Marine Weather and Tide Forecast for Big Sky, MT

DISCUSSION
/Issued 211 PM MDT Sat May 17 2025/

- Meteorological Overview:

Southerly flow aloft becomes more established over the Northern Rockies this afternoon and tonight in response the approaching Pacific trough. The combination of diurnal heating and a northward advancing elevated warm front will initiate isolated to scattered showers and thunderstorms over Central and North-central Montana.
With ML CAPE values running around 200 to 300 J/kg, highest over Hill, Blaine, and Fergus counties, this storm activity should not get very rambunctious. An area of more widespread showers and thunderstorms are then expected to move into Southwest Montana later this afternoon. Higher ML CAPE around 500 j/kg is forecast to reach just south of the MT/ID border, so this activity will need to be monitored for gusty winds.

This activity will transition into an area of widespread stratiform rain and mountain snow as it moves northward tonight into Sunday, with Hi-res guidance placing the swath of heaviest rainfall along the Rocky Mountain Front and the adjacent high plains. The overall positioning of mid-level trough features haven't changes from yesterday with the mid- level low moving southeastward from the Pacific NW through ID and eventually into the Great Basin. At the H700 level, one circulation looks to develop late tonight along the Continental Divide before swinging northeastward into Canada on Sunday. This matches well with the newer Hi-Res guidance placing the heavier half to one inch amounts along the Rocky Mountain Front and adjacent high plains tonight through Sunday afternoon. A secondary low then looks to spin up over South-central and Southern Montana, which also matches some of the Hi-res guidance of placing heavier totals for locations east of I15 Sunday into Monday. Interior areas of Central/North- central Montana and the lower elevations of the southwest still look to see rainfall, just a little less, more on the order of a quarter to two thirds of an inch.

Snow levels begin to drop off toward the 6,000 to 7,000 foot level along the Continental Divide and portions of the southwest by Sunday morning. Then the colder air spills eastward with snow levels dropping to the 4,500 to 6,000 foot level for most locations by Monday morning. Throughout the event, there is a mismatch between the lowest snow levels and the heaviest precipitation, limiting the overall potential for impactful snow at and below pass level. With that being said, heavier precipitation will have the potential to lower snow levels by 500 to 1,000 feet, especially during the overnight and early morning hours. Southwestern locations along and west of interstate 15 will be monitored for accumulating snow tonight into Sunday morning as well as the Glacier National Park area before the focus shifts eastward Sunday night into Monday.

Most of the widespread precipitation clears out by Monday, but the overall unsettled weather pattern will continue for the remainder of the week. A progressive westerly flow will send shortwaves through the Northern Rockies for periods of scattered showers and thunderstorms and breezy conditions, though there are uncertainties with the timing and positioning of each system.
Slight below average temperatures moderate seasonal averages.
- RCG

- Forecast Confidence & Scenarios:

Widespread rain and mountain snow tonight through Monday morning...

Model guidance is narrowing down the potential for heavier precipitation to a few areas, The Rocky Mountain Front/adjacent high plains and areas east of I15, especially over and neat the Central Montana island ranges. This roughly matches the NBM probabilistic guidance with a greater than 40% chance for an inch or more liquid equivalent precipitation. These areas are forecast for half to one inch totals. The Exceptions are in Southwest Montana and the northern high plains east of the Glacier National Park area. These excepted areas in addition to the interior areas of Central/North- central Montana and the remaining lower elevations of the southwest still look to see rainfall totals on the order of a quarter to half inch. West and southwest locations will see their peak rainfall tonight into Sunday while the heaviest rain holds off until later Sunday into Monday for Central/North-central areas east of I15. Overall, impacts are expected to be limited, but the situation will be monitored for potential hydrologic impacts, including unexpected water rises.
Please see the hydrology section for more information.

Confidence in accumulating snow at or below pass level has lowered with the colder air looking to be more disjointed from the core of heaviest precipitation for much of this event. The coldest H700 temperatures will remain over the eastern portion of upper Idaho and not reach Southwest Montana until the precipitation is wrapping up Sunday afternoon and evening.
Probabilistic guidance hasn't changed much with the greatest chances (70% +) for 2 inches of snow or more still above 7,000 feet. Of course areas receiving heavier precipitation, even briefly, may see their snow levels briefly drop an additional 500 to 1,000 feet. With summer activities starting to ramp up and still some lower of snow levels not out of the cards, I'm giving more consideration for winter products. Winter weather advisories have been sent out for the higher terrain along the Continental Divide, with the strongest confidence along the Rocky Mountain Front, including Glacier National Park area, and then the mountains in northwest Beaverhead County. impacts will be primarily for cold and wet outdoor recreation, but slippery and slushy roads can be expected as well, particularly for the secondary forest roads. More consideration will be given over the next 24 hours for winter products over and near the central island ranges as well as the Gallatin/Madison mountain ranges for Sunday night into Monday. - RCG

AVIATION
18/00Z TAF Period

Showers and thunderstorms will continue to develop across Idaho and Southwest Montana through the remainder of the afternoon and early evening hours, with these showers and storms lifting north to northeast through the remainder of the evening hours tonight.
Main threat from any thunderstorm, especially between 00-03z Sunday, will be gusty and erratic wind gusts of between 30-40kts.
Those terminals most at risk of seeing these showers and storms will be KWYS, KEKS, KBZN, and KLWT (due to the proximity to the Snowy Mountains). By late this evening and early Sunday morning precipitation will become more stratiform in nature, with widespread rain lifting north and across the remainder of Central and North Central Montana. Predominately VFR conditions will prevail through 06-12z Sunday, with IFR/MVFR conditions expected thereafter as precipitation becomes stratiform in nature.
Mountains will become increasingly obscured through 06z Sunday, with complete obscuration expected thereafter. - Moldan

Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information.

HYDROLOGY

A Pacific weather system will bring periods of widespread rain and mountain snow tonight through Monday morning. The precipitation will begin in the form of scattered showers and thunderstorms over Southwest Montana this afternoon before gradually transitioning to areas light to moderate rainfall as it expands into northern areas tonight and on Sunday. The most persistent rain and mountain snow will shift eastward and mostly impact areas east of interstate 15 Sunday night into Monday morning.

Total liquid precipitation amounts will generally fall between a half and one inch with totals in excess of an inch along the Continental Divide and over Central/North-central locations east of interstate 15. Snow levels begin to lower in a west to east fashion on Sunday, falling to the 5,000 to 6,000 feet level by Monday morning, lowest along the Continental Divide and Southwest Montana. A few rivers are forecast crest near bankfull towards the end of the event. This includes the Gallatin River at Logan and the Jefferson River near Three Forks. Water levels for rivers and creeks originating over the Rocky Mountain Front and the Central Montana mountain Ranges will also be monitored. Flooding impacts are generally not expected; however, those near waterways or areas normally susceptible to flooding should remain situationally aware of their surroundings in case localized heavier rainfall brings unexpected water rises. - RCG

PRELIMINARY POINT TEMPS/POPS
GTF 45 54 41 60 / 90 100 50 20 CTB 41 51 38 58 / 40 100 50 10 HLN 43 54 38 62 / 100 90 30 20 BZN 43 52 32 59 / 80 100 50 20 WYS 34 47 27 52 / 80 100 70 20 DLN 40 49 29 57 / 90 90 20 10 HVR 44 56 40 57 / 60 90 90 60 LWT 42 56 36 52 / 60 90 90 60

TFX WATCHES/WARNINGS/ADVISORIES
Winter Weather Advisory from 6 AM to 6 PM MDT Sunday for East Glacier Park Region-Elkhorn and Boulder Mountains-Southern Rocky Mountain Front-Upper Blackfoot and MacDonald Pass.

Winter Weather Advisory from midnight tonight to 6 PM MDT Sunday for Northwest Beaverhead County.