textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Diminishing Showers: Light rain and high-elevation snow will taper off this evening with minimal impacts.

- Fog & Low Clouds: Valley fog and low stratus are the primary forecast concerns tonight through Wednesday. If clouds linger into the afternoons, daytime temperatures will stay cooler (40s instead of 50s).

- Low-Impact Late Week: Weak weather systems on Thursday and Friday may bring light precipitation, but significant impacts are not expected.

Through Wednesday: Widespread light precipitation continues early this afternoon as a weak closed low tracks across southern Idaho. Activity will gradually diminish this evening, leaving only isolated showers overnight. Snow levels currently between 6,500 and 7,500 feet will fall to 5,500 to 6,000 feet by this evening, though winter weather impacts will be negligible.

Skies will partially clear from north to south (from northwest Montana into central Idaho) overnight as the low departs. However, lingering boundary layer moisture combined with building high pressure will establish a favorable environment for valley fog and low stratus tonight into Monday morning. As upper-level ridging settles overhead through Wednesday, strong subsidence will promote persistent nighttime and morning inversions across the Northern Rockies. Temperature forecasts will heavily depend on inversion strength; if valley stratus lingers through the afternoon, high temperatures will be suppressed into the mid-to-upper 40s, compared to the 50s expected in fully mixed, sunnier locations.

Thursday through Sunday: A weak disturbance will bring a chance of showers on Thursday, with light snow accumulations largely confined above 5,000 feet and minimal impacts anticipated. Ensemble clusters indicate another minor disturbance passing through the region on Friday with more showers but it still appears low impact.

By next weekend, ensemble guidance strongly favors the development of a large-amplitude ridge over the Intermountain West, signaling a trend toward above-average temperatures. However, about half of cluster members depict a flatter ridge axis, which would allow Pacific moisture to clip the Idaho Panhandle and northwest Montana by Sunday. The NBM supports this flatter scenario, indicating a 20 to 30 percent probability of measurable precipitation by the 8th.

Pattern overview for the 2nd week of November: The hemispheric pattern features deep troughing over the northeast Pacific, which acts to advect mild maritime air into the western U.S. and brings sporadic precipitation into the Northern Rockies. While this setup supports above-normal temperatures aloft, a significant forecasting caveat for the Northern Rockies is the rapidly decreasing late-fall sun angle. Despite the warm air mass, weak insolation may fail to mix out the boundary layer, leading to stubborn valley stratus and fog that could easily keep daytime temperatures below their full potential. There are hints in an increased potential for widespread precipitation influenced by atmospheric rivers around the 12th or 13th.

AVIATION

A mid-level closed low continues to bring widespread rain showers along and east of the US-93 corridor in western Montana towards the Continental Divide. Forecast models and observations suggest this low will track east of the divide by this evening/overnight. The majority of shower activity will diminish during this time, with valley stratus and fog being the main operational concerns overnight into Monday morning. Recent rainfall increases confidence for stratus and/or fog impacts for KMSO, KBTM, and KHRF overnight, especially with forecast soundings suggesting a saturated boundary layer. Further north, KGPI will begin to experience clearing skies, particularly after 02/0600Z, which points towards the risk of dense valley fog. This will be trend will be monitored closely with the 00Z TAF issuance.

MSO WATCHES/WARNINGS/ADVISORIES

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