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.

The Northern Rockies will be under a zonal westerly flow pattern through this week, with gusty westerly winds impacting valley areas each afternoon and evening. Snow levels have fallen near valley floors today following the passage of a cold front. Snow accumulations through Tuesday will focus across area mountain ranges, with milder surface temperatures (generally 30s) and the March sun angle, limiting accumulations for valley areas. Nonetheless, travelers should prepare for changeable conditions, especially across area passes as roadways become snow covered at times through Tuesday.

A long duration atmospheric river event takes center stage Wednesday into Saturday, with confidence continuing to increase for significant impacts from heavy snow and strong winds across backcountry and mountain pass areas above 5,000 feet. Confidence in this atmospheric river is high, with ensemble guidance suggesting >90% probability for Integrated Vapor Transport (IVT) values to reach 250 kg/ms along the divide over a 36-48 hour period, centered on late Wednesday into early Saturday. This is a trend to a longer duration event compared to the forecast yesterday. Furthermore, the IVT values progged by model guidance are near the 95-99% percentile of climatology, pointing towards a significant atmospheric river event. These potentially historic (for March) IVT values are being driven by a significant jet stream aloft, with EPS ensembles pointing towards 700mb winds (winds near 10,000 feet MSL) of 65-75kts (75-85 mph), which would be near an all-time high for March. The combination of strong winds and high atmospheric moisture will lead to significant mountain precipitation, with widespread precipitation totals of 3-6 inches along the Idaho-Montana border and northwest Montana ranges. Favored crests within the Bitterroot, Clearwater, and Swan Ranges may see precipitation totals approaching 6-8 inches over a 72-hr period ending on Saturday.

The atmospheric river will focus travel impacts across the highest mountain passes, such as Marias, Lolo, and Lost Trail Passes as snow levels rise throughout Wednesday nearing 4,000-4,500 across northwest Montana and 5,000-6,000 feet along and south of I-90. Winter Storm Watches have been issued for the Glacier Park region, and elevations above 5,000 feet in the southern Clearwater Mountains, and Bitterroot and Sapphire Mountains, with blizzard-like conditions expected at times. Blizzard conditions are of particular concern for Marias Pass, late Wednesday night into Thursday morning, where the probability for wind gusts in excess of 50 mph is greater than 80%. Snow amounts will be measured in feet, with the highest accumulations above 5,000 feet along and north of the I-90 corridor. Those heading into the backcountry should exercise extreme caution given the severity of this atmospheric river event and should monitor the latest avalanche forecasts.

Finally, valley wind impacts are a notable risk with this system. High wind watches have been issued for the Kootenai/Cabinet and Butte/Blackfoot region Thursday for strong to locally damaging winds. Model guidance suggests a surface low will track along the USA/Canada border late Wednesday night into Thursday morning. The passage of a surface cold front, along with strong cold air advection aloft will increase the risk for stronger winds aloft to mix down to the surface. Currently, the highest probabilities (50% or higher) for gusts of 50 mph or higher are focused across Lincoln County in northwest Montana and along the divide in southwest Montana, generally from Garrison through Anaconda and Butte. Future shifts will monitor the need closely for further wind highlights as confidence changes for convective winds to impact other areas. /AL

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. The biggest impact for aviation through this evening will continue to be strong wind gusts and wind shear. Tomorrow will be similar but with wind speeds trending lower for a day, so there is a better chance for some showers to become organized tomorrow.

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 evening for Butte/Blackfoot Region.

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