textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Atmospheric River Thursday into Friday, bringing wind impacts and heavy mountain snow.

- Winds return area wide by this afternoon, with gusts increasing to 30-50 mph overnight into Friday. Higher gusts along the divide and in the mountains.

- NEW: Winter Storm Warning for Lolo and Lost Trail Passes Thursday-Friday PM.

Thursday through Friday: GOES satellite imagery early this morning shows the next Atmospheric River (AR) advancing into the Pacific Northwest. High-resolution guidance remains in excellent agreement regarding a tightening pressure gradient by this afternoon as the AR plume shifts inland.

*Wind: The initial wind threat begins early this afternoon for North-Central Idaho, specifically the Grangeville area. There is a high (75%) probability for gusts exceeding 50 mph on the Camas Prairie and along US-95, with a 25% chance of localized gusts reaching 60 mph. By tonight, the 700mb jet strengthens to 50-70 kts and surface pressure gradient tightens as a low deepens in southern Alberta, increasing the wind threat area wide (gusts 30-50 mph). Southwest Montana, the Mission Valley, and the US-93/I-90 corridors remain the primary areas of concern as a cold front tracks southeastward overnight into Friday. Probability of gusts >50 mph is currently >50% for these locations. Given the antecedent rainfall and saturated soils, the risk for downed trees and power outages is elevated. Wind advisories or high wind warnings will likely be needed in subsequent packages.

*Precipitation/Mountain Snow: Integrated Vapor Transport (IVT) values are forecast to peak between 400-500 kg/m/s Thursday night into Friday morning, values that rank in the 99.5+ percentile for late December. This moisture surge will focus heavy precipitation over the Clearwater and Bitterroot Mountains. Strong Warm Air Advection will initially drive snow levels up to 5,000-6,000+ ft overnight, resulting in a transition to rain for many valleys. However, the mountain passes (specifically Lolo and Lost Trail) will see intense snowfall rates. A Winter Storm Warning has been issued to account for the combination of heavy snow and blowing snow as the cold front passes.

*Cold Front: By Friday morning, a sharp cold front will sweep through, rapidly dropping snow levels back toward valley floors. This frontal passage may produce a "quick burst" of heavy snow and a sudden west-northwest wind shift, particularly impacting the Continental Divide and Lemhi County. Sharp pressure rises ( 5-7mb/3hrs) along the front will bring gusty winds to valley areas, with isolated gusts of 40-50+ mph.

Saturday and Beyond: The main AR plume shifts south and east by Friday evening. A cooler, unsettled westerly flow will maintain mountain snow showers through the weekend, primarily along the ID/MT border. Looking into early next week, ensemble guidance (80%+) suggests another AR event targeting the West Coast. While the exact trajectory remains under evaluation, current trends point toward a continued active southwest flow pattern for the Northern Rockies, keeping temperatures relatively mild for valleys but maintaining high-elevation snow accumulations.

AVIATION

An intense atmospheric river (AR) will impact the Northern Rockies over the next 36 hours, characterized by significant moisture flux, strengthening low to mid-level jets, and a passing cold front Friday morning.

*Wind/Turbulence: A tightening pressure gradient by afternoon will result in widespread surface wind concerns by this evening into Friday. South-southwest winds will gust 15-25 kts across most valley terminals (KMSO, KGPI, KBTM) starting after 18/2100Z. Higher concern exists for Camas Prairie (near Grangeville), where gusts may exceed 450-50 kts. Expect Mechanical Turbulence and Low-Level Wind Shear (LLWS) as a 50+ kt 700mb jet moves overhead tonight. The risk for peak gusts >40 kts increases Friday morning for the I-90/US-93 corridors in SW Montana (KHRF, KBTM, KSMN) ahead of and along the frontal passage.

*Ceilings/Visibility: VFR conditions this afternoon will deteriorate to MVFR this evening and overnight as the AR plume arrives. Primary impacts will be obscured terrain and heavy precipitation over the mountains.

*Icing: icing is a concern within the AR plume tonight, especially between 5,000 ft and 12,000 ft MSL.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Winter Weather Advisory from 1 PM this afternoon to 11 AM MST Saturday for Lower Clark Fork Region...West Glacier Region.

Winter Storm Warning from 5 PM this afternoon to 5 PM MST Friday for Bitterroot/Sapphire Mountains.

ID...Winter Weather Advisory from noon today to 10 AM PST Saturday for Southern Clearwater Mountains.