textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Breezy winds 15 to 25 mph on Wednesday will cause choppy conditions on area lakes. A few thunderstorms are possible in southwest Montana and Lemhi County.

- Warmest temperatures of the summer this weekend through early next week.

- Low relative humidity and increased west winds will result in elevated fire weather concerns on Saturday and Sunday

A weak cold front moving through western Montana and north-central Idaho will briefly suppress the current high-pressure system today. While conditions will remain generally warm and dry, this frontal passage will generate breezy westerly winds, particularly across northwest and west-central Montana. Sustained wind speeds of 15 to 25 mph are expected, which will likely create choppy and hazardous boating conditions on area lakes, with Flathead Lake being of primary concern. Concurrently, thunderstorm activity will increase across southwest Montana and Lemhi County compared to yesterday. The primary hazard associated with these storms will be sudden, gusty outflow winds. High-resolution models indicate a 20 to 30 percent probability of wind gusts exceeding 40 mph with storms along the Interstate 90 corridor from Garrison Junction to Butte, and along Interstate 15 south of Butte. There remains a lower probability of around 10 percent that isolated gusts could reach up to 55 mph in the most intense storms.

High pressure will rebuild across the region heading into the weekend, temporarily reducing thunderstorm activity for Thursday and Friday while initiating a significant warming trend. By Sunday, daytime highs are forecast to reach the 90s across the valleys of western Montana, with triple-digit heat expected in the lower Salmon and Clearwater river valleys. The combination of an approaching upper-level jet streak bringing increased westerly winds, low daytime relative humidity, and soaring temperatures will lead to elevated fire weather conditions across much of the Northern Rockies by Saturday and Sunday.

This early-season heat wave also introduces an increased heat risk for vulnerable populations, such as the elderly and the very young, prompting the potential issuance of Heat Advisories. Notably, the most abnormally hot conditions may develop along the Continental Divide into southwest Montana. Current projections show a 20 percent chance for temperatures in Butte to reach 100 degrees on Sunday, which would tie an all-time historical record set on July 22, 1931.

Looking toward the middle of next week, forecast guidance indicates a potential shift in the prevailing weather pattern. Between July 13th and 15th, a significant surge of subtropical moisture is projected to enter the region. This influx of moisture would elevate the risk for widespread, rain-producing thunderstorms across western Montana and north-central Idaho, providing a distinct change from the dry, and windy conditions anticipated over the weekend.

AVIATION

VFR conditions are expected to widely prevail across the airspace today, though a weak cold frontal passage will introduce localized aviation hazards. West to northwest surface winds will increase across northwest and west-central Montana terminals, with sustained speeds of 15 to 25 mph (15 to 20 knots) anticipated. Meanwhile, scattered convective activity will develop across southwest Montana and Lemhi County. The primary threat to aviation operations from these thunderstorms will be erratic, strong outflow boundaries. High-resolution guidance indicates a 20 to 30 percent probability of convective wind gusts exceeding 35 knots near flight routes and terminals along the I-90 and I-15 corridors south of Garrison Junction, with a 10 percent probability of localized gusts reaching up to 50 knots in the most intense cells.

MSO WATCHES/WARNINGS/ADVISORIES

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