textproduct: Great Falls

KEY MESSAGES

- Two waves of impactful snowfall develop this weekend into Monday as temperatures trend significantly cooler across the region.

- Initial concern through the day today and into tonight will be along the Rocky Mountain Front and across Central Montana.

- Concern for heavy snowfall expands further south into Southwest Montana Sunday into Sunday night.

DISCUSSION

- Meteorological Overview:

A complex evolution to the upper level pattern will continue through today, initially with a large upper level trough with multiple embedded vort maxes sinking southward from the Pacific Northwest, elongating as it does so. At the surface an area of low pressure centered in North Dakota has pulled a cold front southward across the plains early this morning. The combination of this front, favorable forcing, and ample moisture has resulted in a round of precipitation developing across much of North Central and Central Montana. Precipitation in the form of rain across many areas will continue to transition to snow as cold air advection near the surface wins out. A precipitation transitions to snow expect the snow characteristic to be of the wet and heavy variety, especially at lower elevations, which raises concern for trees with leaves on them still seeing snapped branches, potentially impacting power lines. - AM/Moldan

Colder air continues to sink southward into Southwest Montana heading into Sunday, though there remains some uncertainty with respect to just how far it makes it. Aloft the complex evolution continues, with the upper level troughing elongating further, while also interacting with the remnants of Rachel as they lift northeastward ahead of the trough axis. Ample moisture associated with not only the main troughing, but the remnants of Rachel will support the expansion of precipitation southward into Southwest Montana during the day Sunday as an upper level jet moves overhead. The extent of impactful snowfall across Southwest Montana will follow where the coldest air both aloft and at the surface ends up settling. Banded precipitation looks to persists into Monday morning across portions of Central and Southwest Montana before subsiding through the day.

The region looks to dry out for at least a couple days as the upper level troughing consolidates once again across the desert Southwest. Uncertainty grows a bit toward Thursday, though the majority of ensemble members remain dry Thursday toward the weekend. -AM

- Forecast Confidence & Scenarios:

Snowfall through Sunday night/Monday morning...

Overall confidence in snowfall amounts continues to increase for areas north of the US Hwy 12 corridor, along/near the Montana Hwy 200 corridor, and west of the US Hwy 89 corridor across Central and portions of North Central Montana. Here Hi-Res guidance has been consistently showing a nearly stationary band of moderate to heavy snow through much of the next 48 hours, with additional orographic enhancement from the mid-slopes of the Little Belts and Rocky Mountain Front out onto the adjacent plains above 3500 feet.

As has been the case from previous forecasts snowfall amounts will vary drastically from grassy/elevated surfaces to roadways/paved surfaces given relatively warm ground temperatures and the effects of a higher October sun angle (during the daylight hours) as compared to the heart of the winter season. With that being said, surface temperatures along and west of the I-15 corridor over the plains of Central and North Central Montana have already fallen into the mid- to upper 30s, which will be favorable for snowfall to accumulate on roadways in these areas by the late afternoon hours today as solar insolation decreases. Along with the impacts to travel to the region the heavy, wet nature of the snow has the potential to cause considerable tree damage where foliage still remains. Additionally, this heavy, wet snow accumulating on powerlines and breezy north winds could result in isolated power outages.

Current suite of winter weather statement remain valid outside of the East Glacier Park Region and Northern High Plains where the Winter Weather Advisory has been upgraded to a Winter Storm Warning due to a trend in higher snowfall amounts. Additional focus was given to areas south and east of a Bozeman to Dillon, and the Helena Valley for the potential need to upgrade the current advisories to warnings. Ultimately concerns about snow levels and/or marginal surface temperatures still remain and confidence in warning level amounts/impacts remain too low for an upgrade.

Low Temperatures Monday night/Tuesday morning...

Clearing skies, light winds, and a relatively fresh snowpack will likely result in the coldest temperatures of the upcoming week, with a 25% chance that lows fall into the teens across Southwest through North Central Montana. Model guidance is notorious for under-performing with respect to low temperatures given the aforementioned conditions, so it is reasonable to believe that lows could fall as low as the single digits above zero at some locations during the timeframe. - Moldan

AVIATION

10/18Z TAF Period

Widespread precipitation and low ceilings will continue to overspread the region. Across central and northern Montana, rain will transition to snow through the afternoon into the night. Continued breezy conditions will further contribute to reduced visibilities and mountain obscuration particularly concerning terminals KCTB, KGTF, and KHLN. Across southwest Montana, the rain/snow transition will be delayed towards the end of the TAF period. Reduced visibilities are still anticipated, as widespread light to moderate stratiform rain develops. Terminal KWYS is of particular interest with weak instability potentially allowing for showers to be more convective in nature which would pose the risk of erratic winds in the outflow.

-JR

PRELIMINARY POINT TEMPS/POPS

GTF 27 32 24 35 / 100 100 100 60 CTB 23 31 22 35 / 100 100 90 40 HLN 28 32 26 38 / 100 100 100 50 BZN 29 34 25 38 / 100 100 100 50 WYS 30 43 23 41 / 90 100 100 70 DLN 28 30 23 37 / 90 100 100 40 HVR 27 36 24 38 / 100 100 100 80 LWT 29 34 25 35 / 100 100 100 80

TFX WATCHES/WARNINGS/ADVISORIES

Winter Storm Warning until 6 AM MDT Monday for East Glacier Park Region-Gates of the Mountains-Northern High Plains-Southern High Plains-Southern Rocky Mountain Front-Upper Blackfoot and MacDonald Pass.

Winter Weather Advisory from 6 PM this evening to 6 AM MDT Monday for Bears Paw Mountains and Southern Blaine-Beaverhead and Western Madison below 6000ft-Canyon Ferry Area-Eastern Glacier, Western Toole, and Central Pondera-Eastern Toole and Liberty-Fergus County below 4500ft-Gallatin Valley-Gallatin and Madison County Mountains and Centennial Mountains-Helena Valley- Madison River Valley-Missouri Headwaters-Northwest Beaverhead County-Ruby Mountains and Southern Beaverhead Mountains-Western and Central Chouteau County.

Winter Storm Warning from 6 PM this evening to 6 AM MDT Monday for Big Belt, Bridger and Castle Mountains-Cascade County below 5000ft-Eastern Pondera and Eastern Teton-Elkhorn and Boulder Mountains-Judith Basin County and Judith Gap-Little Belt and Highwood Mountains-Meagher County Valleys-Snowy and Judith Mountains.