textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Periods of haze and smoke persist across the region into tonight. Low rain shower and storm chances.

- Monsoon moisture surge returns Tuesday through Thursday. Localized flash flooding and a few severe thunderstorms possible.

- Hot temperatures are expected this week, with highs reaching the 90s up to 106 degrees in the lower elevations of Idaho.

This afternoon through Wednesday...Upper level monsoonal moisture will continue to stream across the Northern Rockies into Tuesday morning, bringing persistent high and mid-level cloud cover and isolated light showers. Smoke and haze from wildfires in Oregon and Washington will intermittently degrade air quality, though a developing easterly pressure gradient tonight may offer some localized clearing.

Atmospheric moisture deepens significantly on Tuesday, with precipitable water (PWAT) values climbing to 200-250% of normal for the region. However, abundant morning cloud cover will likely limit surface instability and overall thunderstorm potential. By Tuesday afternoon, high-resolution guidance indicates scattered thunderstorm development from Lemhi County into Southwest Montana, where brief heavy rainfall and erratic wind gusts exceeding 40 mph will be the primary threats.

By Wednesday afternoon, the threat shifts slightly northward. Areas south of U.S. Highway 12 will see an increased risk of locally heavy rainfall capable of triggering debris flows or flash flooding. The overall magnitude of this threat remains highly conditional on morning clearing and the resulting realized instability.

It will also be quite warm to hot. NBM temperatures are on the warmer side compared to most guidance and would bring temperatures near heat advisory levels to the lower elevations of north-central Idaho. However, there is considerable uncertainty regarding how much clouds and smoke may impact the high temperature forecast. Still advisories may be needed for the Wednesday/Thursday time period if confidence increases.

Thursday through the Weekend...Thursday presents a more dynamic setup as the region transitions from deep monsoonal moisture to a drier, more sheared southwesterly flow. This combination of enhanced vertical wind shear and increased instability could support strong to severe thunderstorms, making Thursday a key period to monitor.

By Friday and into the weekend, the synoptic pattern shifts toward near-critical fire weather conditions. Increasing westerly winds, a drying air mass, and a sharp decline in shower activity will combine to elevate fire danger across the region.

AVIATION

VFR conditions will prevail through Tuesday morning. Monsoonal moisture overspreading the region will maintain widespread high and mid- level cloud cover and isolated light showers. By Tuesday afternoon, scattered thunderstorms are possible across Lemhi County and Southwest Montana; however, convective development remains highly conditional on the clearing of morning cloud cover. If these storms materialize, brief MVFR ceilings and visibilities may impact KSMN and KBTM after 21/2200Z. Winds will remain relatively light and diurnally driven through this afternoon. An easterly pressure gradient will develop across western Montana tonight into Tuesday morning, but peak gusts are expected to remain under 20 knots.

MSO WATCHES/WARNINGS/ADVISORIES

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