textproduct: Great Falls

KEY MESSAGES

- Areas of rain and mountain snow decrease in coverage later this morning, with much warmer and drier conditions moving in for most if not all of the holiday weekend.

- An approaching cold front will bring breezy to windy conditions and increased shower and thunderstorm activity Monday into Tuesday followed by a period of cooler temperatures, mountain snow, and lower elevation rain.

UPDATE

/Issued 315 AM MDT Thu May 21 2026/

Forecast is performing well and no update is planned this morning.

DISCUSSION

/Issued 315 AM MDT Thu May 21 2026/

- Meteorological Overview:

Showers and thunderstorms have largely transitioned to areas of stratiform rain this evening and this activity will continue to follow the H500 frontogenesis southward during the overnight hours. Mountain snow has already been observed for locations above 7000 ft and snow levels will drop as low as 5000 ft by dawn in response to H700 temperatures dipping to around -5C to -8C. The colder air aloft is slightly disjointed from the precipitation shield, but most central and southwest locations above 6000 ft will see at least some accumulating snow. The core of precipitation will move into southwest Montana during the coolest part of the morning, resulting in the greatest winter impacts being exerted over the higher terrain of Madison and Gallatin counties, mostly in the form of cold and wet conditions for outdoor recreation.

Lingering precipitation diminishes later this morning as the H500 forcing weakens and moves off to the east. A cool and moist north to northwesterly flow aloft develops in the wake of the departing system and maintains scattered to broken cloud cover, widely scattered showers/thunder, and continued slightly below average temperatures for the remainder of today.

Northwesterly flow aloft will slow the warming process some for the next couple of days before weak ridging and strengthening westerly flow ushers in very warm, dry, and breezy to windy conditions by Sunday and Monday. Lower elevation temperatures warm well into the 80s during this period and there may even be a day with highs near 90 degrees depending on the amplitude of the next trough due in early next week. This next system will initially bring windy conditions, primarily to southwest MT, and increased shower and thunderstorm activity. There will also be at least a period of cooler temperatures and increased mountain snow and lower elevation rain on the backside of this system heading towards middle and end of next week. - RCG

- Forecast Confidence & Scenarios:

Areas of rain and mountain snow through this morning...

Accumulating snow looks to be largely be confined to areas above 6000 to 6500 ft and most significant over the northern portions of the Madison/Gallatin ranges and to a lesser degree along the Continental Divide, roughly from Homestake Pass northward to the Southern Rocky Mountain Front. The highest probabilities for 3 inches of snow or more are still running highest (above 80%) for the higher peaks of northern Gallatin/Madison counties where there is around a 30 to 40% chance for 6 inches of snow or more. Road impacts should fairly be isolated given the short opportunity for accumulation and warm surface conditions, but it will be cold and wet/slushy for those with mountain recreation plans. Total rainfall/liquid equivalent amounts will generally range between a tenth to a quarter inch for most central and southwest lower elevations, though there will be a sharp decreasing gradient to just a few hundreths for the drier southwest valleys along and west of I15. Rainfall amounts in excess of a half inch will be more common for areas over and near higher terrain, especially over the aforementioned higher peaks of the Madison/Gallatin ranges. Precipitation diminishes later this morning and will be replaced by Isolated shower and thunder activity during the afternoon.

Early week thunderstorms and windy conditions followed by more widespread precipitation and mountain snow...

An stronger and unstable southwesterly flow aloft develops ahead of the approaching trough Monday into Tuesday for increased shower and thunderstorm activity and more breezy to windy conditions. The concern for stronger winds will primarily be over southwest MT with some ensembles highlighting a higher amplitude trough and H700 winds approaching 50 kts Monday into Tuesday. Strong to severe thunderstorms will also be a concern, especially if the cold frontal passage is in phase with diurnal heating and the trough undergoes extensive deepening.

As mentioned in previous discussions, closed/split troughs, such as this one, often move a little slower than model guidance expectations. This may result in hotter and drier weather lingering as late as Tuesday. There will also be at least a period of cooler temperatures and more widespread precipitation/mountain snow on the backside of this system heading towards middle and end of next week, particularly if the main circulation stays close enough to exert a notable influence over the Northern Rockies. - RCG

AVIATION

21/12z TAF Period

LIFR/IFR/MVFR conditions across Southwest and Central Montana impacting the KHLN, KBZN, KEKS, and KWYS terminals will persist through 14-19z today, with periods rain and/or snow. As precipitation wanes from north to south CIGS will improve, with improvements to VFR between 21-24z this afternoon. Further north across the plains of Central and North Central Montana MVFR/low- VFR are predominately expected through 18-24z. While a low probability there still remains the potential for patchy fog over the plains where skies have partly cleared, but confidence remains too low to mention at this time. Mountain obscuration will continue through 00-06z Friday. - Moldan

PRELIMINARY POINT TEMPS/POPS

GTF 59 37 69 42 / 10 0 0 0 CTB 60 35 68 39 / 20 0 0 0 HLN 60 36 68 41 / 30 0 0 0 BZN 55 30 66 36 / 80 0 0 0 WYS 50 22 60 29 / 70 0 0 0 DLN 56 31 65 37 / 50 0 0 0 HVR 62 35 72 41 / 20 10 0 0 LWT 54 33 64 39 / 40 10 0 0

TFX WATCHES/WARNINGS/ADVISORIES

None.