textproduct: Great Falls

KEY MESSAGES

- Showers and thunderstorms become more widespread today and Friday with some storms producing localized strong wind gusts, hail, and heavy downpours in addition to lightning.

- Strong and gusty southerly winds will also occur on Friday across Southwest Montana, especially within the north-south orientated valleys and over mountain passes along the Idaho border.

- Showers and thunderstorms continue on Saturday followed by cool, wet, and raw conditions with some high mountain snow Saturday night through Monday.

- Temperatures rebound heading towards the middle of next week, but there will be daily opportunities for scattered showers and thunderstorms.

DISCUSSION

/Issued 1205 AM MDT Thu Jun 25 2026/

- Meteorological Overview:

Shortwave energy and diffluent flow aloft will move over the Northern Rockies today in response to a deepening Pacific NW trough. This will send multiple waves of showers and thunderstorms moving through southwest and north-central MT, most widespread this afternoon and evening. HIRES guidance has been steadily increasing instability over the last few days and now depict at least a brief period with ML CAPE values running in the 700 to 1200 J/kg range. This will be complimented by 25 kt + bulk shear and PWATS approaching three quarters of an inch, resulting in a few stronger multi cellular storms capable of producing strong wind gusts, hail, and heavy downpours. The one caveat that may limit diurnal heating will be cloud cover, especially if the morning convection comes in more widespread than advertised; however, the boundary layer will be quick to recover with both the maximum sun angle and day length observed this time of year.

On Friday southwesterly flow will become firmly established over the Northern Rockies with PWATS approaching and even exceeding the one inch mark for the northern half of the forecast area according to the North American Ensemble Forecast System (NAEFS). Similar to Thursday, cloud cover and convective contamination may compromise diurnal heating and ultimately tame instability for much of the forecast area, though bulk shear values rising to the 30 to 40 kt range will make up for at least some CAPE loss. Ensemble guidance is now starting to single out central/north-central areas along and southeast of the highway 87 corridor for ML CAPE in the 600 to 1000 J/kg range with around 300 to 600 J/kg for southwestern areas east of I15. Should this hold true, then these areas will be highest at risk for strong to severe thunderstorms. Strong flow aloft over the southwest will also play a role in transferring convective gusts to the surface. In addition to the severe threat, the anomalously high PWATS will also add a localized heavy rain threat that may cause localized flooding of normally susceptible areas such as burn scars and flood prone urban areas. Scattered to numerous shower and thunderstorm activity will continue into Saturday via the southwesterly flow remaining in place. Abundant cloud cover should dampen instability, but convectively driven areas of heavier rainfall will still be a threat, especially heading into the afternoon and evening hours.

The main trough will move through the Northern Rockies Saturday night through early Monday. This system is still looking to undergo stretching, shearing, and a low pressure center hand off as it moves east. Even though it appears the main area of forcing will be close enough to bring widespread rainfall to most locations, these transformations it will be difficult to decipher the precise areas of heavier rainfall as will the presence of convective enhancement. The cold core with this system looks to be rather impressive with H700/H500 temperatures running between 2 to 3 standard deviations below average according the the NAEFS. Even though the cold core aloft will be centered a little farther south of the forecast area, there will be instances of high mountain snow over most mountain areas given the colder air aloft and dynamic cooling from heavier precipitation banding. This will primarily be for elevations above 6500 ft during the AM hours Sunday and Monday and will be most widespread over the southwest. There will also be gusty winds with this system, especially on the backside of it on Monday. Overall, expect a cool, wet weekend, especially Sunday and Monday. Once this system exits the region later on Monday, there will be warming and drying heading into the middle and end of next week; however, a rather unsettled west to southwesterly flow aloft will remain in place and passing shortwaves will maintain daily opportunities for scattered showers and thunderstorms. - RCG

- Forecast Confidence & Scenarios:

Periods of strong to severe thunderstorms and localized heavy rain today through Saturday...

The evolution of the morning showers and thunderstorms will play a role in determining the strength of the afternoon and evening storms. Current trends indicate that the first round of storms will end over southwest MT by dawn, over shortly after, while the activity farther north lingers later in the morning and even into the afternoon for locations east of I15. There should be sufficient boundary layer recovery over the southwest and the far eastern portions of central/north-central MT may remain unstable prior to the arrival of the first round. Areas east of I15 will be most prone to strong to marginally severe storms with localized strong wind gusts, hail, and heavy downpours.

Even more more cloud cover and convective contamination will be present on Friday, though the wind shear will be more favorable for storm maintenance should they develop. The current target area for strong to severe storms on Friday will be for central/north-central areas along and east of the highway 87 corridor and to a lesser degree southwestern areas east of I15. While there is still a spread on how unstable the environment will be, the higher end guidance supports sufficient shear and CAPE for rotating supercells. The thunderstorm hazards for Friday will have a higher ceiling than Thursday and may include strong wind gusts, large hail, and heavy downpours. There will also be a little more focus for heavy rains and localized flooding of normally flood susceptible areas such as burn scars and prone urban areas. With that being said, storm motion over 25 kts should shorten storm residence time and the opportunity for flooding. By Saturday, the most robust instability looks to move east of the forecast area, but the threat for localized heavy rain will continue. - RCG

Gusty winds over southwest MT on Friday...

Anomalous, with respect to late June climatology, southerly H700 winds of 30-40 kts will overspread Southwest Montana from the late morning through early evening hours. This timing with the flow orientation, with respect the north-south valleys and mountain passes along the Idaho border, will help to support a period of strong and gusty winds due to mixing and terrain funneling. Additionally, any showers or thunderstorms that develop in this environment will help transfer stronger wind gusts to the surface. Latest NBM 5.0 probabilities support a 70 to 90% chance that gusts exceed 40 mph along and south of a Dillon, to Ennis, to Big Sky line, with even a 50 to 80% chance that gusts exceed 50 mph. Furthermore there is even a 20% chance that sustained winds within the Madison Valley reach 40 mph for a period of time from late Friday morning through the early evening hours. While these probabilities and deterministic guidance do not currently support high wind highlights it will be none-the-less windy on Friday, which will cause some difficulties for those operating high profile vehicles. Those with recreation plans in the mountains should also beware of their surroundings and watch for falling trees, especially if there is lingering damage from the cool season winds. - Moldan

Cool conditions with widespread precipitation and high mountain snow this weekend...

Widespread rain showers and embedded thunderstorms are in the forecast late Saturday through early Monday. The latest NBM probability for 1 inch of rain or greater over the 72 hour period ending 6 am Tuesday is now running between 60 and 80% for virtually the entire forecast area with roughly 50 to 70% chances for 2 inches of rain or greater over the higher terrain of the Northern Rocky Mountain Front and over the Little Belts during the same period. The focus of the heavier rain looks to be generally concentrated over central and southwest MT Saturday through early Sunday before heading northward over the plains and north to the Canadian border Sunday into Monday. Mountain snow looks to be most widespread over the higher terrain of the southwest, but other mountain areas that see convective or heavier precipitation banding will see snow levels artificially lowering, at least on a temporary basis. This includes the Logan Pass area and the King's Hill area in the Little Belts. Snow levels during this entire event should range between 6500 to 8000 feet, lowest during the overnight and early morning hours Saturday night through Monday morning. Anyone with outdoor plans should be prepared for cool and damp conditions that may be dangerous to those not properly dressed. Also forest and rural roads may become muddy or even impassible from the wet conditions. Area rivers, creeks, and streams should be able to handle the widespread 1 to 2 inch rainfall totals, although there will be water rises and localized ponding of water or minor flooding, particularly for the normally susceptible low lying area or other prone areas including sensitive burn scars. - RCG

AVIATION

25/12Z TAF Period

VFR conditions will predominately prevail across all terminals through the 2512/2612 TAF period; however, increasing shower and thunderstorm activity will bring MVFR/low-VFR conditions at times, gusty and erratic winds, and mountain obscuration. While most of the next 24 hours will feature scattered showers and storms, areal coverage and intensity of precipitation will peak between 17-23z Thursday before decreasing thereafter. - Moldan

PRELIMINARY POINT TEMPS/POPS

GTF 76 53 76 50 / 70 30 80 80 CTB 73 51 68 47 / 70 40 80 30 HLN 80 55 75 50 / 70 30 90 90 BZN 77 50 78 46 / 70 40 70 90 WYS 71 42 72 39 / 80 40 40 50 DLN 78 52 73 45 / 50 20 60 90 HVR 78 50 79 50 / 40 30 60 100 LWT 70 46 75 46 / 70 50 80 100

TFX WATCHES/WARNINGS/ADVISORIES

None.