textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Scattered thunderstorms today into Thursday, with lightning and outflow winds being the main threats.

- A cold front arrives Saturday, bringing cooling temperatures, gusty westerly winds, and precipitation, focused along and south of I-90.

The Northern Rockies will be under the influence of a southwesterly flow pattern aloft through Friday. This pattern is being driven by a ridge of high pressure centered over the Four Corners region and a trough of low pressure centered over southern British Columbia. Waves of monsoon moisture will bring modest moisture and instability into the area, opening the door for showers and thunderstorms each day. Warm and dry air near the surface will promote gusty to strong (30-50 mph) outflow wind gusts. The combination of dry storms, lightning, and gusty winds will elevate the potential for new fire starts. The thunderstorm threat will include all of the Northern Rockies this afternoon and evening, with activity shifting into Lemhi County, ID and southwest Montana on Thursday. A Red Flag Warning is in effect for the Palouse/Hells Canyon Region, where hot, dry, and windy conditions are expected this afternoon and evening.

Forecast models have come into stronger agreement for the aforementioned trough to push eastward across the Pacific Northwest into the Northern Rockies by late Saturday into Sunday. A subtle drop in temperatures arrives Friday, with temperatures dropping further on Saturday as a cold front moves through the region. While model solutions still differ on the evolution of the trough and moisture availability for precipitation, the probability for wetting rains has increased from 30-40% (yesterday) to 70-80% this weekend for areas along and south of I-90. Generally speaking, probabilities for precipitation decrease further north towards the US-Canadian border. Waves of monsoon moisture will focus atmospheric moisture and instability along and south of I-90, driving higher chances for showers and thunderstorms as the trough arrives. This moisture will limit the potential for critical fire weather.

AVIATION

Afternoon and evening thunderstorms will be the primary impact to aviation today and Thursday. Convection will initially develop around 26/2100Z near KSMN and KBTM, eventually spreading northward toward KMSO, KHRF, and KGPI this evening. The primary aviation hazards with these high-based storms will be localized, erratic outflow wind gusts of 30 to 40 knots. Localized gusts to 50 kts are a concern near the strongest thunderstorms today. Flight categories will largely remain VFR, though brief MVFR conditions and localized mountain obscurations are expected during direct thunderstorm impacts.

MSO WATCHES/WARNINGS/ADVISORIES

MT...None. ID...Red Flag Warning until 7 PM PDT this evening for Palouse/Hells Canyon.