textproduct: Great Falls

KEY MESSAGES

- There will be patchy dense morning fog for the next day or so, mostly over the southwest valleys and the Milk River Valley.

- Temperatures trend warmer each day, with 60s and 70s returning to most locations by the weekend.

- Gusty westerly winds develop along the Rocky Mountain Front and the plains on Saturday.

- Pacific weather systems look to bring cooler temperatures and periods of rain and snow next week.

UPDATE

Evening satellite imagery shows areas of low clouds/stratus lingering across portions of SW MT, where subsidence beneath a positively tilted upper level ridge has trapped lower level moisture. Main weather concern through tomorrow morning will be the potential for the redevelopment of locally dense fog, primarily for the Gallatin Valley and Missouri Headwaters region north through the Canyon Ferry area. These areas saw patchy fog last into early Tuesday afternoon and are most likely to see the quickest redevelopment tonight and greatest risk for visibility less than 1/2 mile. The risk for patchy fog exists elsewhere across the remainder of southwest MT as well as well as through the Milk River Valley of Hill and BLaine counties. Otherwise, the development of steadier southwest surface winds across western portions of north-central MT through the overnight period will help mitigate the potential for fog there. Hoenisch

DISCUSSION

/Issued 520 PM MDT Tue Oct 13 2026/

- Meteorological Overview:

A closed mid- level low will continue moving through the Great Basin and into the SW CONUS tonight into Wednesday. The CWA will generally be unaffected by this system aside from some spotty rain/snow showers along the MT/WY/ID borders through this evening. Otherwise, light winds and lingering surface moisture will favor some patchy fog redevelopment later tonight into Wednesday morning. The southwest wind protected valleys and the Milk River Valley will see the highest concentration of fog.

A trough and attendant frontal system looks to dive southeastward through the plains Wednesday into Thursday. This shouldn't exert much of a change to the sensible weather other than some gusty pre- frontal winds over the plains followed by a brief northerly wind shift.

Ridging aloft then develops during the second half of the workweek, which will bring a return to above average temperatures heading into the weekend. The ridge will be broken down by a broad trough that quickly passes through the Northern Rockies and brings gusty to localized strong winds to the Rocky Mountain Front and the plains Saturday into Sunday.

Nearly all ensemble suites highlight troughing settling over the western CONUS for much of next week; however, the individual members indicate the troughs will be undergoing shearing, which decreases forecast positioning and timing confidence. Overall, there's a general expectation of cooling temperatures and increased instances of rain and snow next week, especially for mountain areas. - RCG

- Forecast Confidence & Scenarios:

Patchy dense fog again tonight...

Light winds and lingering surface moisture will encourage patchy dense fog to develop again tonight into Wednesday morning. The latest HREF probabilities for visibility falling to less than a half mile are now running above 50% from Gallatin valley northward to Helena and then over the Milk River Valley. Confidence for impactful visibility reductions is highest in Bozeman/Belgrade and nearby areas given today's fog stubbornly lingering into the afternoon hours. A dense fog advisory has been issued for the Gallatin Valley and the neighboring Canyon Ferry and Missouri Headwaters zones. Will hold off on fog products for the Helena area and the Milk River Valley for now, but these areas will continue to be monitored heading into this evening. - RCG

AVIATION

14/00Z TAF Period

VFR conditions will prevail at all but the KBZN and KHVR terminals during this TAF Period. At the KBZN, KHVR, KHLN, and KWYS terminal after 14/04Z fog will affect these terminals. At the KBZN and KHVR terminals fog will at least intermittently reduce visibility down to 1 mile between 14/10Z and 14/14Z. During that time period there is a 20% chance for visibility to be reduced to one half mile. Around the KCTB terminal between 14/10Z and 14/17Z there will be isolated instances of low-level wind shear. -IG

PRELIMINARY POINT TEMPS/POPS

GTF 30 57 32 54 / 0 0 0 0 CTB 28 51 29 48 / 0 0 0 0 HLN 26 58 29 58 / 0 0 0 0 BZN 20 57 27 59 / 0 0 0 0 WYS 16 57 19 59 / 0 0 0 0 DLN 20 53 26 56 / 0 0 0 0 HVR 22 59 30 57 / 0 0 0 0 LWT 21 58 32 55 / 0 0 0 0

TFX WATCHES/WARNINGS/ADVISORIES

Dense Fog Advisory until noon MDT Wednesday for Canyon Ferry Area-Gallatin Valley-Missouri Headwaters.