textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- A sharp cold front will move north to south today, bringing scattered thunderstorms, localized heavy rain, and wind gusts up to 40 mph.

- Building high pressure this weekend into early next week ushers in a strong warming trend, with 80s for western Montana and mid-upper 90s for central Idaho.

- A "dirty" ridge pattern may allow weak shortwaves to sneak through the prevailing northwest flow after Tuesday, introducing low-confidence shower chances.

A potent shortwave and surface cold front dropping out of Canada will track southward today. Convective timing has slowed slightly from previous expectations. Showers and thunderstorms will initiate near the Canadian border by mid-late morning, progressing southward and arriving along the Interstate 90 corridor later in the evening, generally between 6:00 pm and 8:00 pm local time.

Atmospheric moisture is relatively limited, but strong frontal forcing will help to compensate. Moderate storm intensities are anticipated, but isolated, stronger convective cores (particularly in northwest Montana and over the higher terrain) could yield brief, intense short-duration downpours with rainfall rates of 0.30 to 0.50 inches per hour. Small hail and erratic microburst outflow up to 40 mph are other primary hazards. Activity decays rapidly as the front crosses the I-90 corridor, keeping north-central Idaho and southwest Montana largely dry. Cool, breezy northerly flow establishes behind the front on Saturday.

An unseasonably strong ridge of high pressure will shift inland along the Pacific Northwest coast this weekend. This leaves the Northern Rockies positioned under northwest flow aloft with strong subsidence and warming to our west. Valley temperatures will rapidly climb into the 80s across western Montana. Extreme low- elevation river valleys of central Idaho will push into the mid- to-upper 90s by Tuesday.

The ridge axis reappears somewhat flat rather than highly amplified. This structural configuration frequently results in a "dirty" ridge setup, where subtle shortwaves can sneak through the prevailing flow. While current deterministic guidance is mostly dry, this pattern could, at minimum, spark isolated mountain showers into the forecast after Tuesday. Ensembles exhibit very poor agreement regarding the ultimate breakdown of the ridge late next week.

AVIATION

Dry conditions will persist this morning before a cold front dives southward out of Canada. Frontal passage will trigger scattered thunderstorms capable of localized MVFR visibility/ceilings in embedded heavy downpours, small hail, and erratic outflow wind gusts as high as 40 knots. Convective timing initiated at KGPI around and after 12/2000z, reaching KMSO later this evening between 13/0100z and 13/0400z, before weakening into widely scattered showers along the I-90 corridor into the overnight. Post-frontal northerly winds will remain gusty before stabilizing late overnight.

MSO WATCHES/WARNINGS/ADVISORIES

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