textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Burst of moderate to heavy snow this afternoon and evening across Lookout, Lolo, Marias and Lost Trail passes.

- Significant Atmospheric River to bring heavy rain, leading to hydrological concerns and flood risk.

- Gusty winds Monday night into Tuesday morning across northwest Montana, with risk for power outages from downed trees.

- Heavy, wet snow across backcountry and high elevations areas above 5000-7000 feet this week.

While the main focus is the incoming atmospheric river Monday, we are tracking a quick hitting Pacific moisture surge this afternoon and evening. Mild temperatures are keeping snow levels above 4,500 feet, with winter travel concerns being focused across mountain passes, including: Lookout, Marias, Lost Trail, and Lolo Passes. Travelers should expect periods of moderate to heavy snow and slushy roadways. High-res models suggest convective snow showers will develop later this evening as the wave passes, therefore, the worst of the conditions should develop following sunset.

Atmospheric River & Hydrologic Concerns for Monday-Thursday:

Meteorological Details: The weather pattern shifts dramatically as a deep fetch of subtropical moisture, known as an Atmospheric River, surges into the Northern Rockies beginning Monday afternoon. This will be a long-duration event, with periods of heavy precipitation continuing into Thursday. Snow levels will rise Monday afternoon, with rain falling as high as 7000 feet Monday night. This presents a concerning rain on snow situation.

A Pacific cold front will pass Monday night into Tuesday morning, leading to sharp pressure rises and gusty west-southwest winds, especially across northwest Montana, where wind advisories have been issued. Wet ground conditions will lead to greater vulnerabilities for downed trees and areas of power outages. A second surge of moisture arrives Tuesday night into Thursday, with heavy precipitation focusing along north-central ID and the Montana/Idaho border.

The two main forecast challenges are 1) Duration of moisture surge and 2) Snow levels, during this second phase of the atmospheric river event. Model guidance suggests a sharp temperature gradient aloft across the Northern Rockies as this second phase occurs, leading to significant spread within snow levels. Generally, snow levels will be lower during this event, ranging from 4,000-5,000 feet across northwest Montana, 4,500-5,500 feet along I-90, and 5,500-7,000 feet across the southern Clearwater, Bitterroot Mountains, Southwest Montana and Lemhi County. It's worth mentioning, the 10-90th percentile spread between the NBM is on the order of 2000 feet for snow levels, hence there's still a lot of uncertainty within the location of colder air aloft and where snow levels will end up during this second surge. Finally, roughly 1/3 of ensemble models suggest the moisture surge will continue Thursday night into Friday morning. This is a recent trend within guidance, particularly the ECMWF and it's ensemble system. This solution would lead to higher precipitation totals and greater mountain snow as snow levels trend lower towards the end of the event.

*Hydrological Summary: Flood watches have been issued across Idaho and Clearwater Counties in Idaho and across Mineral County, in Montana, where our confidence is highest for the most significant precipitation totals through Thursday. Rises in small streams and creeks, rock and mudslides in areas of steep terrain, ponding of water and/or minor flooding in low lying and urban areas. By Thursday, significant precipitation totals are forecast, with 1 to 3 inches in the valleys and 4 to 8 inches in the mountains, with locally higher totals over mountain crests. Mountain areas in the Clearwater and Bitteroot Ranges will see the heaviest totals during this event.

*High elevation Snow: Significant wet, heavy snow will impact mountain areas, especially above 5,000-7,000 feet. The combination of high precipitation totals, moisture content, and strong winds will lead to changeable avalanche conditions. Those heading into the backcountry are encouraged to monitor avalanche forecasts at: www.avalanche.org.

AVIATION

The next surge of Pacific moisture is moving through eastern Washington and the Idaho Panhandle this afternoon as of 07/2100Z. Widespread precipitation will impact the Northern Rockies as this wave moves through, leading to mountain obscurations, with snow levels generally above 4,500 feet. Area terminals will experience temporary visibility and ceiling impacts as the wave passes, generally from the 08/0000-0400Z timeframe. A significant, long-duration atmospheric river event arrives Monday afternoon, with widespread precipitation.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Flood Watch from Monday afternoon through late Wednesday night for Lower Clark Fork Region.

Wind Advisory from 8 PM Monday to 8 AM MST Tuesday for Kootenai/Cabinet Region...Lower Clark Fork Region.

Wind Advisory from 11 PM Monday to 11 AM MST Tuesday for West Glacier Region.

Wind Advisory from 11 PM Monday to 8 AM MST Tuesday for Flathead/Mission Valleys.

ID...Flood Watch from Monday afternoon through late Wednesday night for Lower Hells Canyon/Salmon River Region...Northern Clearwater Mountains...Orofino/Grangeville Region... Southern Clearwater Mountains.