textproduct: Great Falls

KEY MESSAGES

- Scattered snow shower activity and cool conditions continue through Sunday, with the greatest winter impacts along the Northern Rocky Mountain Front Sunday morning.

- Temperatures begin to slowly trend closer to average heading into the middle of next week; however, additional rounds of mostly mountain snow develop late Sunday into Monday and to a lesser degree again Tuesday into Wednesday.

- General milder and drier conditions move in for the second half of next week, but there will be some passing weather systems with at least brief cool downs and increased mostly mountain snow next weekend.

DISCUSSION

- Meteorological Overview:

A cold core mid- level low will continue to churn over the Canadian Prairies and send multiple shortwaves into the Northern Rockies. This combined diurnal instability to maintain scattered to numerous snow shower activity and well below average temperatures over the next 24 hours. Low level easterly flow looks to bring the most impactful accumulating snow impacts to the Northern Rocky Mountain Front and adjacent foothill/plains tonight into Sunday morning.

This low finally ejects northeastward later on Sunday, but general troughing will remain in place through at least the middle of next week. There will be several shortwaves/smaller scale troughs that bring opportunities for rounds of light, mostly mountain, snow and lower elevation increased shower activity by Sunday afternoon. The departing Canadian low will take the colder air aloft with it, allowing H700 temperatures to warm up to around 0 to -5C areawide by Monday. This will commence a slow warming trend while keeping minor snow impacts mostly in the mountains.

While a majority of ensembles favor rising heights and general warming aloft for the second half of next week, they also support more Pacific troughs passing through the Northern Rockies next weekend. Overall, temperatures look to return to near or slightly above average for the Wednesday through Friday period, followed by cooler and wetter conditions. Similar to the most recent storm, forcing looks smaller scale and transient in nature. This erodes forecast confidence with rain, snow, and temperatures all being determined by these features. - RCG

- Forecast Confidence & Scenarios:

Snow and snow showers through Sunday morning...

Snow shower activity will be most numerous over central and north- central MT tonight into Sunday morning. The latest suite of ensemble data have become more aggressive with low level easterly flow bringing a more concentrated area of light to moderate snow along the Rocky Mountain Front and nearby foothills and plains. The NBM snowfall exceedence probability for 2 inches of snow or more have increased to around a 50 to 60% chance for the E. Glacier and Marias Pass areas. The nearby higher terrain, including Marias Pass have around a 30 to 40% chance for receiving 4 inches of snow more.

A winter weather advisory has been issued tonight through Sunday morning to address winter driving conditions, including snowy roadways and reduced visibility. Some consideration was given to adding the Eastern Glacier County and Southern Rocky Mountain Front zones given the aforementioned easterly upslope flow, but held off for now with the heavier amounts dropping off some heading to the east and southeast. While snow showers will become more widespread across central and north-central MT, impacts look to be limited by spotty snowfall amounts in excess of an inch.

Mountain Snow Sunday night through Wednesday...

The focus then shifts to a shortwave that brings increased mostly mountain snow to central and southwest MT Sunday night into Monday. Warming temperatures aloft look to keep the accumulating snow threat over the higher terrain. NBM probabilistic guidance reserves the highest chances (60 to 70%) for snowfall amounts over 2 inches for areas above 6,000 ft, mostly over the Little Belts and the mountain peaks of Beaverhead, Gallatin, amd Madison counties. The primary uncertainty with this system is during the onset when warm air advection aloft is capable of producing a brief period of heavier banding on the northern fringe of the snow. The window of opportunity to produce heavier snow doesn't appear to be long enough to exert impactful conditions, but this situation will be monitored for the need of winter advisories. Passing shortwaves bring additional rounds of light, mostly mountain, snow through Wednesday, though further warming aloft should preclude widespread impacts. - RCG

AVIATION

25/18Z TAF Period

Area remains beneath a moist and somewhat unstable cyclonic northwest flow aloft through the TAF period. This will maintain lower VFR to MVFR cloud ceilings and mountain obscuration with periods of increased snow shower activity. Scattered showers this afternoon will bring the risk of brief MVFR conditions to most terminals with a period of more widespread snow showers and associated MVFR conditions developing this afternoon near the Rocky Mtn Front and KCTB terminal, potentially expanding to portions of southwest MT tonight through Sunday morning. Hoenisch

PRELIMINARY POINT TEMPS/POPS

GTF 24 39 22 48 / 50 50 30 30 CTB 19 35 15 47 / 70 80 10 10 HLN 23 42 25 47 / 30 40 30 50 BZN 22 44 25 47 / 20 30 60 70 WYS 17 44 21 43 / 30 60 70 80 DLN 23 47 27 48 / 10 20 50 50 HVR 20 41 20 49 / 40 20 10 10 LWT 20 37 22 43 / 30 50 50 50

TFX WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory from 6 PM this evening to noon MDT Sunday for East Glacier Park Region-Northern High Plains.