textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Areas of smoke continue to degrade air quality, with impacts focused along the US-93 corridor this morning.

- Hot, dry, and windy conditions to bring critical fire weather across Southwest Montana today.

- A gradual cooling trend by the middle of next week; rain and thunderstorm chances, focused across western Montana.

Fire Weather and Air Quality:

*A dry, westerly flow pattern continues across the Northern Rockies today. Air quality index readings this morning show unhealthy for sensitive groups to unhealthy AQI along the US-93 corridor in western Montana. Strong valley inversions have developed overnight and will trap smoke through most of the morning. An upper level disturbance will move through the flow this afternoon and evening, enhancing boundary layer mixing and ventilation. This will help break valley inversions and improve smoke ventilation, but will lead to elevated to critical fire weather from hot, dry, and windy conditions. Red Flag Warnings have been issued for the Beaverhead-Deerlodge National Forest, with the strongest winds expected to bring the worst fire weather conditions along and east of the Continental Divide. Similar to yesterday, wind gusts will reach 30-35 mph, with localized gusts near 40 mph for the most exposed locations.

Cooling Trend & Moisture Surge (Wednesday-Thursday):

*A change begins Wednesday as an upper-level trough crosses over the Pacific Northwest. Showers and thunderstorms will initially form along the US/Canada border and the Continental Divide Wednesday before expanding across the region by Thursday. This shift is driven by deep south-southwest flow drawing a plume of monsoonal moisture northward. Primary hazards include gusty, erratic outflow winds, the potential for new fire starts from lightning, and localized heavy rainfall by Thursday as precipitable water values increase near 1 inch. Smoke remains the primary forecast challenge, potentially limiting atmospheric instability and consequently thunderstorm coverage, especially if smokes limits daytime heating. Concurrently, model guidance indicates a surface cold front driving southward along the Divide Wednesday morning but stalling out in northwest Montana. A strong push Wednesday night into Thursday morning will generate a north-northeast wind shift for much of western Montana that may yield temporary relief from dense smoke with moderate confidence at this point.

Extended Outlook (Friday Through Weekend): *A prevailing southwest flow regime will keep thunderstorm chances elevated primarily across western Montana through the weekend, with isolated activity persisting farther north and west. Model sounding indicate drying of the low-levels, allowing gusty outflow winds to be a concern once again.

AVIATION

Wildfire smoke will remain the primary operational hazard today, with surface visibilities between 2 and 6 SM. The most persistent visual degradation will focus along the US-93 corridor, including KGPI, KMSO, and KHRF this morning. An upper level disturbance will pass through this afternoon, enhancing vertical mixing and improving smoke dispersion. Consequently, this will lead to elevated to critical fire weather, with fires upstream of the Northern Rockies expected to become active once again. West-northwest surface winds will gust 15-25 kt by mid-late afternoon, before weakening around sunset.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Red Flag Warning from noon today to 9 PM MDT this evening for Deerlodge/West Beaverhead...East Beaverhead.

ID...None.