textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Significant closed low pressure system moving through the region tonight through Friday bringing widespread snow and cooler temperatures.

- High pressure returns this weekend, bringing warming and drying.

Southwest flow aloft will increase today ahead of an approaching closed low, initiating numerous showers across the Northern Rockies as snow levels rise to 6,000 feet. A cold front is projected to traverse the region Thursday morning ahead of an approaching low pressure system, causing snow levels to plummet to 3,000 feet by midday. Expect a period of moderate to heavy precipitation with the frontal passage, including the potential for isolated thunderstorms. High-resolution guidance highlights a risk for heavy snow bands or snow squalls in southwest Montana mid-morning Thursday. This will likely cause rapid reductions in visibility and hazardous travel on Interstate 90 and Highway 12 between Garrison Junction and Homestake Pass.

As the mid-level circulation shifts northward, the primary TROWAL feature and heaviest precipitation will focus along the Continental Divide in northwest Montana. Significant snowfall is increasingly likely for the Mission Mountains and the Swan region through Thursday night due to persistent wrap-around moisture. Total mountain accumulations of 12 to 24 inches are expected, with the highest totals concentrated in the Mission and Swan ranges. Mountain passes will generally see 4 to 9 inches, while valleys will range from a trace to 4 inches near the Divide. Given the heavy, wet nature of the snow, anticipate localized tree damage and power outages. Winter weather advisories have been issued for portions of the region and may need expansion as we hone in on the mid-level circulation's exact placement. In particular, portions of the Mission valley and the Missoula valley will need monitoring for potentially higher snow amounts Thursday night into Friday morning.

Conditions will improve this weekend as high pressure builds, resulting in a warming and drying trend with temperatures rising approximately 10 degrees above seasonal averages by Sunday.

AVIATION

VFR conditions will prevail through much of today, though increasing southwesterly flow will introduce scattered showers and localized terrain obscurations as snow levels rise to 6,000 feet. Conditions will deteriorate rapidly after 09/1200Z as a cold front traverses the region from west to east. Expect widespread MVFR/IFR conditions due to moderate to heavy precipitation, with snow levels dropping to 3,000 feet by midday Thursday. Of particular concern is the potential for convective snow bands or squalls in southwest Montana (near KBTM and the Divide) mid- morning Thursday, which may cause sudden LIFR visibility and significant vertical wind shear. Breezy winds will occur with and behind the front as well, with gusts of 20 to 25 kts being common. Persistent mountain obscurations and icing will continue through Thursday night, especially along the Continental Divide and the Mission/Swan ranges, as wrap-around moisture maintains low ceilings and heavy snowfall.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Winter Weather Advisory from midnight tonight to 6 AM MDT Friday for Bitterroot/Sapphire Mountains.

Winter Weather Advisory from 6 AM Thursday to noon MDT Friday for Potomac/Seeley Lake Region...West Glacier Region.

Winter Weather Advisory from 6 AM Thursday to 6 AM MDT Friday for Butte/Blackfoot Region.

ID...Winter Weather Advisory from 11 PM this evening to 5 AM PDT Friday for Southern Clearwater Mountains.