as a low pressure system moves onshore over Washington and Oregon, northeast winds will increase across much of the region. At the same time, cold, below-freezing air will push south from Alberta, Canada, further increasing the northeast winds. Flathead Lake will again experience choppy conditions, but from the northeast by Saturday afternoon. Northeast winds of 15 to 25 mph with gusts to 35 mph could pose a danger to docks and boats from Yellow Bay south and west toward the Narrows and Big Arm.

Moisture wrapping around the low pressure system over Oregon will move over the cold air pushing south across western Montana Saturday night. Snow levels will fall to the valley floors along and west of the Divide. The cold air will weaken as it reaches the Idaho/Montana border, so snow levels in central Idaho will remain higher, generally above 4000 feet. Temperatures falling into the 20s at MacDonald Pass, combined with gusty northeast winds and wet, heavy snow, will overcome the warm ground and make travel conditions slick. This wet, heavy snow may also cause impacts to power lines and trees. Other locations, including Georgetown Lake, Butte, and Homestake Pass, could also experience winter-like conditions. The highest probabilities for 3 inches of snow or greater exist in these areas.

The heaviest precipitation will be with a narrow band that is currently forecast to be over the Anaconda/Pintler Range and spreading east over Homestake and MacDonald Passes. If this band were to shift further north, areas around Seeley Lake and the Bob Marshall Wilderness could see a higher impact, which would also allow rain to change over to snow in places like Missoula. However, impacts should be minimal in the lower elevations, aside from the chill.

A Winter Storm Watch is currently valid for areas encompassing Homestake and MacDonald Passes and the Georgetown Lake area near the Anaconda/Pintler Ranges. Lower elevations, including Butte and Anaconda, will also see accumulating snow, though to a lesser extent. There are Winter Weather Advisories out for the mountains north and west of this region, including the Bitterroot and Sapphire Ranges and the Seeley-Swan region.

On Sunday, subtropical moisture from the remnants of Hurricane Rachel (which is currently situated southwest of the Baja California) is still on track to move northwards towards the Yellowstone and Wyoming areas. The ensemble weather modeling systems are still divided in how the upper low will evolve later Sunday into Sunday night. This is a key feature that decide how far north and west the moisture will advect. We continue to be on the edge of it, with lingering snow possible Sunday into Sunday night.

As the weather system pulls away, Monday may bring the first widespread hard freeze of the season. Western Montana may see lows in the upper teens to low 20s, while the lower elevations in north-central Idaho get close to freezing.

AVIATION

A dry cold front will bring gusty west to southwest winds today. Terminals GPI, MSO, and BTM may experience the strongest gusts between 25 and 35 knots. Gusty northeast winds will develop on Saturday. A winter storm will bring low ceilings, visibility and snow along the Divide and points west Saturday night through Sunday night.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Lake Wind Advisory from noon today to 9 PM MDT this evening for Flathead/Mission Valleys.

Winter Weather Advisory from 6 PM Saturday to 6 PM MDT Sunday for Bitterroot/Sapphire Mountains...Potomac/Seeley Lake Region.

Winter Storm Watch from Saturday evening through Sunday afternoon for Butte/Blackfoot Region.

ID...None.