textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Juicy, scattered showers and thunderstorms this afternoon and evening will produce localized lightning, downpours, small hail, and erratic outflow winds up to 45 mph.

- A Pacific front Friday brings heavy rainfall potential to north- central Idaho, focusing from the US-95 corridor through the Camas Prairie and northeastward along Highway 12.

- Widespread weekend rainfall across western Montana, elevating risks for hydrologic responses and rockfalls especially near and along the Continental Divide.

Abundant boundary layer moisture (dewpoints in the mid-to-upper 50s) combined with afternoon heating will trigger pulse thunderstorms this afternoon and evening. Stubborn low-level clouds and stratus have slightly delayed heating across the Mission and Flathead valleys and points west, but this cloud cover is now steadily eroding. Once full insolation is realized, rapid cellular development will initiate off the terrain and drift slowly from southeast-to-northwest though about 10pm local time tonight. Stronger cells will carry localized threats of lightning, small hail, heavy downpours and strong winds. Storm coverage diminishes for Thursday afternoon's period of convective instability shifting primarily north of I-90. We are still monitoring elevated fire weather concerns for Thursday afternoon in southwest Montana (primarily south of Dillon) due to gusty winds and relative humidities likely dropping into the teens.

A landfalling Pacific trough moves inland for Friday, focusing a band of heavy rain primarily across north-central Idaho along the US-12 and 95 corridors and secondarily into western Montana.

This trough will kick the Great Basin closed low northeastward into the Yellowstone region by Saturday. Strong model agreement places central and western Montana under a robust deformation zone within the northwest quadrant of the parent low, tapping into a deep southeasterly moisture plume. Expect prolonged, efficient rainfall Saturday morning through Sunday. Because recent and progged rains have already begun to prime soils, and the ongoing high-elevation snowmelt regime, this weekend system introduces concerns for elevated stream and river responses as well as risks for rockfall in canyon terrain. WPC maintains a Marginal Risk for excessive rainfall across all of western Montana this weekend. Snow levels will drop from above 10,000 feet down to near 6,000 feet by Sunday morning along the Divide.

AVIATION

Moist southerly flow continues across the Northern Rockies ahead of a cut-off low meandering around central California. Scattered showers and thunderstorms will continue to initiate across southern Idaho County eastward into southwest Montana and gradually work northwestward into this evening but overall less coverage compared to yesterday (Tuesday). Gusty winds to 40 kts and hail are possible with the stronger thunderstorms.

Patchy fog/stratus may develop on Thursday morning where rain occurs today but once again much less coverage compared to the low clouds experienced this morning.

Another round of afternoon showers and thunderstorms is expected on Thursday but focused more across northwest Montana.

MSO WATCHES/WARNINGS/ADVISORIES

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