textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Showers and thunderstorms through this evening.

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

- Significant hypothermia risk across backcountry and high elevation areas with snow and cold temperatures above 6,000 feet.

Atmospheric instability has increased with surface heating across western Montana this afternoon, with recent mesoscale analysis highlighting 500-1,000 J/kg of instability. Radar and GOES Satellite images show scattered showers and isolated thunderstorm activity early this afternoon. As we move into the evening, thunderstorm coverage will expand, generally focusing along and north of I-90 and along the divide. The strongest thunderstorms will develop north of I-90, in northwest Montana, where deeper instability supports small hail (up to 0.50 in), heavy rain, and outflow winds up to 40 mph.

A cold front remains on track to sweep across the area Friday morning, bringing a drastic change in the weather through the weekend. Widespread rain showers will accompany the frontal passage, with rain focusing across north-central Idaho during the morning, before spreading through western Montana by the afternoon. High-resolution models suggest thunderstorms will develop across central Idaho along the frontal boundary, before spreading across Lemhi County and southwest Montana during the afternoon and evening. Snow levels will fall through the weekend as a closed low slowly moves in south-central Idaho and cold air aloft moves overhead. Snow will focus above 6,500 feet across Lemhi County, ID mountain ranges, the Bitteroot, Sapphire, Anaconda-Pintler, Mountains and along Continental Divide Saturday evening into Monday morning. Winter Storm Watches have been issued for these areas, given cold temperatures and wet heavy snow bringing an elevated risk for hypothermia and difficult travel across backcountry areas. 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 70-90 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, with the heaviest rain focusing across Glacier Park and along the divide. Widespread rain will continue across northwest Montana for much of the day Monday, before the low departs eastward on Tuesday. Model guidance has come into better agreement on this lows track, with precipitation amounts trending higher. Median (most-likely) precipitation amounts of 0.50-1.00 inches are forecast across northwest Montana near the Idaho border, with amounts increasing to 1.00-2.00 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

Scattered thunderstorms will move through western Montana through this afternoon. High resolution guidance depict activity waning by 26/0000Z, but there is a chance that there could be a few stragglers towards the Divide/KBTM around that time-frame. Lightning, gusty winds, small hail and brief heavy rain will be the impacts with these storms. Thereafter, a large upper level trough will move over the Pacific Northwest later Friday and as it does this, showers will be on the uptick along with gusty westerly winds with a frontal passage. The best instability to bring a chance for thunderstorms on Friday exists from southern Lemhi County into southwest Montana towards KBTM. If KSMN is able to be in the warm sector a little longer, then southwesterly winds could be a little stronger, gusting to 20-25 knots by Friday afternoon. Other terminals will see showers at times with gusty winds. This system will bring lower ceilings and widespread precipitation over the weekend.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Winter Storm Watch from Saturday evening through Monday morning for Bitterroot/Sapphire Mountains...Butte/Blackfoot Region.

ID...Winter Storm Watch from Saturday evening through Monday morning for Eastern Lemhi County...Western Lemhi County.