textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Widespread showers and thunderstorms this afternoon and evening, with gusty outflow winds of 30-50 mph, brief moderate rainfall, and lightning, strongest storms south of Highway 200.

- Another round of storms Wednesday capable of 30-50 mph outflows and heavy rain rates up to 0.5 inches per hour.

- Persistent active pattern late week into the weekend under southwesterly flow, with a 20 percent chance of a tropical moisture tap on Sunday or Monday.

As low pressure off the Pacific northwest coast begins to shift inland, a moist southwest flow aloft will transport monsoon moisture from the Desert Southwest into north central Idaho and western Montana. Showers and thunderstorms will develop through north central Idaho and southwest Montana between 1 and 3pm MDT and shift northwards into the rest of western Montana through 10 pm MDT tonight. With thunderstorms moving into the hot and dry airmass already in place, gusty outflow winds will be the main threat. High resolution models generally show gust potential between 30 to 50 mph with storms.

By Wednesday, as the low pressure system continues to move eastwards, a cold front will move through the region, first through north central Idaho in the morning, then into western Montana late morning through early afternoon. Another round of thunderstorms is expected to develop ahead of and with the cold front. Storms will be more moist on Wednesday, with stronger storms capable of half an inch of rainfall in an hour or less. With strong forcing aloft, Wednesday's set up is slight better to favor strong to locally severe thunderstorms in the afternoon, especially in northwest Montana near the Continental Divide where a later frontal passage will correspond with max daytime heating. High resolution models show localized gusts up to 60 mph will be possible. It does appear the threat for strong storms will end earlier in the day, possibly as early as 6pm. The good news: There is high confidence (80 percent) of widespread wetting rain (0.10 inch or greater) through Thursday morning other than for far southwest Montana.

Across far southwest Montana gusty winds and low afternoon relative humidity Wednesday afternoon and evening are expected to create critical fire weather conditions through Wednesday afternoon through Thursday afternoon, before the cold front finally passes through Thursday night.

Southwesterly flow aloft will persist Thursday into Friday as a broad upper-level trough settles over the Pacific Northwest. This pattern will keep temperatures near to slightly below late-summer averages alongside periodic showers and thunderstorms, particularly during peak afternoon and evening heating. Guidance remains in solid agreement that troughing will linger over the region through the weekend, with roughly 20 percent of ensemble members indicating a potential tap of significant tropical moisture Sunday or Monday. Regardless of whether this tropical plume materializes, the Northern Rockies will remain embedded in an active, unsettled weather pattern through the weekend.

AVIATION

Thunderstorms capable of strong outflow winds (25-45kts) are the primary weather concern through this evening. Convective initiation remains on track across central Idaho by early this afternoon, with storms tracking northeast into western Montana through mid-evening. Primary operational hazards include frequent lightning and erratic outflow winds of 25-45 kt. Although convective instability diminishes northward toward the I-90 corridor, linear storm organization will likely sustain strong outflow boundaries pushing into I-90 terminals and potentially into portions of northwest Montana. Showers and isolated thunderstorms may linger through late evening.

By 15Z Wednesday, high-resolution guidance indicates a secondary wave of convection initiating across southwest Idaho and lifting northeast across the Northern Rockies through mid afternoon. Outflow winds of 30-45 kt will once again serve as the primary aviation hazard.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Red Flag Warning from noon Wednesday to 8 PM MDT Thursday for East Beaverhead.

ID...None.