textproduct: Great Falls

KEY MESSAGES

- Temperatures well below normal over the plains of Central and North Central Montana on Friday, with above normal highs in the valleys of Southwest Montana.

- Moderate to heavy snow develops overnight tonight and through the day on Friday over Central and North Central, especially east of the I-15 corridor.

- Several days of windy conditions are expected next week over North Central MT.

DISCUSSION

- Meteorological Overview:

The Arctic front, which surged south across the plains of North Central and Central Montana between 15-18z this morning, will continue to progress (albeit slower and modified with respect to temperatures) south towards the US Hwy 12 corridor in Central Montana through the remainder of the afternoon and early evening hours before stalling. Strong low level frontogenesis associated with this front combined with Pacific moisture overrunning the Arctic airmass was helping to support scattered bands of precipitation largely in the form of snow over the plains and mountains of North Central and Central Montana; however, brief periods of rain or freezing rain were occurring along the immediate frontal boundary. This freezing rain threat will gradually subside through the remainder of the afternoon hours as temperatures aloft cool and the Arctic airmass deepens.

As the best forcing shifts south with the front precipitation will temporary end from north to south through the afternoon hours today, with most of precipitation being focused south of the Montana Hwy 200 corridor and north of the US Hwy 12 corridor through the evening hours tonight. By the late evening hours tonight a mid-level warm front will then begin to lift north to northeast over the Arctic airmass that is in place. This northward progression of the mid- level warm front will help to reorientate the band of snow across Central Montana back to the north and over North Central Montana in northwest to southeast orientation through the day on Friday. The northward progression (i.e. speed) of this mid-level warm front will strongly dictate how much snow eventually falls across portions of Central and North Central Montana (i.e. slower progression will allow for more snow while a faster progression will lead to less snow), especially along and west of the I-15 corridor to the Rocky Mountain Front where low-level drying due westerly flow aloft will be occurring. Snow will gradually end from southwest to northeast from the late afternoon/early evening (Central Montana) hours on Friday to the early morning hours on Saturday (North Central Montana). - Moldan

- Forecast Confidence & Scenarios:

Accumulating Snow through Friday/Friday night...

High End Snowfall (10% chance of occurring) : A slower moving mid- level warm front helps to support a prolonged period of moderate to heavy snow across all of the plains of Central and North Central Montana, with snowfall amounts approaching 12" over the plains of Central and North Central Montana east of the I-15 corridor and 12- 20" in the mountains of Central and North Central Montana.

Low End Snowfall (10% chance of occurring) : A faster moving mid- level warm front limits the time for moderate to heavy snow, especially along the I-15 corridor where downsloping westerly winds lead to snow accumulation of generally less than 1". Across the remainder of the plains to the east snowfall accumulations generally range from 2-5", with 5-10" in the mountains. - Moldan

Strong winds early next week...There is already and 80 percent chance that winds could exceed warning criteria along the Rocky Mountain Front and in the Cut Bank area Mon and Tue and a 60 percent chance on Wed. Brusda

AVIATION

11/18Z TAF Period

The cold front continues to make its way south with winds shifting to the north followed by a rapid drop in temperatures. Precipitation so far has started off as freezing rain and from there transitioned to a rain/snow mix before finally becoming snow once the colder air filters in. As of now, these conditions will impact KGTF, KLWT, and KHLN over the next few hours.

There is a chance for a stray shower to make its way to KEKS and KBZN, however, confidence is overall low in impacts to the terminal with most of the rain/snow expected to stay in the mountains.

Mountain obscuration will be a concern through the entire period. -thor

PRELIMINARY POINT TEMPS/POPS

GTF 2 11 5 29 / 80 90 50 10 CTB -4 4 -2 22 / 50 90 90 0 HLN 21 34 26 43 / 90 80 10 10 BZN 33 45 33 46 / 80 70 10 10 WYS 29 37 21 37 / 50 50 10 10 DLN 41 51 33 49 / 10 10 0 0 HVR -7 0 -8 6 / 60 90 100 10 LWT 5 10 4 29 / 100 100 70 20

TFX WATCHES/WARNINGS/ADVISORIES

Cold Weather Advisory from 6 PM this evening to 10 AM MST Friday for Eastern Glacier, Western Toole, and Central Pondera-Eastern Toole and Liberty-Hill County-Northern Blaine County.

Winter Storm Warning from midnight tonight to 5 AM MST Saturday for Fergus County below 4500ft-Judith Basin County and Judith Gap.

Winter Weather Advisory from 4 AM Friday to 5 AM MST Saturday for Eastern Glacier, Western Toole, and Central Pondera-Eastern Pondera and Eastern Teton-Northern High Plains.

Winter Weather Advisory until 6 PM MST this evening for Bears Paw Mountains and Southern Blaine-Western and Central Chouteau County.

Winter Storm Warning from 4 AM Friday to 5 AM MST Saturday for Bears Paw Mountains and Southern Blaine-Eastern Toole and Liberty-Hill County-Northern Blaine County-Western and Central Chouteau County.

Winter Storm Warning from midnight tonight to 6 PM MST Friday for East Glacier Park Region-Little Belt and Highwood Mountains- Snowy and Judith Mountains-Southern Rocky Mountain Front.

Winter Weather Advisory until midnight MST tonight for Fergus County below 4500ft-Judith Basin County and Judith Gap-Little Belt and Highwood Mountains-Snowy and Judith Mountains-Southern Rocky Mountain Front.

Winter Weather Advisory until 6 PM MST Friday for Big Belt, Bridger and Castle Mountains-Cascade County below 5000ft-Gates of the Mountains-Meagher County Valleys-Southern High Plains- Upper Blackfoot and MacDonald Pass.