textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Today through Friday Fog & Low Cloud Potential: Strong high pressure increasing likelihood of valley inversions.

- Pattern Shift: A Pacific system will tap into atmospheric river moisture beginning Saturday, snow levels stay relatively high but precipitation amounts very light.

- Further cooling early to mid next week with potential for snow levels to lower further.

Today through Friday: High pressure builds into the Northern Rockies today and will persist through Friday, setting up strong valley inversions and stagnant conditions. The primary forecast challenge over the next few days will be the extent and duration of fog and low stratus. Recent precipitation has left abundant low-level moisture across most of the area, with the exception of Lemhi County and southwest Montana. Therefore, as areas of partial clearing occur this morning, areas of valley fog will likely develop. If this fog becomes deeply entrenched it may be stubborn to mix out, though most guidance suggests enough warming and dry air aloft to dissipate the majority of it by midday.

Looking ahead to tonight and Thursday night, incoming high clouds could help limit new fog formation. However, strong radiational cooling is still favored, meaning valley fog is likely to return each night. Additionally, with Flathead Lake surface temperatures still around 50F, the cold, stable airmass will likely allow the Lake to fuel stratus development across the Flathead and Mission valleys (and potential even more widespread). Finally, if fog and stratus become more widespread or persistent than expected, daytime valley high temperatures will likely fall short of current forecasts.

Saturday through Next Week: The next chance for precipitation arrives Saturday morning as southwest flow establishes itself and a decaying atmospheric river clips the Northern Rockies. A weak cold front will cross the region, but models have trended drier with what was already expected to be a light precipitation event. Current ensembles show less than a tenth of an inch of liquid equivalent, even in typically favored wetter locations. While snow levels will hover between 5,500 and 6,000 feet, winter weather impacts will be negligible, with only a dusting of snow expected over mountain passes.

For next week, ensemble guidance strongly favors a split-flow pattern. While successive upper-level troughs and atmospheric rivers will impact the West Coast, the bulk of the dynamic energy and moisture will dive south into California and the Desert Southwest. For the Northern Rockies, this translates to generally light precipitation and a return to seasonable temperatures (which will feel notably colder, roughly 10 to 15 degrees cooler than our recent mild stretch). Mountain passes may see occasional light snow, but widespread or significant travel impacts are not supported by the majority of ensemble clusters. One thing to watch, about 25% of ensemble clusters support a weak closed low Tuesday, this would allow for moderate pass level snow impacts, stay tuned!

AVIATION

High pressure builds across the Northern Rockies today and will persist through Friday, leading to stagnant conditions and strong valley inversions. Recent precipitation has left abundant low- level moisture across much of the region, with the exception of Lemhi County and southwest Montana. Consequently, as mid- to upper-level clouds clear this morning, expect areas of dense valley fog to develop, resulting in localized IFR/LIFR conditions.

If this fog becomes deeply entrenched, it may be stubborn to mix out. However, most guidance suggests sufficient daytime heating and dry air entrainment aloft will lift and dissipate the fog by midday, returning most terminals to VFR. Looking ahead to tonight through Friday, periods of high clouds could occasionally limit widespread fog development. Despite this, strong radiational cooling is still expected to drive recurring overnight and early morning valley fog. Furthermore, with Flathead Lake surface temperatures remaining near 50F, the cold and stable airmass will all= ow the lake to act as a localized moisture source, fueling low stratus and fog across the Flathead and Mission valleys (potentially impacting KGPI) through the end of the week.

MSO WATCHES/WARNINGS/ADVISORIES

MT...None. ID...None.