textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Strong valley inversions will persist for several more days

- Coldest temperatures this forecast period Friday into Saturday

- Uncertainty in the forecast past Saturday

A continuation of a persistent pattern of dry high pressure is expected through the weekend. Lower valleys can expect morning fog and low stratus, while higher valleys will likely break out of valley inversions each day.

Temperatures will slowly cool, to a few degrees below normal for this time of year, Friday through Saturday morning. This will be due to an Arctic air mass east of the Continental Divide seeping into Western Montana. This air mass is originating from the continental Arctic, which simply means it will be cold and generally dry. That won't rule out a light shower or flurry as the cold front moves through.

Saturday onward the forecast has considerable uncertainty. While the NBM, which is the starting point of our forecast, is locked into a strong ridge of high pressure redeveloping. There is a small percentage of outlier models that have precipitation Monday into Tuesday, with a temperature profile that would suggest snow for most elevations. These outliers when averaged out against the drier models, leave a forecast of slight chance or 25% or less for precipitation, widespread across our region. Something to watch as the models come better into focus. AI modeling, which is not part of the NBM, is predicting a light precipitation event.

AVIATION

Low stratus and fog lingers across most of northwest Montana (north of Highway 200) and in the lower elevations of north central Idaho, including the Camas Prairie in Idaho. These areas will likely see little change in conditions, other than that the stratus deck may lower with time this evening and overnight as more stable conditions develop aloft. This may be most prominent for airfield KGPI between 21/0400z and 21/1500z. The rest of western Montana and north central Idaho will likely continue to see generally clear skies. There is some question on how things will evolve specifically for KMSO. Recent drying in the boundary layer makes fog unlikely. However, with low stratus to the north over the Mission valley and to our west near Saint Regis and Plains, there is some risk that stratus will spread into KMSO after 21/0600z. Confidence in this occurring is low, but will be monitored for future forecasts. A weak wave of energy moving through western Montana tomorrow morning (21/1500z) may decrease stability enough to lift the stratus deck slightly in northwest Montana, though likely will not scour the stratus out.

MSO WATCHES/WARNINGS/ADVISORIES

MT...None.

ID...None.