textproduct: Great Falls

KEY MESSAGES

- Light mountain snow and lower elevation rain will continue through the day.

- Cooler temperatures will persist through Tuesday morning before more seasonal temperatures return mid-week.

- Another system arrives on Wednesday bringing another round of light mountain snow and lower elevation rain as well as a chance for afternoon thunderstorms.

UPDATE

/Issued 423 AM MDT Sun May 17 2026/

Early morning update has been published, with the primary adjustments being to lower high temperatures by several degrees where precipitation will be the most persistent through the day and to increase PoPs based upon latest radar and Hi-Res model trends. Overnight low temperatures have cooled off several degrees colder than forecasted where precipitation has/had fallen, which has increased confidence that along with strong cold air advection behind the southward surging cold front this morning, and dynamic cooling processes, that high temperatures will struggle to warm today where precipitation falls most persistently. Otherwise the remainder of the forecast remains on track and no other adjustments were made. - Moldan

DISCUSSION

/Issued 524 AM MDT Sun May 17 2026/

- Meteorological Overview:

An upper level trough will continue its way across the region bringing a chance for light mountain snow and lower elevation rain throughout the day. As the trough starts to depart, northerly flow behind it will keep temperatures well below normal through Monday. More zonal flow returns on Tuesday, allowing for seasonal temperatures to return. Just as temperatures start to gradually warm up, a shortwave trough is expected to move into the region on Wednesday bringing another chance for showers and thunderstorms. Mountain snow will generally stay above pass level for this storm with only slight probabilities of snow levels dropping low enough to provide impacts at pass level.

Towards the end of the week cluster guidance points towards upper- level ridging building along the west coast bringing a return to northwesterly flow over Montana. The result will be more seasonal to above-normal temperatures with generally dry conditions persisting across the region.

- Forecast Confidence & Scenarios:

Sunday Snow:

Overall, snowfall amounts have decreased compared to previous model runs with the mountains and valleys of Southwest Montana expected to see an additional 1 to 3 inches with isolated higher amounts possible along the peaks. For the Northern Rocky Mountains and the Little Belts, 2 to 4 inches are possible at pass level with higher amounts expected along the peaks. But generally, the models have moved away from the previously forecasted 6 to 12 inches along the peaks. This is not to say it is completely out of the range of possibilities. But the probability of occurrence now sits at around 10-20%.

While there is still some potential for enhanced snowfall along north-facing slopes, the models have backed off significantly compared to previous runs. In particular, the probabilities for snowfall amounts greater than 6 inches have decreased significantly across the region. That being said, there is still enough snowfall expected to produce impacts that are worthy of the current suite of Winter Weather Advisories. So no changes additions or subtractions were made to the advisories with this forecast package.

Wednesday Precipitation:

Given that this shortwave trough is not expected to be as cold as the current system moving through, light snow will be primarily confined to the higher peaks of the mountains. Locations at and below pass level may pick up a few flakes but nothing looks particularly impactful at this time. Lower elevations will see a chance for rain throughout the day with a chance for thunderstorms in the afternoon/evening hours. Currently, the Euro focuses more of the instability in Central Montana whereas the GFS pushes it towards Southwest Montana and the eastern Hi-Line. So the generalized 15-20% chance for a thunderstorm across a large portion of the region matches the current confidence of suggesting there could be something but there is still a bit of uncertainty as to where it will happen. -thor

AVIATION

17/12Z TAF Period

A large upper level disturbance moving over the Western US will continue to spread low to mid-level cloud cover across the Northern Rockies through the 1712/1812 TAF period, with the lowest CIGS over the valleys and plains of Central and North Central Montana. Precipitation coverage and intensity will be the most persistent in easterly and northerly upslope areas of Central and North Central Montana, especially near the KGTF and KLWT terminals. While MVFR/low-VFR conditions will generally dominate over the next 24 hours there will be times of LIFR/IFR conditions, most notably at the KCTB, KGTF, and KLWT terminals. - Moldan

PRELIMINARY POINT TEMPS/POPS

GTF 44 30 53 29 / 80 20 10 0 CTB 43 25 55 30 / 70 10 0 0 HLN 46 32 55 32 / 60 10 10 0 BZN 48 27 51 26 / 30 20 20 0 WYS 38 22 44 19 / 10 20 0 0 DLN 47 25 51 26 / 30 20 0 0 HVR 46 29 54 28 / 80 40 10 0 LWT 42 27 45 25 / 80 60 40 0

TFX WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory until midnight MDT tonight 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.