textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Gusty west winds Friday afternoon creating near-critical fire weather conditions across Southwest Montana, choppy conditions at area lakes, and hazardous conditions for empty, high-profile vehicles on I-90 Deer Lodge to Butte, and I-15 south of Butte.

- Strong Northeast winds develop Saturday causing choppy waves that could pose a danger to docks and boats on the southwestern portion of Flathead Lake. Isolated power outages are possible across northwest Montana through Saturday night.

- The first Winter Storm is expected to bring snow, cold temperatures and gusty northeast winds to the mountains and high valleys from west- central to southwest and central Montana late Saturday through Sunday and Sunday night. This could highly impact backcountry recreation, hunters and travel.

Today and tomorrow will be the last summer-like days for us for a while. On Friday, a relatively dry cold front will move across the region. The westerly wind gradient will tighten and southwest winds of 15 to 25 mph will bring a 1 to 2 foot chop on Flathead Lake. Stronger winds of 20 to 30 mph with gusts to 45 mph are possible from Lemhi County to southwest Montana. Besides the dangers of cross-winds on north-south highways, the combination of dry and windy conditions will create near-critical fire weather conditions across southwest Montana.

On Saturday, as a low pressure system moves onshore over Washington and Oregon, a northeast wind gradient will develop across much of the region. At the same time, a cold, sub- freezing airmass will push south out of Alberta Canada which will enhance the northeast winds even more. Flathead Lake will again experience choppy conditions, but from the opposite direction (from the northeast) by Saturday afternoon. Northeast winds of 15 to 25 mph with gusts to 35 mph could pose a danger to docks and boats from Yellow Bay south and west towards the Narrows and Big Arm. The lake is at 2,892.52 feet which is 0.48 feet below full pool.

Moisture wrapping around the trof over Oregon will overrun the cold air that will be pushing southwards across western Montana Saturday night. Snow levels will be falling to valley floors along and west of the Divide. The cold air will start to lose steam as it banks up against the Idaho/Montana border, so snow levels in central Idaho will be higher generally 3000 to 4000 feet. Temperatures falling into the 20s at MacDonald Pass, combined with gusty northeast winds and snow will be enough to overcome the warm ground and could make travel conditions slick. Other locations including Georgetown Lake, Butte, and Homestake Pass could also experience winter-like conditions. The highest probabilities for 3 inches of snow or greater exist in these areas. Depending on how far north the moisture travels, even rain could changeover to snow for places like Missoula but may be minimal impact come Sunday morning except for the chill in the air! This is a very rare circumstance, but moisture from the remnants of Hurricane Rachel will track northeastwards from California and Nevada towards the Yellowstone and Wyoming area. We'll be just on the edge of further moisture and lingering snow later Sunday into Sunday night...i.e. Butte and Homestake Pass on I-90 may hold on to snow Sunday night!

As the weather system pulls away on Monday, high temperatures will be 5 to 8 degrees below average.

AVIATION

VFR conditions will continue across the Northern Rockies terminals through today under high pressure. Light terrain driven winds and mostly clear skies are expected, with passing high clouds. Strong valley inversions will promote the development of localized morning mist/fog near bodies of water, including area rivers and lakes. A dry cold front will bring gusty west to southwest winds on Friday. Terminals GPI and BTM may experience the strongest gusts between 25 and 35 knots.

MSO WATCHES/WARNINGS/ADVISORIES

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