textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Near-critical to critical fire weather this afternoon and evening from hot, dry and breezy conditions.

- Wildfire smoke will continue to impact the region.

A large ridge of high pressure will maintain hot, dry conditions across western Montana and north-central Idaho into early next week. An upper level wave seen in GOES Water Vapor imagery approaching the Pacific Northwest coast this morning is projected to flatten the ridge by this afternoon and evening. As the wave passes, mid-level winds will increase, yielding stronger afternoon wind gusts. The combination of heat, single-digit to low- double- digit relative humidity, gusty winds, and receptive fuels will drive critical fire weather conditions. Red Flag Warnings remain in effect, with peak concerns focused across southwest and west- central Montana, as well as the Palouse and Hells Canyon regions.

Wildfire smoke remains the primary forecast challenge. Wildfires remained active upstream in Oregon and Washington overnight, with plumes of smoke extending downstream into the Northern Rockies. Widespread smoke will delay valley inversion breaks, with dense smoke remaining trapped through the morning hours. Notably, persistent smoke cover may attenuate solar radiation and suppress atmospheric mixing into the early afternoon, locally capping maximum temperatures and wind gusts below forecast for some areas. Nonetheless, the incoming upper level wave will help improve vertical mixing, focusing the fire weather threat of hot, dry, and windy conditions to the mid-afternoon to early evening timeframe.

Subtle monsoonal moisture advecting from the south will introduce a low chance of isolated thunderstorms by late afternoon through this evening, mainly across southwest Montana and southern Lemhi County. Due to an exceptionally dry sub-cloud layer, rainfall will largely evaporate as virga, creating the risk for dry lightning ignitions and sudden, erratic microburst gusts between 40 and 55 mph.

Monday through Next Friday:

Confidence is gradually increasing that mid-level moisture will settle in by mid-to-late next week, sustaining periodic thunderstorm chances rather than producing widespread, soaking precipitation. Higher confidence exists for temperatures moderating back toward seasonal normals in the mid-to-upper 80s. A major weather pattern change, such as a strong, deep storm system from the Pacific Northwest, is necessary to significantly suppress fire activity and clear the smoke. Current forecasts show only a 20 to 30 percent chance of such a system arriving by the middle to late portions of next week.

AVIATION

Widespread wildfire smoke remains the primary aviation hazard. Wildfires remained active upstream in Oregon and Washington overnight, with plumes of smoke extending downstream into the Northern Rockies. Sharp valley inversions this morning have trapped smoke near the surface, with areas of MVFR to localized IFR visibilities (along with locally reduced ceilings). Daytime heating will promote boundary layer mixing and yield modest surface visibility improvements by this afternoon; however, enhanced ventilation will also invigorate regional fire activity, generating thicker smoke plumes aloft. Additionally, high density altitude will be a prominent operational concern as afternoon temperatures reach the 90s and low 100s.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Red Flag Warning until 9 PM MDT this evening for Bitterroot... Deerlodge/West Beaverhead...East Beaverhead...East Lolo.

Red Flag Warning from noon today to 9 PM MDT this evening for Flathead Reservation...West Lolo.

ID...Red Flag Warning until 11 PM PDT this evening for Palouse/Hells Canyon.