textproduct: Great Falls

KEY MESSAGES

- Locally breezy conditions develop Saturday ahead of a colder Spring system moving in late Saturday into Sunday.

- Mountain snow, with a mix of lower elevation rain and snow Saturday night into Sunday.

DISCUSSION

/Issued 529 AM MDT Sat May 16 2026/

- Meteorological Overview:

Longwave troughing over much of the western CONUS will bring well below normal temperatures and widespread precipitation to the Northern Rockies through the weekend, with below normal temperatures lingering into the first half of the upcoming work week along with scattered showers. High temperatures in the 50s today will fall into the upper 30s to mid-40s by Sunday thanks to a strong cold front pushing south from Canada tonight through Sunday morning. This cold front combined with increasing synoptic lift will help to support a shield of precipitation (mountain snow and lower elevation rain/snow) developing from late tonight through the day on Sunday. Prior to this shield of precipitation a line of showers and isolated thunderstorms is expected to develop by the early morning hours today, with these showers and storms slowly sliding east through the afternoon and early evening hours, especially for areas along and south of the Montana Hwy 200 corridor. By the late afternoon/early evening hours tonight snow levels will begin to fall across Southwest and Central Montana, which will allow precipitation to change over to snow for elevations above 5000ft. Snow levels will continue to fall into the day on Sunday thanks to the aforementioned cold front pushing south; however, the effects of a high May sun angle is likely to limit snowfall accumulation across lower elevations, especially between the hours of 10 AM and 8 PM Sunday. Precipitation will gradually decrease in areal coverage and intensity from west to east Sunday night, but daily chances for showers will linger through Tuesday. Several nights, mainly Sunday night and Monday night, are likely to see low temperatures fall into the upper teens to 20s across all elevations. - Moldan

- Forecast Confidence & Scenarios:

Precipitation (rain or snow) through Sunday night...

Latest ECMWF EFIs support the potential for an unusual precipitation event (some snow mixing in at times across lower elevation) with respect to climatology along the northern slopes of the Gallatin/Madison Mountains and Gallatin Valley, portions of the Montana Hwy 200/US Hwy 87 corridor from Great Falls to Lewistown, northern slopes of the Bears Paw Mountains, and eastern slopes of the Rocky Mountain Front. Below is the NBM5.0 probabilities for rainfall accumulations from 6am Saturday through 6am Monday for select cities from North Central through Southwest Montana.

48 Hour Rainfall Probabilities LOCATION 0.10" | 0.25" | 0.50 " | 1.00" Browning 85% | 65% | 15% | <5% Cut Bank 75% | 40% | 10% | <5% Havre 90% | 85% | 60% | 25% Great Falls 90% | 80% | 55% | 20% Lewistown 100% | 95% | 95% | 55% Helena 90% | 60% | 25% | 10% Bozeman 95% | 90% | 40% | 10% Dillon 55% | 35% | 15% | <5% Ennis 95% | 95% | 75% | 20% West Yellowstone 50% | 15% | 5% | 0%

Accumulating Snow from Tonight through Sunday evening...

Latest ECMWF EFIs support the potential for an unusual snowfall event with respect to climatology for the northern slopes of the Little Belt Mountains and eastern slopes of the Rocky Mountain Front and immediate eastern plains out to the US Hwy 89 corridor from Choteau to the Piegan Port of Entry. Here EFIs range from 0.5 to 0.75, with Shift of Tails (SOTs) of 1. Further east over the plains near the northwestern slopes of the Bears Paw Mountains including the Havre Area the SOTs climb to around 2.5, which indicates that at least several ensemble members are suggesting the potential for an "extreme" event with respect to climatology (May climatology vs anytime during the winter). A closer look at the Havre Area reveals that 5 ECMWF ensemble members do support snowfall accumulations of 2 inches or more here, with the most extreme member suggesting upwards of 6 inches through the timeframe. While accumulating snow can't be ruled out for the Havre Area, especially the mid-slopes of the Bears Paw Mountains, the overall timing of the snowfall (during most of the day on Sunday) does not seem favorable for snowfall accumulations on road surfaces to impact travel. For this reason a Winter Weather Advisory was not issued. Further west and south across the previously mentioned Rocky Mountain Front and Little Belt Mountain Areas a Winter Weather Advisory has been issued for the mountains as confidence is high enough that snowfall accumulations will amount to enough to impact travel and outdoor recreation, especially through Sunday morning. Over the adjacent plains out to the US Hwy 89 corridor confidence was not high enough to issue an Advisory at this time, but future shifts may consider to do so should models trend higher with snowfall amounts and colder with temperatures.

Below is the NBM5.0 probabilities for snowfall accumulations from 12am Saturday through 12am Sunday for select mountain passes across North Central through Southwest Montana.

24 Hour Snowfall Probabilities LOCATION 2" | 4" | 6" | 8" Marias Pass 60% | 15% | 5% | <5% Rogers Pass 80% | 55% | 25% | 10% Kings Hill Pass 75% | 35% | 15% | 10% Lewistown Divide 30% | 15% | 5% | <5% MacDonald Pass 40% | 15% | 10% | 5% Deep Creek Pass 10% | <5% | <5% | <5% Boulder Hill 25% | 5% | <5% | <5% Elk Park Pass 10% | <5% | <5% | <5% Homestake Pass 10% | <5% | <5% | <5% Bozeman Pass 15% | 5% | <5% | <5% Chief Joseph Pass 30% | 10% | 5% | <5% Big Hole Pass 10% | 5% | <5% | <5% Monida Pass <5% | 0% | 0% | 0% Raynolds Pass <5% | <5% | 0% | 0% Targhee Pass <5% | 0% | 0% | 0%

- Moldan

AVIATION

16/18Z TAF Period

A mix of rain and mountain snow will continue across portions of the region this afternoon, mostly across Central and Southwest Montana. A few thunderstorms will develop this afternoon, but confidence in one of these impacting a terminal was too low to mention in TAFs at this time.

Snow slowly becomes the favored precipitation type later tonight and into tomorrow in many areas, particularly behind a cold front arriving from north to south Sunday morning and afternoon. Mountains will become increasingly obscured through the TAF period. -AM

PRELIMINARY POINT TEMPS/POPS

GTF 55 33 40 28 / 40 40 90 70 CTB 55 30 38 25 / 10 60 80 10 HLN 50 33 42 31 / 80 40 80 50 BZN 58 29 45 28 / 80 90 30 30 WYS 52 25 35 20 / 30 60 30 20 DLN 57 26 44 22 / 30 40 50 30 HVR 59 35 43 29 / 30 50 80 70 LWT 53 31 39 27 / 80 90 80 90

TFX WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory from midnight tonight to midnight MDT Sunday night for Big Belt, Bridger and Castle Mountains-East Glacier Park Region-Elkhorn and Boulder Mountains-Little Belt and Highwood Mountains-Southern Rocky Mountain Front-Upper Blackfoot and MacDonald Pass.

Winter Weather Advisory from 5 PM this afternoon to 6 AM MDT Sunday for Gallatin and Madison County Mountains and Centennial Mountains.