textproduct: Great Falls

KEY MESSAGES

- Mainly mountain showers today, with dry and cool conditions elsewhere.

- Another cold night is in store tonight across the valleys of Southwest and Central Montana, with a high chance for a hard freeze.

- Temperatures moderate through rest of the work week, but daily chances for showers exist.

UPDATE

Lingering northerly flow aloft has resulted in a few areas of more persistent upslope snow in a few mountain areas. The only change this morning was to increase probability of precip and snow amounts in these areas through the remainder of the morning. Precipitation becomes more showery in nature this afternoon as daytime heating ensues, and largely looks to be confined to the mountains and adjacent areas. -AM

DISCUSSION

/Issued 517 AM MDT Mon May 18 2026/

- Meteorological Overview:

One final day of well below normal temperatures can be expected as cool, cyclonic northwest flow prevails over the Northern Rockies, with additional opportunities for predominately mountain precipitation, especially along the Rocky Mountain Front. By this afternoon upper level ridging over the Eastern Pacific will begin to amplify, with overall rising heights across much of the Western CONUS. While this ridge will amplify over the Eastern Pacific it will fail to build east and over the Northern Rockies through the work week, which will help to keep Southwest through North Central Montana beneath continued northwest flow aloft. While temperatures will moderate beneath the rising heights aloft there will be daily chances for showers and storms throughout the work week, especially from Wednesday morning through Thursday morning when a compact shortwave dives southeast and over the Northern Rockies within the aforementioned northwest flow aloft. This shortwave looks to bring the best chance for widespread precipitation this week, with most lower elevations seeing light rainfall up to 0.10" and the mountains seeing between 0.10" to 0.25". The exception to this will be the northerly upslope areas of Central Montana, Island Ranges of Central Montana, and Continental Divide where precipitation amounts of between 0.10" to 0.25" and 0.25" to 0.50" are possible for plain and mountain locations, respectively. This weekend the upper level flow will become quasi-zonal, which will lead to increasing surface winds and warming temperatures, with most locations climbing above to well above normal. - Moldan

- Forecast Confidence & Scenarios:

Cold Temperatures Tonight...

Upper level ridging building in over the Northern Rockies combined with overall light surface winds and mostly clear skies will setup ideal radiational cooling processes across much of Southwest through North Central Montana tonight, with the exception of the immediate Rocky Mountain Front where southwest surface wind will be just strong enough to inhibit surface decoupling. With below normal temperatures already in place thanks to this weekends disturbance temperatures will cool efficient tonight; with a 50% or greater chance that values fall below freezing across most locations and 70% or greater chance for a hard freeze across most valley locations in Southwest and Central Montana. Those with early season gardening interest should be prepared to take protective measure for sensitive vegetation.

Below is the NBM5.0 probabilities for certain temperatures for select locations across North Central, Central, and Southwest Montana tonight.

Low Temperature Probabilities LOCATION <36F | <32F | <28F Browning 100% | 55% | <5% Cut Bank 95% | 60% | 20% Havre 100% | 80% | 40% Great Falls 95% | 60% | 25% Lewistown 100% | 100% | 85% Helena 85% | 45% | 5% Bozeman 100% | 100% | 85% Dillon 100% | 100% | 70% Ennis 100% | 100% | 90% West Yellowstone 100% | 100% | 100%

Gusty Winds and Choppy Water this Memorial Day Weekend...

Increasing south to west surface winds over the holiday weekend, specifically on Sunday and Monday, is likely to lead to choppy water conditions on area lakes, reservoirs, and long fetches of rivers. NBM5.0 probabilities for wind gusts in excess of 40 mph across most of Southwest through North Central Montana range from a 20-40% chance on Sunday and between a 40-60% chance on Monday, with the exception of the Rocky MOuntain Front where a 40-75% chance exists both days. Those with plans to recreate on waterways should be prepared for choppy conditions, especially considering that local waterways will be significantly colder than the ambient air temperature due to snowmelt. These choppy conditions and cold water temperatures will create an increased risk for drowning should a person fall into the water without proper safety equipment. - Moldan

AVIATION

18/12Z TAF Period

Primary concern throughout the 1812/1912 TAF period will be the potential for fog at the KGTF and KCTB terminals through 14-16z this morning; however, confidence in this occurring is low so for now kept conditions to temporary MVFR with the potential for VLIFR/LIFR conditions. Otherwise IFR/MVFR/low-VFR CIGS will improve to low-VFR/VFR through 16-20z today. Mountain obscuration will persist through 00-06z tonight. - Moldan

PRELIMINARY POINT TEMPS/POPS

GTF 52 29 63 40 / 10 0 30 10 CTB 54 31 63 38 / 20 10 20 10 HLN 55 29 62 38 / 10 10 20 0 BZN 51 22 60 34 / 10 0 10 0 WYS 41 16 51 24 / 10 10 20 0 DLN 48 23 60 30 / 10 10 20 0 HVR 54 26 66 41 / 10 0 50 10 LWT 46 23 59 36 / 40 0 30 0

TFX WATCHES/WARNINGS/ADVISORIES

None.