731 FXUS65 KTFX 122154 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 254 PM MST Thu Nov 12 2020 .SYNOPSIS... A strong a moisture rich westerly flow aloft will maintain windy conditions and mountain snow through Saturday. Areas of drifting snow will continue until the existing snow is either scoured away or melts. Breezy and seasonally mild weather continues into next week, along with periods of mainly mountain snow. && .DISCUSSION... Area web cams reveal ongoing drifting and blowing snow across North-central Montana, mainly along and west of I-15. Wind gusts are gradually climbing toward 35 mph in this area, and closer to 50mph along the Rocky Mountain Front. An increasing surface pressure gradient is responsible for the winds. Winds are forecast to increase further overnight along the Rocky Mountain Front as subsidence follows behind the exiting shortwave ridge aloft. A high wind warning is now in effect for the Rocky Mountain Front, with the expectation that winds will exceed 58 mph in this area and produce continued impacts from cross winds and drifting snow. Along with the increasing cross-barrier wind, the strong westerly flow will bring a deep layer of Pacific moisture up to the Continental Divide. The associated orographic forcing will produce a prolonged period of mountain snow up to, along, and adjacent to the Continental Divide over the Northern Rockies, and over other ranges of Southwest Montana. Accumulating snow is expected over Marias Pass and may extend out to Highway 89 along the Rocky Mountain Front. Total accumulations are expected to exceed 6 inches at pass level, but will be spread out over a two- day period. A winter weather advisory is in effect to address the potential hazards of slippery travel over mountain passes in falling and blowing snow. The Pacific moisture and associated mountain snow expands further into Southwest Montana during the day Friday and into Saturday, with similar impacts expected for the higher mountain passes of Southwest Montana, including the West Yellowstone area. The lee side surface low will eject off the Rocky Mountain Front early Saturday, causing an expansion of high winds across North- central Montana. Although widespread strong wind gusts are expected, there remains some uncertainty regarding the threshold for reaching high wind warning criteria of 58 mph. A high wind watch has been added to highlight the expanding area of strong winds. Check for later forecast updates regarding the additional potential for high winds on Saturday. All these weather impacts will decrease in coverage and intensity late Saturday into Sunday as a shortwave transient ridge crosses the region. However, periods of mainly mountain snow may continue in this moist and unsettled northwesterly flow. Ensemble forecasts reveal a generally dry week ahead outside of mountain areas, and overall seasonally cool temperatures in the 30s and 40s. However, the forecast ensemble retains a rather large degree of spread in their wind and temperature elements. The uncertainty in the longer range forecast stems from timing differences among a series of fast moving waves in the energetic westerly flow. && .AVIATION... 1045 AM MST Thu Nov 12 2020 (12/18Z TAF period) VFR conditions are expected to prevail over the next 24 hours at most TAF locations; however, low level clouds and light snow over KWYS will continue to lead to IFR/MVFR conditions through at least the afternoon hours today. Mid- and upper level cloudiness will then increase from west to east over the area between 06z-12z Friday as moist, and strong westerly winds aloft overspread the Northern Rockies. These strong westerly winds aloft will lead to further increasing of the surface winds (from the breezy winds that are observed today) after 12z-15Z Friday. In addition to increasing surface winds there will also be increasing chances for mountain wave turbulence and low level wind shear over/at most of the region/terminals after 00Z Friday. - Moldan Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .PRELIMINARY POINT TEMPS/POPS... GTF 23 41 28 37 / 0 10 20 10 CTB 18 37 23 33 / 0 20 30 30 HLN 17 42 27 38 / 0 30 40 10 BZN 11 40 22 37 / 0 40 60 20 WYS 8 30 15 26 / 10 100 100 50 DLN 11 35 18 32 / 0 70 80 20 HVR 18 40 25 35 / 0 10 30 40 LWT 18 42 26 35 / 10 10 10 20 && .TFX WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory from 3 PM Friday to 5 PM MST Saturday above 6500 feet for Beaverhead...Gallatin...Madison. High Wind Watch from late Friday night through Saturday afternoon Cascade...Central and Southern Lewis and Clark... Eastern Glacier...Eastern Pondera...Eastern Teton...Judith Basin...Meagher. High Wind Warning from 3 AM Friday to 6 PM MST Saturday Northern Rocky Mountain Front...Southern Rocky Mountain Front. Winter Weather Advisory from midnight tonight to 2 PM MST Saturday above 4500 feet for Northern Rocky Mountain Front... Southern Rocky Mountain Front. && $$ http://www.weather.gov/greatfalls