textproduct: Great Falls

KEY MESSAGES

- Light snow showers will continue along the Northern Rockies through Monday evening.

- More widespread snow arrives late Monday into early Tuesday across southwest Montana and becomes widespread by mid-morning.

- Periods of moderate to heavy snow and gusty winds possible in the mountains with lighter snow amounts and blowing snow a concern at lower elevations.

- Active pattern and much colder temperatures continue through at least Thursday.

DISCUSSION

- Meteorological Overview:

Monday will remain generally warm and calm across the region with light mountain lingering in the Northern Rockies with impacts expected to remain above pass level through Monday evening.

Increasing winds aloft will mix down breezy to gusty winds across Southwest MT starting on Monday. A broad, deepening trough from western British Columbia combining with a small wave from the Great Basin region will bring a more active and colder period to the region Tuesday through the week. The first, quick hitting shortwave and low pressure system begins to move in late Monday night/early Tuesday morning. This will bring a wave of minor to moderate mountain snow, with light snow at lower elevations through Tuesday evening. A mid and low level jet with this system will bring gusty winds to Southwest MT and the plains, which will bring blowing snow and visibility reduction concerns with snow Tuesday.

There will be a brief lull in snow late Tuesday evening. Though with the broad trough will making it's way through there will be additional instances of light to moderate snow Wednesday through Friday. The Tuesday system is expected to pull in colder air from Canada, and will bring near to below average temperatures through at least Wednesday and Thursday. -Wilson/thor

- Forecast Confidence & Scenarios:

Potential for Snow Squalls:

Both the GFS and the NAM are confident in potential frontogenetic snowbands forming Tuesday morning and moving from southwest to northeast across the region. That being said, there are still significant differences in the timing of when snow squalls may start to form and how they will propagate across the region. Right now the greatest risk is across southwest Montana with a concern for the I-90 corridor on Tuesday morning. Motorists should be prepared for snow squall conditions such as sudden reduced visibility and slick roads.

Winds/Blowing Snow Tuesday:

Wind gusts along the Rocky Mountain Front, downslope of the Sweetgrass Hills, and along the gap between the Little Belts and the Highwoods may see gusts up to 50 mph through the afternoon on Tuesday. Additionally, most locations across the region are likely to see gusts up to 40 mph. In combination with falling snow, periods of blowing snow are likely across mountain passes and the northern plains on Tuesday. In particular, there is a potential for near-blizzard conditions along the Hi-Line in the afternoon and evening. Right now the limiting factor east of Havre will be snowfall amounts but even if the region only picks up a couple inches, with the wind that is expected, periods of reduced visibility are likely. Because confidence in definitive blizzard conditions is low, no changes will be made to the current products. But this will be something to watch with future updates along with the additional concern for snowbands/snow squalls down in southwest Montana.

Watch/Warning Updates:

No changes were made to the products with this forecast, however, there is a potential additional Winter Weather Advisories may be needed down the road. The only thing lack in confidence is timing and whether or not impactful amounts will occur, particularly in the Little Belt and Big Belt zones.

When it comes to the mountain zones along the Continental Divide and in southwest Montana, there is too much variability in the models to decide whether or not any of the advisories should be upgraded to a warning. The most noticeable change with this update was snowfall amounts increased quite a bit along the Montana/Idaho border. But given that this is a very recent change and is not completely supported by the larger suite of models, there is not enough confidence to upgrade any zones in southwest Montana to a warning at this time. Should future model runs confirm the higher amounts then this situation will be re- evaluated accordingly. -thor

AVIATION

16/12Z TAF Period

VFR conditions generally prevail for the TAF period, though clouds will begin to lower at KEKS and KBZN by 17/12z as our next weather system approaches. While steadier southerly winds continue at KEKS, variable surface winds elsewhere also become more southerly late tonight into Monday. Widespread IFR to LIFR conditions are expected Tuesday as our next weather system pushes through the area, though that will be after 12z. - RCG/Ludwig

PRELIMINARY POINT TEMPS/POPS

GTF 49 32 57 29 / 30 10 0 20 CTB 45 23 51 20 / 10 0 0 20 HLN 51 28 53 28 / 50 20 0 20 BZN 52 27 54 31 / 20 30 0 30 WYS 40 18 40 23 / 10 10 20 90 DLN 51 26 51 31 / 10 0 0 60 HVR 45 26 53 28 / 20 10 0 0 LWT 43 28 51 30 / 40 20 0 0

TFX WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory from 2 AM to 8 PM MST Tuesday for Eastern Glacier, Western Toole, and Central Pondera-Eastern Toole and Liberty-Gates of the Mountains-Northern High Plains- Southern High Plains.

Winter Weather Advisory from 2 AM to 5 PM MST Tuesday for East Glacier Park Region-Elkhorn and Boulder Mountains-Northwest Beaverhead County-Southern Rocky Mountain Front-Upper Blackfoot and MacDonald Pass.

Winter Weather Advisory from 11 PM Monday to 5 PM MST Tuesday for Gallatin and Madison County Mountains and Centennial Mountains-Ruby Mountains and Southern Beaverhead Mountains.