textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Long duration atmospheric river event Wednesday night into Saturday.

- Very difficult backcountry conditions with strong winds and heavy snow, leading to areas of blizzard conditions.

- Risk for strong, locally damaging winds Thursday, focused across northwest Montana and along the divide.

Breezy west winds and light showers throughout today with below normal temperatures is forecast.

Late Wednesday through Saturday, a long duration high intensity atmospheric river event is on track to affect the Northern Rockies. Strong to damaging winds throughout Thursday morning are expected in western MT. Widespread 50 to 100% probabilities of wind gust 50 mph or stronger are forecast in western MT, valleys orientated northwest to southeast have lower probabilities, 20 to 50%. Every mountain range has a 50 to 70% probability of gusting 60 mph or stronger, there will localized gusts as high as 80 mph. Winds will be slightly weaker in north- central ID but strong winds are still forecast. Watch for tree damage and potential power outages from the winds.

This will be a very wet system. As with most Atmospheric Rivers they originate in the subtropics and are warm systems. Wednesday snow levels quickly rise to around 5000 feet and remain there through Saturday I-90 and south. Snow levels north of I-90 will be roughly 1000 feet lower. Specific snow forecasts, even at this time frame, will not be reliable. However, 3 to 6 feet of snow in the highest elevations of the Clearwater, Bitterroot, and Mission Mountains including almost all ranges of northwest Montana is moderate probability. Travel or recreation in the back country during this timeframe will be dangerous to impossible, expect blizzard conditions at times, downed trees, and feet of heavy, wet snow. Please check road reports and follow local avalanche center guidance. Typical areas for rock slides; US-12 either side of Lolo Pass, I-90 vicinity of Lookout Pass, US-200 east of Missoula, and US-93 either side of Lost trail Pass, are already experiencing rock slides. This trend will not improve through the weekend.

Valleys will be trickier: if the wind forecast materializes, storm total precipitation will likely verify at the 25th percentile due to severe valley downsloping, widespread 0.50 to 0.75" over 72 hours would be reasonable. Conversely, if precipitation verifies at the 75th or high percentile, 1 to 2" of rain, then winds will likely be subdued by moderate precipitation rates. Lemhi County ID and in the vicinity of Butte have the lowest precipitation totals in the current forecast. While typical low lying and poor drainage areas will be susceptible to ponding of water, no rivers are currently forecast to reach action stage or higher.

Please keep apprised of future forecasts in case potential impacts change.

The active weather will continue throughout the weekend and into early next week with less intensity.

AVIATION

A strong jet stream over the region in zonal flow is helping to drive strong, gusty, westerly winds. The longer days and higher sun angle as we approach the equinox are also helping to destabilize the boundary layer. As a result, some showers are trying to form in the region, but the wind shear is so strong compared to the instability that it keeps tearing the updrafts apart before they can organize. The result has been, and will continue to be very light showers with sprinkles or flurries and visibility reductions down to 6SM at times. The biggest impact for aviation through this evening will continue to be strong wind gusts and wind shear.

Late Wednesday through Saturday, the Northern Rockies airspace will be impacted by a high intensity Atmospheric River system.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Winter Storm Watch from Wednesday evening through Saturday morning for Bitterroot/Sapphire Mountains...West Glacier Region.

High Wind Watch from Wednesday evening through Thursday morning for Kootenai/Cabinet Region.

High Wind Watch from late Wednesday night through Thursday morning for Lower Clark Fork Region.

High Wind Watch from late Wednesday night through Thursday evening for Butte/Blackfoot Region.

ID...Winter Storm Watch from Wednesday afternoon through Saturday morning for Southern Clearwater Mountains.