textproduct: Great Falls

KEY MESSAGES

- Very warm, dry, and breezy to windy conditions continue through Wednesday

- Isolated afternoon and shower and thunderstorm activity develops today and will increase in coverage on Wednesday.

- Much cooler temperatures, locally heavy mountain snow, and lower elevation rain and snow move in Wednesday night through Friday.

DISCUSSION

/Issued 316 PM MDT Tue Apr 21 2026/

- Meteorological Overview:

Deep southerly flow becomes established over the Northern Rockies through Wednesday in response to an approaching Pacific Trough. This will bring increasing surface winds and widely scattered shower and thunderstorm activity, particularly on Wednesday. A 40 to 50 kt southerly low level jet and tightening surface pressure gradient will encourage gusty to localized strong wind gusts to southwest and portions of central MT on Wednesday, especially the narrow north to south oriented southwest valleys and exposed ridge tops. There may be just enough moisture for a few isolated convective showers this afternoon and evening, but this activity will generally be most widespread on Wednesday. CAPE doesn't look overly impressive for Wednesday's storms at a couple hundred J/kg, but increasing winds aloft may encourage surface momentum transfer and cooling aloft may be sufficient for some pea size hail within the stronger cores.

H700/H850 temperatures peak today and will be running around 2 standard deviations above average for this time of year. This is combined with the rising April sun angle is warming lower elevation temperatures to near or at record highs in the upper 70 to lower 80s. Temperatures will be on the warmer side on Wednesday as as well, but increasing cloud cover and cooling aloft will commence the return to chilly conditions that kicks in to high gear by Thursday.

A closed mid-level circulation will move northeastward into the region on Wednesday, but the colder air aloft and associated more widespread rain and snow looks to hold off until Wednesday night into Thursday. North to northwesterly flow aloft becomes cold enough for snow at all elevations over central and north-central MT by Thursday morning. Snowfall amounts and overall impacts still look to be greatest over the north/northwest facing foothills of the central island ranges. There will also be accumulating snow along the Rocky Mountain Front and over the higher terrain of southwest MT. The snow will become more scattered and convective in nature during the day on Thursday with 30 to 50 kt H700 flow bringing gusty to strong northwesterly surface winds, mostly along the Hi-Line and the plains east of I15.

A secondary mid- level circulation moves in by Thursday night and brings additional snow and snow shower activity. Most ensemble guidance is now latching on to a smaller scale low developing along the MT/ID/WY borders. This will serve to enhance a more concentrated brand of accumulating snow over southwest Montana for Friday morning, particularly for the mountains south of I90. Broad troughing will then continue through the weekend and into early next week, maintaining below average temperatures while offering daily opportunities of rain/snow shower activity, especially for areas over or near higher terrain. - RCG

- Forecast Confidence & Scenarios:

Increasing winds and widely scattered showers and thunderstorms through Wednesday...

Modest moisture and instability are already moving in via the shift to southerly flow aloft. Moisture availability is lacking with dew points less than 40F, but a few isolated showers or storms can't be ruled out amid diurnal heating and lifted index values up to around 1. Localized gusty winds will be the primary hazard with any shower or storm though this evening.

The southerly flow strengthens tonight into Wednesday and brings gusty to strong winds to southwest and portions of central MT. The best opportunity for sustained winds over 40 mph or gusts over 60 mph will be over the south to north oriented C/SW valleys, including the Madison Valley and Judith Gap area in addition to the southwest exposed ridge tops. A high wind warning was send out only for the Madison Valley; however, any area within the forecast area may see instantaneous wind gusts over 50 mph in or near any shower or storm. As mentioned in the previous section forecast CAPE or 'thunderstorm energy' has gone down from previous model runs, leaving localized wind gusts as the most obvious hazard, but colder air aloft will be moving on Wednesday and may result in some instances of small hail or graupel beneath the stronger cores. Widely scattered showers and thunderstorms may develop as early as Wednesday morning over the southwest, but in general convective activity will be most widespread in the afternoon and evening hours.

Snow and winds on Wednesday night through Friday...

Cold northwesterly flow on the backside of the low pressure system will ramp up the snow and surface winds Wednesday night into Thursday. Northern areas will be most impacted Thursday morning, especially over and near the central island ranges where the combination of winds over 50 mph and heavy wet snow wet snow look to cause instances of power outages and tree damage in addition to brief whiteout conditions. There will be less impactful conditions in other mountains areas along the Continental Divide, southwest MT, and the Bear's Paw Mountains. The inherited winter storm watch was retained in it's entirety, but the end time was extended to Friday at noon (more on this later). There was some consideration given to add the Bear's Paw zone to the watch. but declined for now given these areas being on the far northern extent of the snow. Winter weather advisories were added for all other mountain areas. Most lower elevations were not assigned winter weather products despite there being at least some snowfall. Borderline temperatures and only a short window of opportunity for accumulations/impacts before diurnal heating reduces pavement impacts were the primary reasons to hold off for now.

There will be be gusty to strong winds that develop over much of central/north-central MT by Thursday with H700/H850 winds approaching 50 kts. A high wind watch was issued for much of the Hi- Line east of I15 and areas southeastward into Fergus County. Locations on the southeast aspects for the Sweetgrass Hill, Bear's Paw, and the Snowies will be most susceptible to wind gusts over 60 mph.

A secondary wave move though Thursday night into Friday and delivers another round of snow, but with less wind. This one will impact southwest MT the most, especially over the higher terrain of Madison and Gallatin counties and even in the Gallatin Valley. Upper level temperatures will be much colder with this system, so lower elevation accumulation will be more common. In response to this secondary system, all the end times for the winter weather watches and advisories no go through noon on Friday. Additionally, a winter storm watch was added for the Gallatin valley Thursday night and Friday morning due to the cold northerly upslope flow and nearly stationary frontogenesis. Other southwest lower elevations will be considered in the for advisories for Friday morning in the near future. - RCG

AVIATION

22/00Z TAF Period

VFR conditions will prevail at all terminals during this TAF Period. Wednesday afternoon/evening a cold front will move through and bring a wind shift and rain/snow showers and isolated thunderstorms to North-central, Central, and Southwestern Montana. Between 22/21Z and 22/24Z there is a 15 - 30% chance for thunderstorms at all terminals. The cold front will lower ceilings and bring periods of mountain obscuration to all terminals Wednesday afternoon and evening. Wednesday afternoon isolated heavy rain/snow showers have a medium chance to briefly reduce visibility to MVFR-levels or lower. Between 22/04Z and 22/16Z there will be isolated instances of low- level wind shear and mountain wave turbulence across North-central, Central, and Southwestern Montana. -IG

PRELIMINARY POINT TEMPS/POPS

GTF 50 70 30 41 / 20 60 100 80 CTB 42 67 28 41 / 10 70 90 50 HLN 47 66 34 43 / 10 80 100 100 BZN 43 62 33 45 / 20 60 100 90 WYS 32 47 28 42 / 20 90 100 100 DLN 43 57 34 48 / 10 80 90 80 HVR 49 78 32 44 / 10 30 90 60 LWT 52 70 28 38 / 20 40 90 80

TFX WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory from 6 PM Wednesday to 6 PM MDT Thursday for East Glacier Park Region-Northern High Plains.

High Wind Watch from late Wednesday night through Thursday afternoon for Bears Paw Mountains and Southern Blaine-Eastern Toole and Liberty-Fergus County below 4500ft-Hill County- Northern Blaine County-Western and Central Chouteau County.

Winter Weather Advisory from midnight Wednesday night to 6 PM MDT Thursday for Bears Paw Mountains and Southern Blaine.

Winter Weather Advisory from 6 PM Wednesday to noon MDT Friday for Elkhorn and Boulder Mountains-Gates of the Mountains- Northwest Beaverhead County-Ruby Mountains and Southern Beaverhead Mountains-Southern Rocky Mountain Front-Upper Blackfoot and MacDonald Pass.

Winter Storm Watch from Wednesday evening through Thursday afternoon for Big Belt, Bridger and Castle Mountains-Gallatin and Madison County Mountains and Centennial Mountains-Little Belt and Highwood Mountains.

Winter Storm Watch from late Wednesday night through Thursday afternoon for Fergus County below 4500ft-Judith Basin County and Judith Gap-Snowy and Judith Mountains.

High Wind Warning from 3 AM to 6 PM MDT Wednesday for Madison River Valley.

Winter Storm Watch from Thursday evening through Friday morning for Gallatin Valley.