textproduct: Great Falls

This forecast discussion was created in the public domain by the National Weather Service. It can be found in its original form here.

SYNOPSIS

An active pattern will continue across the region to begin the work week, with strong winds and accumulating snow expected.

DISCUSSION

Key Points:

- Generally quiet conditions for the rest of today with light rain/snow showers possible

- Periods of strong winds in combination with heavy snow will create blizzard conditions Monday and Tuesday along the Rockies and Marias Pass

- Significant snow expected along the mountains of central and north-central Montana Monday afternoon through Tuesday night

- The first accumulating snowfall of the season for most lower elevations of central and north-central Montana is likely Tuesday morning through Tuesday night

Impacts by region:

Rocky Mountain Timing and Impacts:

- Strong winds and heavy snow will combine to create blizzard conditions along the higher elevations of the Rockies including Marias Pass

- Timing of the greatest impacts are between mid-afternoon Monday through Tuesday afternoon

- Sharp cut-off of the heaviest snow amounts expected for locations below 5000 feet

Changes to Headlines

- Blizzard warning for the Rockies followed by a Winter Weather Advisory

- Winter Weather Advisory for the Rocky Mountain Front

When looking at the combined timing for the strongest wind gusts along with expected snowfall rates, there are major concerns for blizzard conditions developing along the Rockies. Winds up to 80 mph are possible with at least 4 inches of snow expected along Marias Pass. This has the potential to create a very dangerous situation along Hwy 2 with travel becoming difficult to nearly impossible Monday afternoon through Tuesday morning. When looking at the previously issued winter storm watch, the drop off of snow amounts heading into the lower elevations is such that while far western portions of the Rocky Mountain Front zones may experience some reduced visibility, the snow totals are not there to warrant either a blizzard warning or a stand alone winter storm warning. Thus a winter weather advisory was issued through the event for the Rocky Mountain Front to account for the expected lesser impacts and lower elevations.

The timing of the impacts for the Rockies start with the wind, followed approximately 6 hours later by the snow and then the wind tapering off with snow continuing for another 6 to 12 hours. Because the main threat is really the wind combined with the snow, once the wind starts to taper off a bit late Tuesday morning into Tuesday afternoon, the concerns for impacts due to snow start to taper off as well. An additional 2 to 4 inches is possible after blizzard conditions subside, however, these amounts are expected to occur over a total of 6 hours with an hourly rate of half an inch of snow at best during the most active times. As a result, a winter weather advisory was added to the end of the blizzard warning to account for addition snow.

Regarding the high wind warning for the lower elevations of the Rocky Mountain Front into Cut Bank, there is no need to combine a wind and snow threat at this time so that warning will be allowed to exist in its current form with winter weather advisories spanning the same area.

Mountains of Central and North-Central Montana Timing and Impacts (including the Bears Paw, Little Belts, Big Belts, Snowies, and Judith Mountains):

- Snow will start late Monday night but the main impacts are expected Tuesday morning through Wednesday morning

- Significant impacts are expected along the Little Belts with Kings Hill Pass a major concern for impacts during this event

Changes to Headlines

- Upgrade Little Belts to a Winter Storm Warning

- Added Cascade County to the watch to account for expected amounts and impacts to the southeastern portion of the county including Hwy 87 corridor

- Add a Winter Weather Advisory to the Big Belt Mountains and lower elevations of Meagher county

Model guidance continues to maintain a similar threat along the mountains of central and north-central Montana. The most significant and consistent threat for heavy snow is the Little Belts with high confidence persisting in moderate to major impacts. Because this zone has maintained such high confidence in the impacts and snow rates/totals, the decision was made to upgrade this zone to a winter storm warning.

When looking at the portion of the winter storm watch including the Bears Paw Mountains as well as Fergus and Judith Basin counties, there is still a potential with some upsloping to get the necessary amounts for warning criteria. The main uncertainty with these zones is still the timing of the heaviest snow and how strong the upsloping will be. There is enough confidence to keep the watch but with all things considered in the remaining uncertainty the decision was made to keep these zones at a watch for now and wait for another round of updated models to confirm or deny whether these zones need to be upgraded to a warning or kept at advisory level. Regarding Cascade county, although Great Falls proper is expected to stay out of the highest snow amounts, updated model guidance keeps pushing the higher snow amounts into southeastern Cascade county posing a threat to the Hwy 87 corridor. Given the expected impacts of snow and wind through this area, the Cascade county zone was added onto the winter storm watch.

A bit of a wild card for this event is the Big Belts. Probabilistic guidance has the Big Belts ranging from as low as 40% to as high as 90% for 6 inches or more and as low as 20% to as high as 80% for 8 inches or more. But looking at the spatial aspect of the highest probabilities, it is quite small and really only includes the highest elevations of the range. So based on that alone, a winter storm watch or warning does not fit the situation. But what makes this such a wild card is the zone including the Big Belts also includes Bozeman Pass. And Bozeman Pass stands to have minor to moderate impacts with around a 50% chance of 4 inches or more and a 20% chance of 6 inches or more.

When balancing the less-than-ideal probabilities of meeting warning criteria along the Big Belts with the potential public impacts of Bozeman Pass, for right now the best fit decision is still to choose an advisory over a warning. All that being said, this will be an area to watch with future updates as adjustments in future model data may point to the need for an upgrade.

Southwestern Montana Timing and Impacts:

- Snow will start Monday evening and taper off Wednesday morning

- Two rounds of heavier snow are expected with the first round Monday night followed by a brief lull with lighter snow and a second push of heavier snow Tuesday afternoon

- Impactful snow along the higher elevations of the Madison and Gallatin ranges are expected as well as the mountains surrounding West Yellowstone

- Most significant road impacts are expected along the Bozeman, Raynolds, and Targhee passes as well as US 191 from Bozeman to West Yellowstone

The greatest potential for impacts will be along Raynolds and Targhee passes as well as US 191 from Bozeman to West Yellowstone with slippery road conditions possible. At this time, snow amounts are expected to stay below what has been observed during previous events this past week. Thus, the concern for significant impacts remains low and no products will be issued for the area at this time.

Lower elevation Plains and the Hi-Line:

This event is expected to bring the first snow for many lower elevation locations. There is an aspect of impacts simply because it is the first event, however, overall snow totals are generally remaining less than 2 inches at the moment. While this may warrant an advisory at a later date, for now the decision was made to hold off on issuing products across the plains and Hi-Line until higher confidence in snow amounts is reached.

-thor

AVIATION

04/00Z TAF Period

Expect VFR conditions for the TAF period. The only exception is there is a slight chance for light rain/snow showers in Southwest MT/North Central MT (30%) this evening. These showers will be a hit or miss to terminals. Right now, I've only put chances in KBZN, where they could get MVFR visibilities if hit with a shower. A cold front moving through the area Monday will bring breezy to gusty winds across terminals (with the strongest winds as KCTB and KGTF). -Wilson

Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information.

PRELIMINARY POINT TEMPS/POPS

GTF 26 49 35 41 / 0 10 20 80 CTB 25 45 31 39 / 0 10 40 70 HLN 27 49 31 42 / 10 10 40 80 BZN 20 45 26 40 / 10 0 70 80 WYS 5 30 13 28 / 20 30 90 90 DLN 18 43 22 36 / 0 0 50 30 HVR 23 50 30 43 / 0 10 50 70 LWT 23 47 28 39 / 0 0 50 70

TFX WATCHES/WARNINGS/ADVISORIES

High Wind Warning from 8 AM Monday to 8 AM MST Tuesday for East Glacier Park Region-Eastern Glacier, Western Toole, and Central Pondera-Northern High Plains-Southern High Plains-Southern Rocky Mountain Front.

Blizzard Warning from 1 PM Monday to 8 AM MST Tuesday for East Glacier Park Region-Southern Rocky Mountain Front.

Winter Weather Advisory from 1 PM Monday to 11 PM MST Tuesday for Northern High Plains-Southern High Plains.

Winter Storm Watch from late Monday night through late Tuesday night for Bears Paw Mountains and Southern Blaine-Cascade County below 5000ft-Fergus County below 4500ft-Judith Basin County and Judith Gap-Snowy and Judith Mountains.

Winter Storm Warning from 5 AM Tuesday to 5 AM MST Wednesday for Little Belt and Highwood Mountains.

Winter Weather Advisory from 8 AM Monday to 11 PM MST Tuesday for Upper Blackfoot and MacDonald Pass.

Winter Weather Advisory from 5 AM Tuesday to 5 AM MST Wednesday for Big Belt, Bridger and Castle Mountains-Meagher County Valleys.

Winter Weather Advisory from 8 AM to 11 PM MST Tuesday for East Glacier Park Region-Southern Rocky Mountain Front.


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