Polson, Montana Weather Forecast Discussion

209
FXUS65 KMSO 182017
AFDMSO
Area Forecast Discussion
National Weather Service Missoula MT
217 PM MDT Thu Sep 18 2025
.DISCUSSION...
KEY MESSAGES:
-  Warm, sunny, dry conditions continue through Saturday.
-  Westerly winds with gusts 30 to 40 mph Sunday could create
   hazardous, choppy conditions on area lakes, especially Flathead
   Lake where a wind advisory may be needed.
-  Low-lying areas could experience their first frost Monday
   and/or Tuesday morning (30-70% chance).
-  Brief ridge builds early next week before a more unsettled
   pattern could return from Thursday onwards.
The ridge over the region continues to provide sunny skies and
temperatures 10-15 degrees above average through Saturday. Patchy
fog is possible in some of the valleys. Frost is possible in the
high elevation valleys.
Attention then turns to a Pacific trough set to move across the
region on Sunday. There continue to be some timing differences in
the guidance regarding the main cold front passage, but the window
appears to be opening for the later morning through the afternoon
hours. Regardless of exact timing, winds are expected to ramp up
significantly through the afternoon and evening. Current numbers
suggest the potential for wind gusts ranging between 25 and 40 mph
across the region. Gusty, erratic winds are possible with showers.
Hazardous waves of 2 to 4 feet is possible on Flathead Lake and
this could necessitate a lake wind advisory.
Moisture remains limited, but forcing with the front and some
marginal instability with the trough, could support scattered
showers and a few weak thunderstorms. The best instability is found
across northwest Montana and from the Bitterroot Valley south
into Lemhi County and east towards Butte. Any showers that develop
will be capable of mixing down stronger gusts, potentially to 40
mph.
Behind the front, a cooler airmass settles in for early next
week.There is some question as to how quickly clouds will clear
out Sunday night, which will be a key factor for overnight lows.
Have trended temperatures a little cooler, especially for Tuesday
morning, to highlight the potential for frost in the typical low-
lying, cold- prone valleys. A warm ridge quickly builds back over
the region for the middle of the week.
Ensemble guidance then depicts a possible shortwave trough ejecting
out of California and tracking north into our area by Thursday,
which could bring another chance for showers. Confidence
decreases into next weekend, but there is a growing signal for a
broader breakdown of the ridge, pointing toward a higher
probability of a cooler and more unsettled period.
&&
.AVIATION...High pressure continues to dominate the weather for
the region through Saturday. With longer nights to cool off and
dew points in the 40s in most valleys, patchy fog is a concern
Saturday morning. On Sunday, an approaching cold front will bring
the main aviation impacts. Expect increasing southwest winds with
gusts of 25 to 35 kts developing through the afternoon.
&&
.MSO WATCHES/WARNINGS/ADVISORIES...
MT...None.
ID...None.
&&
$$

NWS MSO Office Area Forecast Discussion