textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Widespread smoke continues to degrade air quality, with the highest impacts across northwest Montana.

- Elevated to near-critical fire weather across Southwest Montana today.

- A gradual cooling trend by the middle of next week; rain and thunderstorm chances, primarily along the Continental Divide.

As with the last several days, the Northern Rockies will remain under a dry westerly flow aloft through Tuesday. A large ridge of high pressure remains centered to the south, with the Northern Rockies situated on the northern periphery of the ridge. Fire weather remains a concern, with hot, dry, and breezy conditions bringing areas of near-critical to localized critical fire weather, focused along and east of the divide in Southwest Montana. Areas in the vicinity of Dillon have the highest probability (40-50%) of reaching Red Flag Warning criteria as gusts reach 20-30 mph by mid-late afternoon.

Wildfire smoke continues to degrade air quality and visibility across the region. Unhealthy to very unhealthily Air Quality Indexes have been observed along and north of the I-90 corridor in northwest Montana. These impacts are driven by significant wildfire activity upstream in Oregon and Washington. Looking ahead to this afternoon, deeper mixing associated with an upper level disturbance will help to provide improved ventilation and perhaps improvements to visibility. This will largely depend on how active wildfires become upstream.

Model guidance continues to highlight a cooling trend by Wednesday into Thursday as a trough of low pressure moves southeastward from British Columbia. Shower and thunderstorm activity will initially focus along the USA-Canadian border and divide in northwest Montana Wednesday afternoon, before spreading southward across much of the area by Thursday. This change will be driven by increased atmospheric moisture and instability as deepening south-southwest flow moves monsoon moisture into the region. Furthermore, model guidance suggests a cold front will track southward along the divide Wednesday night into Thursday morning, bringing a north-northeast wind shift across western Montana.

By Friday and into next weekend, the focus for thunderstorm activity will shift south into Southwest Montana as the region transitions into a southwesterly flow pattern.

AVIATION

Wildfire smoke will continue to bring visibility impacts, with observations fluctuating between 2 to 6 miles. The lowest visibility is expected northwest Montana, north of the I-90 corridor. Light and variable winds this morning will become west-northwest, with gusts of 15-20kts by late afternoon as an upper level disturbance enhances vertical mixing. High density altitudes will also persist at higher elevation airfields with warm temperatures in the afternoon and early evening hours.

MSO WATCHES/WARNINGS/ADVISORIES

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