textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Easterly gap winds strengthen tonight into Sunday morning. Anticipate choppy lakes and strong canyon winds.

- Widespread rain and mountain snow above 7,000 feet late tonight into early Monday, mainly through central Idaho and northwest Montana.

- Drying and warming conditions by mid-week.

Satellite imagery early this afternoon across the Northern Rockies highlights a cirrus cloud shield moving overhead under a westerly flow aloft. A broad low pressure system is seen off the Oregon coast. Easterly winds observed earlier this morning across western Montana will weaken and shift to the west-northwest by mid-afternoon into this evening. Cooler temperatures will be felt across northwest Montana and along the divide today as the easterly gradient this morning ushered in a colder airmass than felt previously the last few days.

Looking ahead, the low pressure system will move inland tonight into Sunday morning, tracking across eastern Washington, before moving along the USA-Canadian border Sunday night into early Monday morning. Easterly winds will once again develop after sunset tonight across western Montana and will trend stronger than last night. The strongest gusts (30-45 mph) will be felt on western slopes of the Whitefish, Swan, and Mission mountains and in canyon/gap locations. The winds will generally peak during the late evening to early morning hours on Sunday as the low tracks into eastern Washington and the pressure gradient is maximized. Those planning to head outdoors Sunday morning should be prepared for choppy conditions on area lakes and cool, blustery conditions in the backcountry.

The low pressure's northern track continues to focus precipitation amounts across north-central Idaho and northwest Montana, where half an inch to around an inch of rain is forecast across the higher terrain, with lesser amounts (0.10-0.50 inches) in valley areas. Snow levels will fall near 7,000 feet by Sunday night in these areas. Backcountry recreationists should prepare for cold, wet, and raw conditions that will significantly increase the risk of hypothermia for anyone caught unprepared. Further south and east, including the Missoula area, precipitation amounts will be notably less (< 0.10 inches). Southwest Montana, east of the divide (east and south of I-15 and I-90), will remain dry with gusty southwest winds, resulting in critical fire weather, where a Red Flag Warning is in place for Sunday afternoon and evening.

The system will depart to the east by Tuesday, transitioning the region to a generally warmer and drier weather pattern through mid-week. Despite the overall drying trend, a few weak atmospheric disturbances may still trigger isolated, light showers. Tuesday morning stands out as having the potential to be one of the coldest mornings yet this Fall. The higher valleys of southwest Montana and areas along the Continental Divide will see low temperatures near or below freezing. Frost is also a potential concern for the Missoula and Bitterroot Valley and higher mountain valleys within the Clearwater Mountains of Idaho.

AVIATION

VFR conditions will continue through this evening across western Montana and north central Idaho ahead of an approaching low-pressure system. Aviation operations will be impacted by increasing easterly winds, as winds develop this evening (13/0100-0300Z), and peak tonight into early Sunday morning. Gusts of 30-45 mph will develop near favored canyons and western facing slopes, likely inducing localized mechanical turbulence and areas of low-level wind shear. Flight conditions are expected to deteriorate late tonight through Sunday as widespread showers and isolated thunderstorms overspread the airspace. This incoming system will lead to widespread mountain obscurations and periods of MVFR to locally IFR ceilings and visibilities, with the most significant flight impacts concentrated over north-central Idaho and northwest Montana where the heaviest precipitation is forecast.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Red Flag Warning from noon to 9 PM MDT Sunday for East Beaverhead.

ID...None.