textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Strong Cold Front today: An extreme cold front will sweep across the region this morning. Expect a rapid transition from rain to snow, and damaging winds with gusts of 40-60 mph in valleys (higher in mountains).

- Snow Thursday and Friday: 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.

- Mountain snow Saturday

Satellite imagery early this morning is showing a low pressure circulation moving into central British Columbia. This circulation is pushing a strong cold front from west to east south of the low pressure center. The front will strengthen as it moves through the Pacific Northwest and arrives in the Northern Rockies this morning. The primary impact from this feature, well-advertised over the last several days, will be strong, damaging winds. Timing for arrival will be about 5am PST in Idaho and Clearwater counties, moving east to western Montana by about 7am MST, and becoming widespread throughout the Northern Rockies including southwest Montana and Lemhi County by about 9am MST. Southerly winds early in the event will turn westerly with the frontal passage. Westerly gusts of 50 mph will be widespread. High resolution ensemble models indicate a 50 to 90 percent chance for gusts of this magnitude across most valleys in the Northern Rockies. In the higher terrain such as the Continental Divide and the Bitterroot crest, there will be about a 50 percent chance of exceeding 70 mph. A secondary threat today will be the transition from rain to snow behind the front. Snow accumulation will favor northwest Montana, especially eastern Flathead County. Marias Pass will see about 6 to 12 inches of snow by tonight. A Winter Storm Warning is in effect for the Glacier Park region due to the combination of high winds and snow, while the remainder of the Northern Rockies will be under a High Wind Warning through today.

Moving on past today, enough cold air holds on through Thursday for snow accumulation in the higher terrain by Friday morning. Moisture is still supplied from the Pacific Ocean by an upper level jet positioned over the Northern Rockies. Cool temperatures will keep snow levels low enough for valley snow through Thursday, while forcing for ascent will come from the terrain with the exception of some weak overrunning of Pacific moisture over residual cold air in the valleys. Accumulations over mountain passes such as Lolo, Lookout, Marias, and Lost Trail will be in the 5 to 10 inch range. A Winter Storm Watch is in effect for the Clearwater, Bitterroot, and Sapphire ranges for Thursday and Friday.

Snow levels rise again on Friday afternoon, then come back down to valley floors for Saturday. The mountains hang onto snow into Saturday as the Clearwaters and Bitterroots are favored for another several inches of snow by Saturday morning. The next period of interest for precipitation will be Sunday into Monday, though model agreement is still uncertain.

AVIATION

Wind will be the primary impact to aviation over the next 24 hours. A potent cold front is approaching the Northern Rockies from the west. Southerly winds early will turn westerly behind the front, arriving in north-central Idaho by about 17/1300Z and western Montana by about 17/1400Z. Expect gusts of 40 to 50 knots near the ground, 60 knots over the terrain. Low-level windshear will also develop early this morning and persist throughout today. Snow will obscure terrain, while rain will turn to snow by about midday at all terminals. Precipitation, including blowing snow, will produce reductions in visibility.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Winter Storm Watch from late tonight through late Friday night for Bitterroot/Sapphire Mountains.

High Wind Warning until 5 PM MST this afternoon for Bitterroot/Sapphire Mountains...Flathead/Mission Valleys... Kootenai/Cabinet Region...Lower Clark Fork Region... Missoula/Bitterroot Valleys.

Winter Storm Warning until 11 PM MST this evening for West Glacier Region.

High Wind Warning until 8 PM MST this evening for Butte/Blackfoot Region...Potomac/Seeley Lake Region.

ID...High Wind Warning until 2 PM MST /1 PM PST/ this afternoon for Eastern Lemhi County...Lower Hells Canyon/Salmon River Region...Northern Clearwater Mountains... Orofino/Grangeville Region...Southern Clearwater Mountains...Western Lemhi County.

Winter Storm Watch from late tonight through late Friday night for Southern Clearwater Mountains.