562 FXUS65 KMSO 220828 AFDMSOArea Forecast Discussion National Weather Service Missoula MT 228 AM MDT Mon Sep 22 2025
.DISCUSSION...
KEY MESSAGES:
- Widespread valley frost possible Tuesday morning, especially western Montana.
- Warming and drying trend through late week.
The cold front that brought widespread showers and isolated thunderstorms to the region yesterday has moved east of the Continental Divide this morning. Isolated rain showers have lingered overnight on the backside of the trough and will continue to move east through the morning hours. For most, today will be mostly sunny with afternoon temperatures around normal for this time of year; however, smoke from wildfires in Washington may cause hazy skies, especially in north-central Idaho and west- central Montana this afternoon and Tuesday.
With clearing skies, diminishing winds, and a cool, dry airmass moving in, conditions will become favorable for the first widespread valley frost of the season Tuesday morning. Sheltered, low-lying areas across western Montana and the higher valleys of north-central Idaho have the highest potential for seeing temperatures drop to near or below freezing. The probability for frost is highest (90%) in the Deer Lodge, Butte, and Anaconda areas, with lower probabilities (50-70%) for the valleys along Highway 93.
High pressure will build back over the Northern Rockies mid-week, causing high temperatures to moderate. Afternoon temperatures are expected to return to the low to mid-80s in western Montana and low 90s in north-central Idaho by Wednesday. These temperatures will remain 10 to 15 degrees above normal until Friday, when the ridge begins to flatten, ushering in another day of breezy westerly winds.
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.AVIATION...VFR conditions will prevail for most of the period across western Montana and north-central Idaho. Lingering isolated rain showers on the backside of a departing trough may bring brief lowering ceilings along the Continental Divide through 15Z this morning. Clear skies and calm winds overnight will be conducive to widespread valley frost development by Tuesday morning.
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.MSO WATCHES/WARNINGS/ADVISORIES... MT...None. ID...None. &&
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NWS MSO Office Area Forecast Discussion