textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Dramatic shift to colder and wet weather today into the weekend, as temperatures fall 20-30 degrees.

- Significant hypothermia risk across backcountry and high elevation areas with snow and cold temperatures above 6,500 feet.

A cold front has tracked eastward across north-central Idaho and western Montana, with the boundary currently stalled along the Continental Divide in Lemhi County and southwest Montana. Radar and surface observations show widespread rain showers across the Northern Rockies early this afternoon. The highest rain totals thus far have been observed across the Clearwater Mountains, with 0.15-0.25 inches of rain so far. Rain showers will focus along and south of I-90 this evening.

Widespread cloud cover has inhibited instability and convection so far this afternoon across Lemhi County and southwest Montana, where high-resolution models suggest the highest probability (15-25 percent chance) for thunderstorms. Instability will increase by mid-late afternoon with further surface heating, reaching 100-250 J/kg. A few stronger thunderstorms may develop, with wind (peak gusts near 40 mph) being the primary concern given strong forward storm motion of 40-50kts. This threat will be isolated, perhaps limited to a storm or two given the lack of stronger instability.

Heading into tomorrow, a closed low will stall across south- central Idaho, with snow levels will falling as cold air moves overhead aloft. Model guidance suggests a broad mesoscale rain band will form on the north-northwest area of the low, with snow levels dropping near 6,000-6,5000 feet across Lemhi County, ID mountain ranges, the Bitteroot, Sapphire, Anaconda- Pintler, Mountains and along Continental Divide Saturday evening into Monday morning. Winter Storm Warnings have been issued for these areas, given cold temperatures and wet heavy snow bringing an hypothermia concerns and difficult travel across backcountry areas. Furthermore, heavy, wet snow may lead to downed trees, potentially blocking access to backcountry roads and travel.

Lower elevations will experience widespread rain Saturday into Sunday morning as the upper level low spins along and south of the I-90 corridor. Locations south of a line from Missoula to Helena will see the highest rain amounts during this period, having a 60-80 percent chance of picking up 1 inch or more of rain by Sunday morning. Those planning outdoor recreation on any small streams and rivers, should be prepared for rapidly changing flows and much cooler temperatures.

Looking ahead to Sunday, the closed low will pick up speed, tracking through western Montana, before settling east of the divide through Monday morning. As the low tracks along and east of the divide, precipitation shift northward in Northwest Montana. The heaviest rain will focus along the Continental Divide, in Glacier Park, and along the west slopes of the Swan and Mission Mountains. Widespread rain will continue across northwest Montana for much of the day Monday, before the low departs eastward on Tuesday. Rain amounts have continued to trend higher for these areas, with 24-hour (ending 6pm Monday) median (most-likely) precipitation amounts of 0.50-1.00 inches forecast across northwest Montana near the Idaho border, with amounts increasing to 1.25-2.25 inches along the US-93 corridor from Eureka, through the Flathead and Mission Valleys. The highest totals are expected across Glacier Park, and the Mission and Swan Mountains, where 2.00-3.00 inches could fall. Those heading into Glacier Park Sunday into Monday should prepare for cold and raw conditions, especially at Logan Pass and the higher elevations.

Global ensemble guidance suggests unsettled weather will continue Tuesday into Wednesday as Pacific disturbances move across the Pacific Northwest into the Northern Rockies. These systems will keep showers and thunderstorms in the forecast.

AVIATION

An upper level low will bring showers and lower ceilings to the region through Monday. For Today, there will be periods of showers with intermittent lower ceilings. There may be enough instability for embedded thunderstorms from south of Grangeville east towards southwest Montana and south towards Lemhi County. These storms will be weak, but could produce gusty winds to 40 knots, lightning and in-flight graupel. Steadier rainfall will develop on Saturday and there could be patchy fog in the valleys, though westerly breezes could keep that threat down. Low stratus could form by sunrise Saturday in places that saw the moisture, but would improve by 27/1800Z(noon). Mountain obscuration will increase later today through Saturday as snow levels begin to fall towards 7000 feet from central Idaho into western Montana.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Winter Storm Warning from 6 PM Saturday to 9 AM MDT Monday for Bitterroot/Sapphire Mountains...Butte/Blackfoot Region.

ID...Winter Storm Warning from 6 PM Saturday to 9 AM MDT Monday for Eastern Lemhi County...Western Lemhi County.