textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Atmospheric River to bring widespread precipitation, high elevation snow, and gusty winds Tuesday into Wednesday

- Mountain snow showers Wednesday night into Thursday, with minor accumulations.

- Warming trend Friday into the weekend, with above normal temperatures.

A ridge of high pressure will move out of the region later tonight into Tuesday, shifting the upper-level flow to the southwest. A quick shot of subtropical moisture (a short-lived atmospheric river) will move into the region Tuesday afternoon and overnight, primarily targeting north-central Idaho and northwest Montana. This system will bring 0.5 to 1.5 inches of precipitation to the higher terrain along the Montana/Idaho border, particularly in northwest Montana.

A cold front will push through the area Tuesday night into Wednesday. As this front moves in, snow levels will drop to around 3,000 feet across northwest Montana. Further south, ahead of the main cooling, snow levels will steadily rise to around 7,000 feet across Lemhi County and southwest Montana by Wednesday afternoon. Snow levels will then fall region-wide overnight Wednesday into Thursday morning, meaning any lingering showers during this period will transition to mainly snow. Mountain passes, especially Lookout and Marias Passes, could see a few inches of new snow throughout the event, though travel impacts are expected to be minimal.

The biggest concern with this mid-week system will be the winds on Wednesday. A 40 to 60 knot low-level jet will set up over the Northern Rockies, bringing a broad area of breezy to windy conditions. The strongest winds are expected across Lemhi County into southwest Montana on Wednesday afternoon. While there is a 25 to 50% chance for valleys of western Montana may see wind gusts of 30 to 40 mph, the higher terrain along the Continental Divide could experience gusts up to 60 mph. Lemhi County and the Butte/Anaconda area currently have a 40% to 60% chance of seeing gusts in excess of 40 mph.

Additionally, parts of southwest Montana will become very dry Wednesday afternoon, with relative humidity values dropping into the teens. The combination of low humidity and strong winds will lead to critical fire weather conditions, prompting the issuance of a Fire Weather Watch.

Following the frontal passage on Wednesday, temperatures will dip below normal for this time of year on Thursday, accompanied by a few lingering showers. A ridge of high pressure will then rebound over the region by the end of the week and into the weekend, initiating a warming and drying trend. Temperatures will climb back above normal by Saturday.

AVIATION

A low amplitude ridge will keep VFR flying conditions over the region through today. Areas of high clouds will continue to stream overhead. Westerly breezes will develop by early afternoon, with periodic gusts of 10-15kts for area terminals. Cloud ceilings will gradually lower with time into Tuesday as a frontal system nears the Northern Rockies. An atmospheric river is expected to bring precipitation across northwest Montana starting by 1800Z Tuesday (12 PM MDT) with mountain obscuration likely by the afternoon and evening hours. This area of precipitation will shift southwards across north-central Idaho and western Montana by Tuesday night. Also mid-level westerly winds will range between 50 and 60+ knots Tuesday evening through Wednesday, so mountain wave activity is expected over and near the Continental Divide. On Tuesday afternoon many terminals may see breezy southwest wind gusts generally under 25 knots, except the more stable conditions from the rain in northwest Montana(KGPI) will limit much wind.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Fire Weather Watch from Wednesday afternoon through Wednesday evening for East Beaverhead.

ID...None.