textproduct: Great Falls

KEY MESSAGES

- Showers and thunderstorms increase in coverage each afternoon through the remainder of the week.

- Trending cooler with more widespread precipitation this weekend into early next week.

- Impacts to recreation this weekend, including the risk for higher

DISCUSSION

/Issued 205 PM MDT Tue Jun 23 2026/

- Meteorological Overview:

An active pattern looks to unfold over the next week or so across the Northern Rockies. Departing upper level troughing is resulting in a zonal to slightly northwest flow aloft ahead of weak shortwave ridging. This ridging largely looks to suppress convection through the remainder of the day, though some terrain based showers still seems reasonable.

The first of several embedded waves within a building southwest flow aloft Wednesday will result in increased shower and thunderstorm coverage in the afternoon. Areas along and south of US-12 look to have the greatest probability to see thunderstorms.

A stronger wave arrives heading into Thursday, which will result in greater coverage of showers and thunderstorms across the entire region. This wave does look to arrive slightly ahead of peak afternoon heating, so there is some uncertainty in the magnitude of instability that develops before convective initiation occurs.

Forcing becomes even stronger heading into Friday as a large upper level disturbance moves southeast off the BC coastline toward WA and vicinity. While the kinematic environment improves as this troughing approaches, the thermodynamic environment looks to become a bit more questionable - largely due to ample cloud cover associated with the stronger forcing. While showers and thunderstorms are forecast across the region Friday, confidence in a stronger thunderstorm remains lower given this factor. Keeping along with the theme of better kinematics, there does look to be some gusty synoptic winds Friday, mainly south of I-90.

The core of the upper level disturbance moves across the Northern Rockies this weekend. Temperatures trend cooler as a result, presently looking coolest on Sunday. The main takeaway for the weekend into early next week is cooler with precipitation around at times. Confidence in specific precipitation amounts remains low given the uncertainty with respect to the track of the upper level low. Temperatures aloft look cool enough to support mountain snow where the upper level low ends up moving over but again, confidence in specifics remains low. -AM

- Forecast Confidence & Scenarios:

Increased shower and thunderstorm activity Wednesday through Friday...

Shortwave energy interacting with a seasonably unstable atmospheric profile will result in increased shower and thunderstorm activity during the second half of the workweek. Wednesday's storms look to be the most benign of the three days with showers and storms mostly confined to and near the higher terrain of central and southwest MT. Instability increases on Thursday for more widespread showers and thunderstorms, but there are some disagreements on the strength and evolution of the shortwave and the shear it will bring to the table. Most members favor a weaker wave with less forcing and shear availability for widespread strong to severe storms, though the forcing looks sufficient for more widespread precipitation. On paper, Friday looks to have the best overall kinematic environment for stronger storms given increased shear/forcing from the approaching Pacific NW trough; however, there's nothing really standing out as unusually strong or intense for this time of year from an instability perspective. Impacts for all three days will be mostly for those with outdoor recreation plans and will include localized strong gusty winds, hail, downpours in addition to lightning. -AM/RCG

Strong and gusty winds over southwest MT on Friday...

The combination of deep layer mixing and the strengthening southwesterly flow may bring gusty to strong winds to southwest MT, particularly over the higher terrain and the south to north oriented valleys. Ensembles seem to be trending with the stronger winds aloft developing to the east of the forecast area, though the set up still favors breezy to windy conditions over the aforementioned areas. The latest NBM probabilities for wind gusts over 50 mph, for most areas south of the I-90 corridor, have fallen slightly to the 20 to 50% range and the probability for 40 mph + sustained winds have largely fallen below 15% at this time. This trend towards lighter winds is fairly new and will be monitored in the coming days. It is also important to note that any showers or thunderstorms that develop on Friday will help transfer stronger wind gusts to the surface. Given increased summer traffic across the area and more people recreating in the mountains, stronger winds will be more impactful than the winds normally observed during the cooler season.

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

Widespread rain showers and embedded thunderstorms are in the forecast for the first half of the weekend with periods of moderate to heavy stratiform rain and even high mountain snow Sunday into Monday. The latest NBM probability for 1 inch of rain over the 72 hour period ending 6 am Tuesday is now running above 60% for virtually the entire forecast area with roughly 15 to 30% chances for 2 inches or more over most lower elevations during the same period. Mountain snow will depend on where the cold core aloft sets up and where any convective or heavier bands of precipitation artificially lower snow levels with dynamic cooling. The cold core looks to settle over the southwestern mountains Sunday into Monday, but the higher terrain along the Continental Divide and even the central ranges will need to be monitored for temporary lowering of snow levels from dynamic cooling. Snow levels during this entire event should range between 7000 to 9000 feet, lowest during the overnight and early morning hours Saturday night through Monday morning. Anyone with outdoor plans should plan 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. - RCG

AVIATION

24/00Z TAF Period

Weak ridging aloft will maintain general VFR conditions tonight, but there will be some scattered areas of low VFR clouds over the central/north-central terminals this evening. A shortwave moves in from the west and brings increased shower and thunderstorm activity on Wednesday, mostly for southwestern areas, south of I90, after 24/18Z. - RCG

PRELIMINARY POINT TEMPS/POPS

GTF 48 75 49 76 / 0 0 10 60 CTB 46 73 47 75 / 0 0 10 60 HLN 50 78 52 77 / 0 10 30 80 BZN 46 79 47 76 / 0 10 20 70 WYS 39 77 39 72 / 0 40 40 70 DLN 46 79 46 77 / 0 20 40 60 HVR 44 75 48 78 / 0 0 0 40 LWT 42 69 45 70 / 0 20 10 80

TFX WATCHES/WARNINGS/ADVISORIES

None.