textproduct: Missoula

DISCUSSION

- Atmospheric River Impacts: An atmospheric river will bring widespread rain and high-elevation snow through today. Saturated soils will likely cause rises on streams and rockslides in steep terrain, particularly in northwest Montana and north-central Idaho.

- Strong Cold Front Wednesday: A sharp cold front will sweep across the region on Wednesday. Expect a rapid transition from rain to snow, and damaging winds with gusts of 50-70 mph in valleys (higher in mountains).

- Winter Conditions Return: Following the front, cooler and unsettled weather will prevail for the remainder of the week, with snow levels dropping to valley floors at times and periods of mountain snow affecting travel.

Low pressure in the northeastern Pacific and high pressure off the coast of California are working together to send a plume of moisture into the northwestern U.S. tonight in the form of an atmospheric river. Initially, warm air will be associated with this system, so precipitation will remain in the form of rain below about 4000-5000 ft in northwest Montana and 5000-6000 ft in southwest Montana and north-central Idaho through tonight. Liquid precipitation totals (24 hr ending Wednesday AM) for hard-hit Lincoln County will be about another one half to one inch in the valleys and one to two and a half inches in the higher terrain of the Cabinets. This rainfall on top of existing saturated soils has elevated the risk for rockslides, mudslides, and rises on creeks and small streams.

A shortwave trough will bring an intense Pacific cold front through the Northern Rockies early Wednesday morning. The most notable impact from this front will be the winds. Widespread damaging westerly winds will gust 50 to 60 mph even in valley locations. These winds will create a risk of downed trees (made easier by sodden soil), power outages, and damage to infrastructure. The front will also crash temperatures, changing rain to snow and freezing wet surfaces. The transition from rain to snow will be abrupt. Bursts of heavy snow or snow squalls could lead to near-zero visibility and difficult travel conditions immediately following the frontal passage in the morning. Higher valleys, mountain passes, and locations along the Continental Divide have the highest risk for these heavy bursts of snow and snow squalls. A Winter Storm Warning will be in effect for the Glacier Park region tonight into Wednesday night.

Unsettled weather will continue behind Wednesdays front. Temperatures on Thursday will stay low enough to keep snow levels at the valley floor until rising again later in the afternoon. Friday morning will see snow accumulation on mountain passes. Lookout, Marias, and Lost Trail passes could see about five to ten inches of snow by Friday morning. Snow levels dont stay low enough for a long enough duration to have much snow in the forecast for the valley floors. A continued unsettled weather pattern with light to moderate precipitation accumulation and cooler, more seasonable, temperatures is expected through the weekend.

AVIATION

High winds this morning will taper by about midday before picking up again this afternoon. Expect gusts of about 20-30 knots this afternoon out of the southwest. Wind direction will shift to southerly ahead of a potent cold front arriving early Wednesday morning. Low level windshear will continue across the Northern Rockies airspace. In addition to wind impacts, precipitation will remain in the form of rain today, resulting in minor reductions in visibility and obscured terrain.

The cold front arriving overnight tonight through Wednesday will present significant wind impacts across the Northern Rockies, as well as heavy snow in northwest Montana. South winds transition to west winds with the frontal passage. High resolution guidance is showing 70 knot gusts out of the west over much of the Northern Rockies airspace by about 17/1300Z.

MSO WATCHES/WARNINGS/ADVISORIES

MT... High Wind Watch from late tonight 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 until 5 AM MST early this morning for Flathead/Mission Valleys...Potomac/Seeley Lake Region... West Glacier Region.

Winter Storm Warning from 8 PM this evening to 11 PM MST Wednesday for West Glacier Region.

Wind Advisory until 8 AM MST this morning for Missoula/Bitterroot Valleys.

ID...High Wind Watch from this 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 tonight through Wednesday afternoon for Eastern Lemhi County...Western Lemhi County.